Note: Descriptions are shown in the official language in which they were submitted.
~L~,6~37~L~
MASKING TECHNIOUES IN CHEMICAL VAPOR DEPOSITION
Background of the Invention
This invention relates to the fabrication of
semiconductor devices via chemical vapor deposition
and, in particular, the fabrication of such devices in
S metalorganic chemical vapor depositions (MO-CVD) with
nonplanar layer characteristics by means o~ mas~ing
techniques employed during growth.
It has been established in research and
development of semiconductor injection lasers having an
active layer and/or cladding layers which are nonplanar
and have spatial variation in their thickness exhibit
improved properties, such as, low threshold current,
linear light output versus current characteristics and
stable fundamental transverse mode control. Such
nonplanar variations are discussed in U.S. Patent
4,335,461 entitled "Injection Lasers With lateral
Spatial Thickness Variations (LSTV) In The Active
Layer" and assigned to the assignee herein~
To date, such nonplanar lasers have been
successfully grown by liquid phase epitaxy (LPE~.
Within the past several years, molecular beam
epitaxy (MBE) and metalorganic chemical vapor
deposition (MO-CVD) have become important processes in
the fa~ri_~lioIl of- ~;incle crystal semiconductor
integrated devices, including injection lasers. M~E is
a growth process carried out under ultra high vacuum
conditions, by the evaporation of the crystal
constituents and dopants and beam deposited on
substrates. MO-CVD is a gaseous crystal growth
technique in which compounds, such as, (CH3)3 Ga, are
caused to react with other gases, such as, AsH3, and
appropriate dopants, in the vapor phase to produce
single crystalline or polycrystalline deposits. These
two procedures have, to a large extent, replaced the
conventional LPE crystal growth techniques, owing to
their improved control over (1) layer thickness, (2)
crystal composition, (3) layer smoothness, (4)
1~ '; 4 'I';'i
lS
abruptness of interfaces, and (5) uniform doping
profiles.
~ EP processes permit nonplanar variations in layex
contours and thicknesses as desired. For example, LPE
growth of channeled substrate lasers produced curved
contours and thickness variations in deposited layers
on the substrate. However, MBE and MO-CVD processes
characteristically do not produce the same type of
growth variations. Depending upon deposit rate, flow
rate, substrate temperature, etc., the deposited layers
or films tend to "match" the contour and shape of the
depositing surface. It would be desirable to start
with a substrate surface with a curved contour having a
curved contour or taper adequate to produce the tapered
variations during growth, as taught in the previously
mentioned U.S. patent. ~owever, it is not readily easy
to fabricate the desired curvature in a substrate prior
to growth. It would be simpler to develop the desired
contour during growth, as done in the past, and obtain
better accuracy and control in the desired contour and
thickness variations that MBE and MO-CVD processes
would provide.
One way of accomplishing these spatial variations
in MBE is by employing a mask having an aperture. The
mask is positioned between the elemental sources and
the substrate. Only elemental materials propagating
through the mas~ aperture will deposit on th~ surface
of the substrate.
But what about masking in MO-CVD processes? One
3U would conclude that an apertured mask in MO-CVD will be
of little help. MO-CVD involves the flow of gases
through a reactor that engage a supported substrate
where pyrolyzation of vapor mixtures of elemental
compounds in these gases occurs. Turbulence is present
in the flow of these gases in the region of the
subs~rate. One would, therefore, postulate that
because of the turbulent nature of the gas Elow in this
region, it would be inept for one to conclude that
apertured masking may be a viable way of producing
3 ~ ~
desired layer spatial variations during MO-CVD growth
processes. With an apertured mask positioned over the
substrate upon which deposition is to occur, the
turbulent motion of gases about and in the mask
aperture would surely lead to uneven and nonunifor~
spatial variations in tapered contours and layer or
film thicknesses.
Summary of khe Invention
According to an aspect o~ this invention, masking
techniques can be successfully employed ln chemical
vapor deposition, such as, MO-CVD. Nonplanar shaped
lay~rs with spatial variations in both uniform
contours, taper and thickness deposited on
semiconductor structures can be produced in MO-CVD
deposition system by introducing a mask in the heated
deposition zone of the system during the pyrolyzation
of vapor mixtures of elemental compounds including the
semiconductor materials to be deposited and wherein the
mask has at least one aperture. The mask may have more
than one aperture and the configuration of the mask
aperture may be of any size or shape, e.g~, curved,
round, parallelogram, trapezoid, triangle, ellipse,
square, etc.
The mask may be a removable mask, positioned over
the structure, e.g., semiconductor injection laser upon
which deposition is to occur, either in spaced relation
to the depositing surface or in engagement with that
surface. The mask may be an integral mask comprising a
deposited layer of the structure or formed in the
structure, such as, formed in a semiconductor
substrate. Whether of the removable or of the integral
type, variations in the mask aperture dimensions and
the spacing relative to the structure surface upon
which deposition is to occur can provide accurate
control of the deslred spatial variations in deposited
layers or films~
Integral masks have the advantage over removable
masks of being fabricated of thinner mask dimensions
(in the low ~ m range). Micro-semiconductor structures
are possible having micro spatial variations. However,
3a
,
the dimensions of composite removable mask structure
can approach the small dimensions of the integral mask.
Spacing of removable masks from khe depositing
surface may be accomplished b~ supportin~ the mask in
spaced relation from the depositing sur~ace. In the
case of integral masking, a well or channel may be
formed in the structure through the aperture of the
mask. This is advantageous in the fabrication of
semiconductor injection lasers because not only can the
one or more layers le.g. the active layer) of the
completed device have desired spatial variations but
also current confinement definition and alignment are
automatically achieved duriny growth in a congruent
manner, which was not previously possible in any other
process.
The masking techniques disclosed may be used in
the deposition of amphorous, polycrystalline or single
crystalline materials and layers.
According to an aspect of this invention, a
semiconductor device may be fabricated by a chemical
vapor deposition to have one or more layers of
predetermined lateral spatial thickness variation. The
lateral spatial thickness variation may be formed by
means of pyrolyzation of vapor mixtures of
semiconductor materials comprising the layer or layers
through an aperture of a mask employed during the
chemical vapor deposition thereof~
A particular example of such a semiconductor
device is a semiconductor injection laser device which
is fabricated by metal organic chemical vapor
deposition (MO-CVD) with the aid of an apertured mask
wherein the active layer and possibly other layers
comprising the laser device are deposited in the MO-CV~
reactor through a mask aperture onto a laser substrate
so that the layers so deposited are characterized by a
la~eral spatial thickness variation or tapered contour
wherein the thickest region of the variation is central
of the mask aperture.
3b
The mask structure may be removable and removed
after deposltion of such layers or may be an integral
part of the device structure, i.e., an integral layer
with an aperture formed therein.
An aspect of the inven-tion is as follows:
A method employed during the fabrication of a semicon-
ductor device to determine the center point of a plurality
of semiconductor layers deposited on a semiconductor
substrate, the deposited top layer thereof characterized
by lateral spatial variation in thickness with at least
one position therealong having a minima or maxima cross
sectional thickness, said position beiny said center
point due to the method used in the deposition of said
layers comprising the steps of
projecting confined radiation onto the surface
of said top layer producing a pattern of interference
color fringes,
determining said center point by examining said
interference fringes produced by said projection and
distinguishing said one position from other areas
of said top layer adjacent thereto by the intensity
or color variation created at said position due to said
interference fringes.
Other objects and attainments together with a fuller
understanding of the lnvention will become apparent and
appreciated by referring to the following description and
claims taken in conjunction with the accompanying drawings.
Brief Description of the Drawings
Figure 1 is a schematic representation of a MO-CVD
reactor system suitable for practising the method according
to this invention;
Figure 2 is a perspcctiYe view of Ihe susccptor of the system shown in Figure 1 with a
gencric illuslralion of the mask as applied to a semiconductor structure upon which
deposi~on is to occur;
S Flgure 3 is a side elevation of the view shown in Figure 2. In this Figurc and subsequent
Figures, the mask according ~o this invention is shown in cross hatched lines for purposes
of clarity;
Figure 4 is a removable mask according to ~his invention, and having a cavity so that the
10 mask aperture is spaced firom the surface of the structure upon which deposition is to
occur,
Figure 5 is ano~er rernovable spaced mask similar to that shown in Figure 4 but
supported in alternate rnanner;
~s
Figure 6 is still another removable, spaced mask similar to that shown in Fïgures 4 and 5
but supported in an alternate manner,
~gure 7 is an inlegral rnask according to this invention, and integral wiLh a
20 semiconductor s~ructure comprising a semiconductor substrate;
Flgure 8 is anolher il]ustration of an integra1 mask according to this inYention integral
with a semiconductor structure comprising a semiconductor layer or film deposiled on a
semiconductor substrat~;
2s
F;gure 9 is another illustration of an inte~ral mask which is formed as part o~
semiconductor struet~re comprising a semiconductor substrate;
Figure lO is a graphic illustration of thc conlour and shape that may be grown through
30 dle apenure of a mask according to ~he method oî ~his inYen~ion;
Fïgurl~s 11 and 18 are diagrammatic illustrations relating lo inlegral mask configurations.
For puJposes of simplicity, the growth is shown, in most c~ses, as a singlc ]aycr bu~
reprcsent~tive, ]loweve~, of or~e or more dcposi~ed ]ayer.s of di~crenl elementa]
35 compounds
Figure 11 is a side elevation of a channelcd scmiconduc~or stnlcture comprising a
subsuate with an inlcgral mask;
Figure 12 is a side elevation of a channeled scmiconductor structure similar to Figure 11
S but having a mask lip or overhang;
Figure 13 is a side elevation of a channcled scmiconductor siructure similar to Figure 12
but having a differently shaped struclure channel;
o Figure 14 is a side eleva~ion of a channeled semiconductor structure simil~r to Figure 12
but having a mesa formed in the channel;
Figure lS is a side elevation of a channeled semiconductor structure similar to Figure 12
but having ano~her channel formed in the main char.nel of the structure;
Figure 16 is a side eleva~ion of a channcled semiconductor structure similar to Figure 12
excep~ that the semiconductor siructure comprises an interrncdiate dcposited layer
belw~en a semiconductor substrate and a dcposited mask;
20 Figure 17 is a side elevation of a channeled semiconductor structure similar to Figure 16
excep~ the deposited interrnediate layer is of thicXer cross-section;
Figure 18 is a side elevation o~ a semiconductor structure similar to Figure 17 but
wiehollt a channel formed in the substrat~;
Figurcs 19 through 31 are diagrammatic i]lustrations relating to rcmovable masl~configurations. For purposes of sirnp1i/ity, tlle growth is shown, in most cases, as a single
layer b~e rcprcscnlativc, ho Ycver, of one or morè deposited layers of different elcmental
compounds:
Figure 19 is a side elevation of a semiconductor stnlcture with a mask having a cavily
similar to mask shown in Figure 4;
Fi~ure 20 is a side elevation of a scmiconduclor s~ruclure with a f~at surfaee mask similar
3S to mask shown in Figure 3:
7~
I igure 21 is a side elevation of a semiconduclor struclure having a mask similar to the
mask of ~'igure 20 except having an vutwardly beveled mask aperlure;
Figure 22 is a side elevation of a semiconductor structure havin~ a mask sirnilar to ~he
5 mask of Figure 20 excepl having an inwardly beveled mask aperture
Figure 23 is a side elevation of a semiconductor structure similar to Figure 22 bu~ having
a mask with an inwardly beYeled mask aperiure and a mask cavity simi1ar to the mask
caYity shown in mask of Figure 19;
Figure 24 is a side elevalion of a semiconductor structure similar to Figure 23 except
provided with a deeper mask cavity;
Figure 25 is a side elevation of a semiconductor structure havin~g a mask aperture simi]ar
5 to the mask aperture in Figure 21 and provided with a mask cavity:
Figure 26 is a side elevation o~ a semiconducaor structure wi~ a composite mas~
structure;
20 Figure 27 is a side elevation of a channeled semiconductor structure having a flat masl~
structure like the mask shown in Figure 20;
Figure 28 is a side elevation oF a channeled semiconductor structure with a cen~ral mesa
disposed in the structure channel and a mask structure like the mask shown in Figure 19:
Figure 29 is a side elevadon of a channeled semiconductor struclure similar to the mask
shown in Figure 28 except having an upwardly disposed masX cavity;
Figure 30 is a side elevation of a channeled semiconduc~or structure with a pair30 poinied shaped mesas centrally disposed in the stn~clure ch~nnel and a mask structure
like the mask shown in Figure 20;
Figure 31 is a side e~eva~ion of a channeled V-shaped semiconductor struclure employing
a mask stmcture like the m~sk disclvsed in Fi~ure 28:
3~
]-igure 32 is a diagrammatic ilius~ration of a double helerostnJctllre injeclion l~scr with a
s
nonplanar ac~ivc rcgion having dcsircd spatial varlialions, grown in a channclcd substra~c
with a rcmovablc mask cmploycd during dcposition:
Fi,gure 33 is a diagrammatic illuslration of anolhcr doublc hctcrostmcturc injcction lascr
5 with a nonplanar active rcgion having dcsircd spatial variations, grown in a channclcd
substralc with a rcmovablc mask cmploycd dcposition;
Figure 34 is a scanning elcctron microscope photomicrograph of a side clcvation a
scmiconductor structure after complction of an initial etching stcp, the structure
o cornprising a scmiconductor subsLrate and two contiguous, dcpositcd scmiconductor
laycrs;
Figure 35 is a photomicrograph of thc s~ructurc shown in Figure 34 aflcr complction of a
sccond e~ching stcp, the struclurc now bcing thc same as that shown in Figure 8;
Figure 36 is a photomicrograph of a side elevation of a scmiconductor structurc similar to
that shown in Figure 35 cxcept the structure inc]udcs scvcral intcnncdiate dcposilcd
scmiconductor tayers;
20 hgure 37 is a photomicrograph of a side elevation of a doublc hc~crostructurc injcction
lascr grown by M3-CVD cmploying an intcgral singlc ~rystalline mask during growth;
i-igurc 33 is a photomicrograph of the ~amc ]ascr shown in Figure 37 but of grcater
magnificaLion
2s
Figurc 39 is a photomicrograph of a sidc clcva~ion of still anothcr doub]e hclcrostructure
injcction laser grown by MO-CVD cmploying an integral polycrystalline mask during
growth;
30 Figurc 40 is a diagrammatic illustJation of a partia1 side clcvation of an injcction lascr
grown by MO-CVD cmploying an inlcgral mask and il]ustrating currcnt confincmcnt and
alignrncnt tcchniqucs in complcLing thc fabrication of thc ]ascr; and
Figur 41 is a diagramma~ic illustration of a parlial sidc clcvation of an injcclion ]ascr
35 g,rown by MO-CVI~ cmploying a rclnnvablc mask and iliustrating currcnt cnnfincmcnt
and alignmcnt Icchniqucs lo~ard complc~ing thc ~abrication of Ihc lascr.
Detailed Description of the Preferred Embodiments
In Figure 1 there is shown a conventional MO-CVD
reactor system 10 for practicing this invention and for
the fabrication of semiconductor devices, such as,
injection lasers. The employment of the mask
configurations and making techniques to be discussed
are not limited to MO-CVD. These masks may be readily
employed in other chemical vapor deposition systems and
in molecular beam epitaxy (MBE). In the case of MBE,
however~ the desired degree of spatial variations may
not be as ea~ily achieved.
System 10 will be described in conjunction with
elemental compounds used in fabrication of GaAs/GaAlAs
injection lasers. However, employing the masking
techniques to be disclosed, any other depositable
materials may be used.
Prior art discussion of MO-CVD systems is found in
an article of Russell D. Dupuis and P. Daniel Dapkus
entitled "Preparation & Properties of Gal_xAlxAs-GaAs
Heterostructure Lasers Grown by Metalorganic Chemical
Vapor Deposition", IEEE Journal of Quantum Electronics,
Vol. QE-15, No. 3, pp.128-135, March, 1979.
System 10 comprises sources 12, 14 and 16,
respectively, trimethylgallium (TMGa),
trimethylaluminum (TMAl~, and arsenic hydride (ASH3).
Sources 12 and 14 are bubbler sources with purified
hydrogen provided fxom source 15. The hydrogen is
bubbled through these sources at a controlled rate via
the mass flow controllers 17. Physical vapor phase
mixtures of these compounds are pyrolyzed in hydrogen
generally between 600 to 850C to form thin solid
films according to the net reaction:
H?
~l-x)[(CH3)3Ga] + X[(CH3)3Al] -~ ASH
Ga(l x)AlxAs + 3CH~
~he metalorganics TMGa, TMAl and DEZn are liquids
near room temperature with relatively high vapor
pressures. Hydrogen gas from source 15 is used as a
carrier to ~ransport these source vapors into vertical
Dr,;l~'
.IL- ~J 7 _ILs.
8a
reactor 18. Susceptor 20 is supported within the
reactor on a rotatable rod 22. The semiconductor
structure 24, upon which deposition is to occur, is
positioned on the top of susceptor 20. The terms
"semiconductor structure" as used herein means a
semiconductor substrate or one or more previously
deposited semiconductor layers on a semiconductor
substrate.
7:~LS
9,
'rhc Rl~' hcating coil 28 providcs hcat to thc dcpositon zonc 30, surrounding susccptor 20
and structure 24, ~o wilhin thc abovc mcntioncd tcmpcraturc rangc Lo pyroly~c thc vapor
phasc mixturcs of the sourcc compounds. The alloy composition of thc dcpositcd film is
contro11cd by thc rclatiYe pat~ial prcssurcs of thc Ga and ~1 mckllorganic source
5 compounds.
For p-typc zinc doping, a source 32 of dicthylzinc (DEZn) is cmploycd and for n-type Se
doping, a sourcc 34 of hydrogcn sclcnidc (H2Sc) is cmployel
0 Tlte flows of mctalorganics and hydridcs arc prcciscly controllcd to dcsired molccular
proportions for introduction into the rcactor 18 by mcans Or the mass tlow controllcrs 17.
The growth ratcs are typically From 1,000-~0,000 A pcr mil)ute. Thc thickness of layers
and the extcnl Or doping can be prcciscly controllcd by the appropriatc timcd scqucncing
of the path flow values 19. Exhaust flow valves 23 arc uscd in purging ~he systcm 10.
Bricfly, the process for forming layers on a s~ructure 24 compriscs thc steps of (1)
evacuating the rcaclor 18; (2) flushing thc rcactor 18 with hydrogcn; (3) hcatillg the
dcposition zone 30 to thc desircd dcpos;tion tempcraturc within thc range of 600C to
850C; (4) cquilibrating the flow gas from thc compound sourccs by connccting the
20 appropriate sources to exhaust while also bubbling hydrogcn through sclcctcd
mctalorganic sources 12, 14 or 32 at a contro11ed rale to cquilibrate the vapor flows at
dcsired ratios; (53 introducing the sclcctcd rcactants into the rcactor 18 for a givcn pcriod
of timc to form a thin film or laycr of dcsircd thickncss on thc exposcd surface of the
structure 24; and (6) thcrcaftcr c~hausting all rcactant gascs from thc rcactor 18 and
25 cooling thc structure while purging the rcactor wiîh hydrogen.
The rcacLant gases entcr the reactor 18 via main flow valvc 25 and sprcad ~hroughout the
physical volume of rcactor. Vnlikc MBE the envirorlmcnt compriscs a vapor phase
mixturc of reactant matcrials that will pyrogcnically rcact in zone 30. Illcrc bcing the
30 physical movcrncnt of gascs in and about the arca of thc susccplor 20 and thescmiconductor structurc 24, thcre are also somc divcrgcnt gas flow crcalcd in this rcgion.
Un;forrn and unobstructcd growths arc, thus, possiblc on thc stnlcturc surface.
Rcccntly thcre have bccn dcvclopmcnts in thc scmiconduct()r fic]d o~ dcsigning and
35 rabricaling scmicunduclor dcviccs v~ith s~rip or boundcd compositcs or mc~a typc huricd
structurcs. 'I'hcsc slructurcs arc furmcd via additionlll and intcnncdialc pr~ccssin, slcps
which usually invoke selective etching. An example of
such a device is an injection laser disclosed in U.S.
Patent 4,371,966 entitled "Heterostructure Lasers With
Comhination Active Strip And Passive Waveguide Strip"
and assigned to the assignee herein. Masking
techniques would be desirable to form these mesa type
patterns or layers directly by deposition to eliminate
intermediate steps of removal of the growth structure
from the process and apply ~elective etch techniques to
form the desired strip or mesa type structures.
Crude forms of masking have been employed in LPE
for growing desired patterns directly on substrates
through mask patterns but with limited success.
Masking techniques have been also employed in MBE with
a good degree of success because growth takes place in
an ultra high vacuum chamber and the beams of elemental
constituents are, for the most part, unidirectional.
In MO-CVD, however, the reactant gases entering the
xeactor are multi-directionalO Attempts to employ
apertured masks in a potentially turbulent environment
is highly suspect of not producing uniform and
desirably contoured deposited patterns via mask
apertures. I have discovered, to the contrary, that
apertured masks may be employed in MO-CVD to form mesa
type patterns through mask apertures having desired
spatial variations in pattern contour and thickness.
These spatial variations are accomplished by several
factors: (1) mask size, (23 dimensional size of the
mask aperture; (3) the thickness of the mask and mask
aperture, and (4) spacing relative to the surface below
the mask aperture upon which deposition is to occur.
Mask structuxes may be either of the removable or
integral type. If o~ ~he integral type, their presence
provides for "automatic" fulfilment of alignment for
location and fabrication of current confinement means
for semiconductor devices, such as, injectlon lasers.
From my development of mask parameters and
structures as well as masking techniques in MO-CVD, I
have found that the non~directional aspect of the
7~
lOa
reactant gases may, indeed, not be as paramount as one
might believe. Although it is not altogether clear why
masking during growth is successful in MO-CVD, it
appears reasonable that one reason for success is that
when the gas components, such as H2 and CH3, dissociate
from the liberated elements or compounds deposited,
they are comparatively of much lighter mass and because
of the thermal dissociation, have attained high kinetic
energy. Because of these two factors, they move at
much higher velocities than other molecular components
and are able to move expeditiously away from the mask
aperture and the deposition zone.
~199~5
Thc simplcst mask structure is s3)own in Figurcs 2 and 3. Mask 26 compriscs a tlat
composi~c having at lcas~ onc apcrlure 27. Mask 26 may be inlegral with slructure 24,
such as, an layer or film, or may be a rcmovablc stnJcLure. The mask 26 may be made of
any number of ma~erials, such as, silicon dioxidc, gallium aluminum arscnide, gallium
arscnide, silicon niLrite, aluminum oxide, etc.
Special considcralion can be given in mask dcsign in ordcr to minimize contamination
and to rcstrict the flow of reactant gases around and from regions undcr the mask. This is
particularly true for removable masks. ln Figure 4 the rcmovable mask 3~ has an outer
0 perimetrical lip 33. Mask 32 a1so has an aperture 34. Positioning of the mask 32 on
suuclure 24 provides for the aper~re 34 to be spaced from the surface 24' on which
dcposition is to occur. The configuration of this particular mask stn cture, as comparcd to
mask 26 in Figure 2, is îhat growsh will be pcrrnittcd to exlcnd over surface 24' bcyond
the confincs or dimensions of the aperture 34.
''
The reactor 18 can bc modificd to include an asscmbly within the reaclor lo provide for
the inscrtion and rcmova] of masks during thc dcposition processes.
An examp]e of the employment of a mask 32 is as follows. Mask 32 was made of si1icon
with <110> orientation. The mask was about 3 mi]s thick (dimcnsion A in Figure 4~ and
the width of the apert~re was S mils wide and 25 mi]s long. The spacing B was about
6~m. The growth on the surface of the structure 24 was of Gaussian shapc about lO mi1s
wide, 30 mils long and 4.4 ,um high. The contour oî the growth ~Yas similar to the
contour pattcrn 66 shown in Figure lQ.
~s
In Figure S, mask 36 is similar to mask 32 in Figure 4 exccpt it is provided wilh a
pcrimetrical lip 37 îor supporting the mask in spaced rclation from surface 24'( of
scmiconductor structure 24. Lip 37 is designed to engage the sur~ace of susccptor 22
]caving no spacing for reactant gases to escape under thc mask lip 37. Mask 36 is also
shown with two apcrtures 35' and 3S".
In Figure 6. mask 3~ is also provided ~o be main~ined in spaccd rc]ation ~rom the
surface 24' of a scmiconductor structure 24. The spaccd rcla~ion, howcvcr, is
accl)mplishcd by thc pcrimclrica] edgc or lip 29 proYidcd on thc scmiconductor structure
24. Mask 38 is p1anar has two apcrtures 3~' and 38".
- 12-
Mask 32, 36 and 38 in Figurcs 4-6 arc all dcsigncd to bc rcmovablc masks, that is, thcy
arc cmploycd during thc growth proccss and may subscqucn~ly bc rcrnorcd prior to the
complction of proccssing in rcactor 18.
s The mask structures shown in Figurcs 7, 8 and 9 arc intcgral masks. In Figure 7, the
scmiconductor structurc 24 is providcd with a channc1 40. A dcpositcd mask 42 isprovidcd with an apcrturc 44 aligncd wi~h thc centcr of channel 40. ~hc mask laycr 42
may comprisc polycr,vstallinc matcrial, a amorphous matcrial or a singlc crystal matcrial.
For example, structure 24 may bc a substrate of ga]lium arscnidc (GaAs). Mask 42 may
lo comp~ise a dcpositcd laycr of SiO2, Si3N4 or A121:)3. A important aspect of masl~ 42 is
the cantilevercd lips 46 extending ovcr thc channel 40 of thc serniconductor sLructure Z4.
- The sclf-aligned mask 42 is made by the vapor dcposition of SiQ2 on thc substrate 24.
Ncxt, an elongated aperture 44 is ctchcd through the SiO2 mask laycr cmploying a SiO;
LS ctch. This is followcd by a sclcctive etch for GaAs to for n the channcl 40 in substrate 24
through the apcrture 44. This two step ctching proccss leaves Ihc mask cantilevcrcd lips
46 ovcr both sidcs of the channcl 4Q With this type of mask structurc, growtb of non-
planar laycred structurcs can easily bc pcrforrncd by MO-CVD. ln the case of thecxarnp7e of the previous paraOraph, the growth through the SiO2 maskcd apcrturc 44 on
the GaAs substrate will be cr~stallinc while the gro~vth on the surfacc of the mask will be
polycrystalline. Discussion conccrning growth will be explaincd in greater dctail in
subscquent figures.
In Figurc 8, semiconductor strucnlrc 24 compriscs a substratc 48 on which is a dcposited
layer 50. Layer 50 rnay comprise~ for cxamplc, Gal xAIxAs. Mask layer 52 is dcposilcd
on laycr SO and will subscqucnt!y bc thc mask structurc for the scmiconductor s~cture
24. Substrate 48, fior example, may bc ~lOU> oricnt~tion, n-dopcd Ga~s. Layer 50 may
be Gao~A106As. Mas1~ layer S2 may be undopcd GaAs. Layer 52, as wcll as other
intcgra3 mask laycrs to be hercinafler discusscd, may proton or ion implantcd or oxyge~
or Gc dopcd ~o rcndcr thcn clcclrically insu1aling. Such a 1aycr may ~o~n part oF the
currcnt confine slructurc of scmiconductor dcrice comprising a pluraliI~ of scmiconductor
laycrs subscqucntly dcposi~cd through apcrture 56 in ch;mncl 57.
Laycr S2 may 3]SO bc n-dopcd CiaAs whilc laycr 50 may bc p dopcd Gal xA3xAs to rorm
a rcvcrsc junction and ~orm part of lhc currcnt conïlncmcnt mcans for a scmiconductor
dcvicc dcpositcd in channcl 57.
13
The preparation of this mask structure for subsequent
growth is accomplished as follows, reference being made
also to the microphotographs of Figures 34 and 35.
Figure 35 is an actual photomicrograph of the structure
illustrated in Figure 8 except for substrate
orientation. With selective masking, an elongated
aperture 56 is etched through the gallium arsenide
layer 52. Figure 34 shows the result of this single
etching step wherein the etchant has also extended a
little into the intermediate layer 50 of Gal xAlxAs.
This first etching step is followed by a second etching
step comprising an etchant for Gal xAlxAs such as, HCL
or HF etchant. The mask aperture 56 now performs the
function of a mask for performing this second etching
step. The etching process, over a selected period of
time, will produce a channel 54 in layer 50 and
extending beneath the elongated edges of the aperture
56 forming the extended cantilever lips 58. The
structure resulting from this second etching step is
shown both in Figures 8 and 35.
The ~dges of the lips 58 can have different angled
contours depending on the crystal orientation of
structure 24. For example, in Figure 8, the upwardly
open bevelled edges are obtained by a ~100~
orientation of the substrate 48 with the etch d channel
psrpendicular to the (011) cleavage plane. On the
other hand, V-shaped edges are obtained by a ~100>
orientation of the substrate 48 with the etched channel
perpendicular to the (011) cleavage plane, as
illustrated in Figures 34 and 35.
The photomicrograph ~hown in Figure 36 is similar
to that shown in Figure 35 except that the structure 24
comprises two addi.tional deposited layers. Structure
24 may, for example, comprise a ~100~ orientation
substrate 48, an undoped layer 50 of GaO 4Alo 6As, a
p-type layer Sl of GaO 4Alo 6As, an n-type layer 53 of
Ga~ 4Alo 6As and the single crystal mask 5~ of undoped
GaAs with the channel etched perpendicular to the (011)
cleavage plane~ Layers 51 and 53 will form a reverse
7~5
14
junction forming part of the current confinement for a
semiconductor device formed in channel 57.
The mask structure need not be formed as an
integral layer or a film on the semlconductor structure
24. As shown in Figure 9, the mask structure may be
actually part of the semiconductor structure 24, per
se. Using a GaAs etch, a mask opening is formed in the
body of the substrate forming a dovetail channel 60
defining an aperture 62 and forming the elongated lips
6~. The channel 60 etched perpendicular to the (011~
cleavage plane is sufficiently deep so as to function
as a mask structure to obtain contoured growth on the
surface of the channel. The profile of the side walls
65 of the channel can be varied depending upon the
etchant as is known in the art. See, for example, the
channel profile in Fi~ure 13.
In all of these removable and integral mask
structures in Figures 4-9, MO-CVD growths may be
performed through the apertures of the masks by the
deposition of materials or compounds from the reactant
gases onto the surfaces of the channels formed beneath
the mask apertures. The extent of the growth, that is,
the height, thickness and curvature of the growth is
controlled by the size and shape of the mask aperture,
the thickness of the mask and the amount of the channel
volume beneath the mask.
A rule of thumb is that the width of the mask
aperture should be greater than thickness of the mask.
This ratio is particularly important in order that a
major portion of the reactant gases make initial
contact and deposit on the channel bottom beneath the
mask aperture before making substantial contact with
the surfaces of the mask aperture edges or channel
extremities. In this sense, the mask thickness should
be comparatively thin, but this dimension also depends
on the mask aperture width.
The thickness, for example, of a removable or
integral type mask may typically vary between 2 to 5
mils with an aperture width between 4 to 8 mils. A
7~
14a
specific example would be a mask 3.5 mils thick at the
- mask lips with an aperture width of 4 to 5 mils,
follows the above mentioned rule. Thinner mask
dimensions are more easily achieved with integral
masks. The thickness and aperture width of integral
masks may typically be 1 to 5 ~ m and 2 to 30 ~ m,
respectively.
By the use of these mask structures, a three
dimensional controlled, contoured growth is possible in
MO-CVD. The preferred mask design for contoured shapes
and configurations is to have a region (e.g. channel
57) under the mask in which the reactant gases can
spread laterally, depositing compounds in the chanffel
volume in a tapered or contoured manner. The channel
volume and mask aperture width selection permit control
over both aspects of spatial variation of the growth -
1) the curved contour and extent of the growth and 2)
the thickness and height of the growth, albelt a single
layer or a plurality of layers. The wider the
aperture, the lower the taper rapidity of the growth,
i e. a more level growth profile, with channel volume
presumed constant. With the same mask and channel
parameters, the spatial variations of the growth may be
reproduced in a continuous and substantially identic~l
manner.
A three-dimensional type profile or pattern 66 of
a contoured growth with a Gaussian shaped cross section
is illustrated in Figure lQ. These mesa like patterns
may be employed in the fabrication of semiconductor
devices requiring three dimensional
a7 ~ ~
contours, such as~ the formation o~aclive regions in injcction lascrs h~ving dcsircd spa~ial
variations in lapcrcd contour and thickncss.
Thc prcccding discussion has bcen in connec~ion with thc fabrication of difrcrcnt typcs of
S mask s~ruclures. ll-e descrip~ion of ~he rcmaining fi~urcs involvcs thc use of various
rcmovable and fixed mask structures in thc dcposition of one or more layers of
scmiconductor compounds in MO-CVD.
ll~e purpose of Flgures 11 through 31 is to illustrate the differcn~ type of growths
lo possible with various ~ypes of inlegral and rcmovable mask configurations. Figurcs 11
through 18 illustrate integral type mask structurcs. In these Figurcs mask ovcrgrowth is
shown since the mask rcmains as an intcgral part of the fabricatcd dcvice. Flgurcs 19
through 31 illustrate rcmovable mask type structures. ln all these Figures, thc masl;
structures are shown cross-hatche~ for purposes of clarity. ~n the fig~rcs rclating to
rcmovablc mask structures, ovcrgrowth on thc mask is not illustratcd since Lhc masks are
removed during or after complction ol growth.
Tl-e growth in Figures 11 through 31 is pcrforrncd in the MO-CVD systcm 10 of Flgure
1 and in most cases is shown as a single 1ayer for purposes Or simplicity. This
represCnLation, however, is a]so intcnded ~o rcprcsent the bu]k of a plura1ity of dcposited
layers, such as, illustrated in Figure 31. Dctai1cd multilayer structures are discusscd in
Figures 32, 33, 37, 38 and 39.
In Fgures 11 through 15, the strl3cturcs shown each comprise an oricnlcd crysta]line
2s scmiconductor (such as, dopcd or undopcd Ga~s) subslratc 70, an oxide (SiO2) or nitride
(Si3N4) mask 72, a polycrystalline growth 74 over thc mask surface and a single crystal
growth 76 dcpositcd through the aperture 78 of the mask 72. The growth 76 forrns a
spatial ~ariation in tapcred contour or rapidity and in thickness, as illustratcd at 80. The
growth extcnds in a uniform contourcd shapc in the substratc channe]s 71, 73 and 75
away from the ccntral axis of the apcrture 78 tov~ard thc channcl cxtrcmitics. /l~lso the
growth extcnds around the cdgcs of thc mask lips 82 and tapcrs on the undcrsurraces of
thc mask lips toward thc channcl exiremities.
In Figurc II, a sclcctivc ctch is pcr~ormcd to Form thc c]ongatcd apcrturc 78 in ]aycr 74.
Ch3nncl 71 is ~onmcd, as by sclcctiYc ctch, in~o thc substratc 70 fonning ch;lnnci 71. Two
diffcrcnt ctchants m~y bc nccdcd for clching thc matcri~]s of thc mask nnd of Ihe
subslratc.
7~i
- 16-
ln Figurcs I2 and 13 the subs~rate channcls 73 and 75 arc formcd by an ctchant that is
not cffcctive on thc mask matcrial as prcviously cxplaincd rclativc to Flgure 7. A two
step etching treatrncnt forms ~he channcls 73 75 and mask lips 82. The diffcrcnce in the
cross-scctional shape of channcls n and 7~ is duc to crystal oricnta~ion of structure 24 as
s known in the arL
In Figure 14 the thickness varia~ion is more pronounced and the tapcr rapidity is
greater as comparcd to prcvious structurcs duc lo thc prescnce of the mcsa 84 forrncd in
channel 73. Mesa 84 is easily formed by slripe masking the ccntral portion of channcl 73
10 and procceding further with the sc~ond ctching s~ep.
In Flgure IS a second channel 86 is forrned in substrate channel 73 employing
conventiona1 selectiYe mask tcchniques. Composi~e layers 76.1 76.2 and 76.3 dcmonstrate
the different shapcd contours that can be formed when scquentially dcpositing diffcrent
elcmental compounds through mas& apcrlure 78. Layer 76.1 is contoured concave due to
the prescnce of channcl 86. However during the growth of 1ayer 76.2 this will cventually
become p]anar due to thc prcsence of the apcrturcd mask 72. Continued growth of ]ayer
76.2 will bccome convex contourcd so that layer 76.2 will have an eye shapcd contour.
The final layer 76.3 has an eveD more convex contoured as the growth rcaches theaperture 78.
In Flgures IS through 18 the structurcs shown each comprise an <IOD> oriented
crystalline scmiconductor (such as dopcd or undoped Gal~s) substrate 70 a single cryst~
laycr 88 (such as for example Ga~ xAIxAs where 0.3~ x<I) a singlc crysta]line mask 90
(such as for example Gal yf~IyAs whcrc Ky<0.3) a single crystal growth 92 ovcr surface
of the mask 90 and a single crystal growth 94 dcpositcd through the aperture 96 of ~he
rnask 90. The growth 94 forrns a spatial variation in tapercd contour and in thickness as
i~lustratcd by the contour 98.
ln the slructures of Figurcs 16 and I7 thrcc diffcrcn~ ctching stcps arc pcrforrncd prior
to growth. First therc is sclective ctching of the mask 90 to form lhc mask apcrture 96.
The complction of Ihis stcp is illustratcd in Figure 34 as prcviously discusscd. The
sccond stcp is thc elching of thc channcl 93 through 13ycr 88 and into thc substralc 70.
lllc third stcp is thc ctching of laycr 88 through thc mask apcrtusc 96 to f~rm ch lnoel
9I in laycr 88 producing thc cantilc~cr lips 95.1he structurc shown in ~i~ure 18 diffcrs
from thosc of Figurcs 16 ~nd 17 in that thc sccond c~ching stcp is not pcrforrncd i.e.
9t7~L~
thcrc is no channcl 93. lhc struclurc of Figurc 18 is the samc as that shown in the
photomicrograph of Figurc 35 cxccpt for initial substratc oricntation.
To bc no~cd from Figurcs 16 through 18 is the diffcrcnccs in thc dcgrcc of tapcr rapidity
98 and growth thickncss of growth 94 duc to diffcrcnccs in the width of thc apcrturc 96,
the thickness of thc mask 90, thc thickness of ]aycr 88, the volumc and width of channel
91 and thc prescncc or abscncc of thc substratc channcl 93.
While the mask structures of Figures 11-18 are charactcrizcd as intcgral, the ovcrgrowth
0 72 and 94 and cvcn the masks 72 and 90 may be rcmoved, as by wet or dry ~pla~rna)
ctching, bcfore comp]etion of furthcr fabrication processes.
In Figures 19 through 3L the structures shown each cornprise an oricntcd crystalline
scmiconduc~or substrate 70, such as, GaAs, a rcmovable mask structure having an
aperturc 105, and a rcsultant growth 100 forrncd on the surface of the substrateemploying MO-CVD system 10. These rcmovabl- mask structures may be fabricated
from Si GaAs, SiC, Graphite, SiO2, Si3N4, '~'23 as well as many other types of
matcria]s. Each of the mask structures shown in ~ese figures has a dif}~rcnt attribute. In
some cases there is a modification to the substrate depositing surface. These dif~crcnt
configurations illustrate how variations in the mask paramelers and geomctry aree~lp~oyed to control and produce desircd spatial variations i11ustrated by the contour 102
of each of the growths 100.
in Figure 19 thc mask 97 is provided with an apcrturc 105 and an undcrgro~ve or
2s channcl forming the lips 107 and chamber 95 when the mask is positioncd on the
surface of the structure 24. Masl~ 97, as positioncd on subslrate 70, lis simi]ar to the
intcgrai mask configuration shown in Figure 18. Thus, thc spatial variation of resultant
growths 98 and 1ûO in thcse Figurcs arc quite sirnilar.
In Figure 20, masl ~9 has a flat configuration. Masl s }01 and 103 of Figurcs 21 and 22
are also flat mask configurations with mask ]û} having an ou~wardly bevclcd mas~apcrturc 105.1 and mask 1a3 having an inwardly bcvclcd mask apcrture 105.2. To bc
notcd is thc diffcrcncc in thc contour of thc growths 102 duc to thc diffcrcncc in thcse
mask apcnurcs. ïllc rapidity of spatial variation rclative to thc contourcd curYature and
thickncss of the growth 100 is quitc pronounccd in ~igurcs 20 and 21 as cornparcd to the
samc spaIial ariation for thc groulh ~00 in Figurc 22.
~.$~t~
- 18 -
In Figurc 23 mask 104 is of similar configuration lo mask 97 of Flgurc 19 but has a
mask apcrturc 105.2 îike ~hat shown in Fi~ure 22. Thc spatial variation rclative to the
tapcrcd contour is similar ~o that Or growth 100 in Figure 22 but is of rcduced taper
rapidity. The tapcr rapidity can bc incrcascd for growth 100 as cvidcnccd by Figure 24
5 by incrcasing thc apcrturc width and thc cxtcnt or volumc of chamber 95 .
In Figure 25 mask 106 has an upwardly bcvclcd apcrture 105.1 in combination with a
mask cavity to form chambcr 95. Mask 106 is also providcd with lips 107 that include
rccess 109. Reccss 109 provides for a thinncr mask thickness at the apcrture 105.1 which
0 will provide a largcr growth 100. To be notcd is the extcndcd nature of the growth 100 as
comparcd to growth 100 in Figure 21 but ha~ ing a larger tapcr rapidity as compared with
the growths of Flgurcs 23 and 24.
In Figure 26 there is shown a composite mask 108 consisting of t~vo componcnts.
Component 108.1 may comprise fior example graphite Si or GaAs. The thinner
componcnt 108.2 may comprise SiC SiO2 Si3N4 or Al203. A prcfcrrcd cornbination o~
malcria]s for componcnts 108.1 and 108.2 is graphite for componcnt 108.1 with SiC for
componcnt 108.2 bccause these matcrials can bc made to match rclativc to strain and
thcrrnal cxpansiorL The advantage of this composite mask is that the largcr component
108.1 is a rigid support for the much thinncr mask component 108.2. The composite
mask 108 is designcd for fabricating much smaller dimcnsional growths wherc the ma~k
aperture 105 may be Icss than 6 um wide and the channcl about 3 um dcep.
Composite masl~ 108 is fabricatcd by first ctching the channcl pattcrn 108.4 in the bottom
2s of component 108.1 of oricntcd crystallinc silicon. Next a film of SiO2 (such as 0.1 to 2
um~ is dcpositcd on the etchcd surfacc forming componcnt 108.2 Thir(i an SiO2
sclective etch is pcrforrncd to form the mask aperture 105. Last the oppositc sur~ace of
componcn~ 108.1 is e~ched to forrn the rcsess 108.5. thc eLchant uscd does not etsh
component 108.2. Composi~c masl~ 108 is a simple struclure to produce and has its
grca~cst ulility in fabricating micro growths 1~.
Figurcs 27 through 31 disclosc rcmovablc masks employcd ith channclcd slructurcs 24.
In cach of thcsc figurcs growth 1~ occurs in thc channcl 111 of the subs~ratc 70 below
thc apcrturc 105 of the mask s[ructurc producing diflcrcnL dcsircd spatial ~ariation in
~he contours 102.
7~5i
In Figurc 27, thc substratc 70 is providcd with a channcl 111. Mas~; 110 has a tlat
configuration. ïlic channcl 111 pcrrnits the growth to sprcad lalcra11y bcnca~h thc mask
during dcposition.
llle configurations of Figurcs 28 and 29 are similar to Figure 27 cxccp~ for the masX lip.
Substrale 70 is also providcd with a channcl 111, as in thc casc of Figure 27, but the
channcl furthcr includcs thc mcsa 113. The mask lips 107 are formcd by sclcc~ivc ctching
a channc7 into a surface of thc maslt struc~ure 112, 114. This cn1argcs the vo1umc of the
charnbcr formcd bclow the mask whcn the mask has becn positioncd on the substrate 70
0 thereby pcrmitting enhancemcn~ of lhe latcral extcnt of the dcposition. 2n Figure 29, the
mesa 113 is narrower in width than mesa 113 in Figure 2B.
In Figure 30, the sLructure 24 and rnask 110 are gcomctrically thc same as that shown in
Figure 27, except that channe1 111 is provided with a pair of mesas 115 having atriangular cross-section. This configuration wi11 providc for h;gh and a~rupt spatia2
variations in the contour 102.
In Flgure 31, substrate 70 has a V-shaped channcl 117. Mask 112 of Flgure 28 is
emp]oycd lo provide the channe2 or spacing 119. Growth ]00 compriscs three layers
100.1,100.2 and 100.3. Additiona2 layers may be dcpositcd on ]ayer 100.3. Bccause of the
mask apcrture ]05, the cx~ent of channe1 119 and the presencc of channel 117, a M~
CYD fabricatcd device may ~e provic2cd ~vith a complctcly buried s~rip in the form of
layer 100.2. For cxarnple, strip lOQ2 may bc dopcd or undopcd GaAs and function as
the s~rip active ]aycr or region in a strip heterostructure injccLion ]aser.
~5
Rcference is now made to Figures 32 and 33 which disc]ose scmiconductor
hetcrostructure injection lasers ~abricated in MO-CVD cmploying the removab]e mask
techniqucs just discussea
~n Figures 32 and 34, hctcrostructurc injcction lascrs 120A and 12013 ct)mprisc substrate
122 of n-~aAs and cptiaxia] growth 126. Growth 126 compriscs n-Gal xAIxAs cladding
laycr 126.1, undopcd GaAs active laycr 126.2 and p-Gal xAlxAs cladding ]aycr 126.3.
Additional Ga~s/GaAlAs laycrs may bc providcd in thc structurc as contact and
atlditional cladding laycrs. as is wcl] known in the ar~
Grt)wlh 126 is rormcd in channcl 124. A rcmovablc mask, such as, mask llO or 112, may
, .
be employed and is positioned on the surface o~
substrate 122 during growth in reactor 18. After
completion of the deposition of growth 126, the
structure is removed from reactor 18 and the mask is
removed. Selecti~e proton or ion implant 128 is
performed to form the insulating barrier, indicated by
the dotted line in each of the Figures, leaving the
semiconductive channel 136 for current confinement to
the active radiation emitting region 126.20 of layer
126.2. Such a current confinement technique is known
in the art. It should be not~d that the implant
penetrates through the active layer 126.2 but is
sufficiently far enough from the lasing region 126.20
so as not to interfere with the operation of laser
120A, 120B.
The metalized layer 130 is, then, deposited on the
top surface of the device and metal contacts 132 and
134 appropriately fixed. In the case of the
metalization 130 in Figure 33, there is a break 138 in
the metalization due to applying the metalization vapor
~rom an angular position as indicated by arrow 139.
These two laser structures demonstrate how
removable masking in MO-CVD permits controlled
optimi~ation of the spatial variation 127 of the
nonplanar active region 126.20 with desired taper
rapidity and active region thickness in accordance with
the teachings of U.S. Patent No. 4,355,461.
Figures 37, 38 and 39 are photomicrographs of
heterostructure injection lasers fabricated in MO-CVD
using an integral mask. The laser structure of Figure
37 differs from that of Figure 39 by the material used
for the mask. In Figure 37~ the mask is single
crystalllne material (Ga~s), whereas in Figure 39, the
mask is an amorphous material (SiO2). As a result, the
growth on the amorphous mask will be polycrystalline
while the growth on the single crystalline mask will be
single crystalline, which is evident from an
examination of these Figures~
7~i
21
An added advantage of the integral mask laser
structures is that the presence of the mask aperture,
which provides for in place, "automatic" alignment over
the desired lasing region of the device. This greatly
simplifies subsequent current confinement procedures
and subsequently applied metalization. There is no
necessity of an intermediate step to determine the
center point oE the deposited growth beneath the mask
aperture.
In Figures 37 and 38, heterostructure injection
laser device 140 is fabricated as follows: One starts
with the structure shown in Figure 35, the fabrication
of which has been previously explained relative to
Figure 8. This structure comprises substate 48 of C100
oriented n-GaAs with the etched channel perpendicular
to the (011) cleavage plane, layer 50 of undoped
GaO 4Alo 6As and mask 52 having an aperture 56 and
etched chamber 57 formed undex mask lips 58. Layer 50
and mask 52 may be fabricated of single crystalline
materials having electrically insulating properties,
such as, oxygen or Ge doped GaAs and GaAlAs.
The Figure 35 structure is next placed on the
susceptor 20 in reactor 18 and layers 1~2-150 are
deposited forming the laser structure 141 in chamber 57
beneath the mask aperture 56. These layers comprise
base layer 142 of n-GaAs, cladding layer 144 of
n-GaO 7Alo 3As, active layer 146 of undoped GaAs
(active region 146.1 being part of laser structure 141
while the remaining portion 146.2 of this layer being
deposited on the mask 52, as in the case of the other
sequentially deposited layers), cladding layer 148 o
p-GaO 7Alo 3As and contact layer 150 of p-GaAs.
Conventional polishing, metalizations for contacts,
cleaning and bonding is then performed. A Cr-Au
metallization is shown at 152 in Figures 37 and 38.
Because of the size of chamber 57, the width of
mask aperture 56 and the control of the deposition
rate, the spatial variation of active region 146.1 may
7~
be controlled in accordance with the teachings of U.S.
Patent 4,335,461.
In Figure 39, the heterostructure injection laser
device 160 is fabricated as follows. A 0.15 ~m thick
SiO2 layer 164 is deposited by electron beam
evaporation on a clean ~100~ oriented Si doped GaAs
substrate 162~ An 8 ~m aperture 166 is then formed in
the SiO2 layer 164 by conventional photolithographic
techniques and plasma etching. Next, about a 3 ~ m deep
channel 168 is etched into the GaAs substrate by 5~
solution of H2SO4:H202 and H2O. The underetching below
the SiO2 mask 164 during this etching step creates the
chamber 170 defined by the cantilevered lips 173 of
mask 164 formed over the channel 164. The extent of
each lip 173 is about 1.5 ~m.
Since the aperture 166 in the mask 164 is narrower
than the extent of chamber 170, the growth rate during
deposition at the center of the channel 168 is faster
than the growth rate in the channel beneath the mask
lips 172.
During growth in system 10~ the following single
crystal layers are sequentially deposited through -the
mask aperture 166: cladding layer 172 of
n-GaO 7Alo 3As, active layer or region 174 of p or n
doped or undoped GaAs, cladding layer of p-GaO 7Alo 3As
and contact layer of p GaAs~
During growth, polycrystalline material,
comprising the compounds of layers 172-178, is
deposited on surface of mask 164 forming a
polycrystalline electrically insulating layer 180.
The presence of the mask aperture 164 causes the
materials to be deposited with a curved tapered contour
in channel 168. Also, as the polycrystalline material
forming layer 180 is deposited on mask 164l the
aperture 166 narrows in width and thereby acts to
increase the thickness of active layer 174 in the
center of channel 168 as compared to lateral regions of
the same layer (al~hough this is difficult to discern
from the micrographs because the thickness variations
22a
are very small~. Thus, the ratio of the diminishing
aperture width to the depth of each of the grown layers
determines the flnal thickness variation that will
occur laterally along each layer. This taper and
thickness spatial variation provides lateral
waveguidance, as taught in U.S. Patent 4,335,461.
The measured light output versus current
characteristics at 300 K under pulsed operation (100
nsec pulse 1 kH2 repetition rate~ of various fabricated
laser devices 160 produced linear optical power output
up to 130 mA and a power output per facet in excess of
15 mW. In some cases, some of the fabricated devices
160 had a current threshold ranging from 32 to 42 mA.
Figures 40 and 41 illustrate how more refined
stripe alignment and current confinement may be
provided in the previously described laser devices 120,
140 and 160.
In Figure 4n, MO-CVD fabricated laser device 190
includes laser structure 191, generally identical to
structure 141 in Figure 37 at 141, deposited on
substrate 48. Deposit of single cxystalline materials,
forming layer 192, on the sur~ace of mask 52 has
occurred during epitaxial growth of laser structure
191. Upon completion of this growth, but prior to
deposit of metalization 152 and contacts 132 and 136,
an electrically insulating layer 194 is deposited over
the entire exposed surface of the device 190, forming
deposited layers 194.1 and 194.2O Conventional
metalization techniques can now be applied. The mask
lips 58 with deposits will serve as a shadow mask and
metalization vapors ~ill not penetrate into the open
regions of chamber 57.
":
Onc rni~ suspcct a scrious drawback if insulating matcrials uscd in lhis growth proccss
in systcr~ 10 mighl causc contamination, e. g., Lhc subscqucnl dcposition of oxidcs or
nitritcs a~cr the dcposi~ion of III-V or Il-VI clcmcnts or compoun~is. ~lowcvcr, Si3N~
laycrs ~94 havc bccn succcssfuly ~rown aftcr thc dcposition of thc 1ascr structurc 191.
S Si3N~3 is onc of thc casicst insulating compounds to grow in thc MO-CV]? systcm 10 at
~his poir~1 since thcir gas mixtures (5% SiH4 in H2 and NH3) arc bclicvcd to bc the Ieast
ellcctcd lby thc background impurties alrcady prcscnt in thc the rcac~or 18. It is bclicvcd,
ha~ this can bc cx~cnded ~o othcr insulating 1ayers, such as, Al;~03, SiO2 and SiC.
~0 Next, by employing an optical microscope, projcclion mask aligncr or clcctronlilhograplly, dc~cnnination can be rcadily rnade of the ccnlcr of laser s~ruc~ure 191
becausc ~f the intcrfcrcnce fringes crcatcd by the microscopc light rcflccting from Lhe
surface of insula~ing layer portion 194.1. Thcse fringcs rcsult rrorn the varia~ion in the
ihickness of layer 194, the color patlern at ccnter point bcing quite distinguishable. For
e~ample" ;n the case of layer portion 294.1 compriscd of Si3N4, the interrcrcnce pattern
could be a decp rich blue co]or at the center point and vary to lightcr blucs and other
ligl~ter co]ors away from the center poinL This method of alignment perrnits theforrnatioin of the stripe l9B in the laycr portion 194.1 by photolithographic and plasma
etch Icchniques. After the formation of stripe 198, the mctalization 152 may be vapor
20 dcposited~ Thus, a very confined curren~ channel can be fabricatcd to confine the current
f~ow 20t~ to a small region of the ac~ive layer 293 of structure 191 thcrcby lowcring ~he
a~rrent ~hreshold of the device 190.
The sam~ process ean be employcd to dctcrrnine thc centcr point of an applicd photo
2~ rcsist lau~r. For example, top layer 194.1 may be a spun photo rcsist laycr. However,
because of the manner of i~s application to structure 191 having spatial variation, i.e.,
curved a~ntour, the photo rcsist laycr will be thinncr a~ the top of this struc~ure as
compare~ to adjacent regions. lllis process for deterrnining the centcr point may be uscd
whcthcr ~is position is of minimum or maximum cross-sec~ional thickness, anel rcgardless
30 of thc pærticular matcrial uscd for the top layer.
llle bca~ of confined radiation may bc polychromatic, monochroma~ic or vcry narrow
bandwid~ or Or single wavclcng~ xamp]cs arc mcrcury vapor lamps or a lascr bcarn.
6)f spcci3~ intcrcst is a ]ascr bcam tlJnable to a wavclcngth within a parIicular b~ndwidth
35 or a dischargc lamp having ccrtain spcc~ral lines which can sclcc~ivcly be fillcred.
- 24-
l~y projccling thc confincd radiation onlo Ihc surfacc Or thc lop laycr 194.1, a paltcrn of
intcrrcrcncc fringcs is produccd, from which a dctcrrnination of thc ccntcr point of thc
]aycr can bc madc distinguishing thc ccnlcr point position by dcfinitivc rcsolution of
intcnsity or color varia~ions crca~cd at this position duc to thc produccd fringcs.
Figure 41 illustratcs thc smploymcnt of this alignment and currcnt confincrncnt
tcchniquc rclativc to rcrnovablc mask cmbodimcnts. Thc ]ascr structurc 214 having active
laycr or rcgion 216 is grown, cmploying, for example, a mask 97 shown in Figure 19 or a
mask 104 in Figure 24, on subs~ra~e 70. With ~he rcmovablc mask still in place~ the
10 c]cctsically insulating layes 218 (such as, Si3N4) is grown.
llle mask is then rcmovcd from the reactor 18 and an additional elcctrically insulatin~
layer 220 is dcposi~cd on the dcvice. Sincc the masl~ has bccn removed. layer 122 will
cover the enLire surfare of the sLructure.
L~
The device 210 is then removcd from thc rcactor 18 and using thc alignmcnt tcchnique
just dcscribcd and convcntional photolithographic and plasma ctch tcchniques, thc s~ripe
222 can be forrncd at the exact ccnter point of lascr stlucturc 214.
20 While the invention has been described in conjunction with spccific cmbodimcnts, it is
evident that many alternatives, rnodifications and variations will be apparcnt to tllose
sk;l1ed in the a~t in light of the forcgoing dcscription. Accordingiy, it is intendcd to
cmbrace all such altcrnativcs. modifications, and variations as fall within thc spirit and
scopc of the appendcd claims. An altcrnaLiYc cxample of a mask structure is the provisio~
2s of a cavity or channel cxtcnding from one end to the othcr in a surface Or a flat mask
struc~ure. This nonplanar surface of the mask is laid face down on Lhe substratc so that
an end facc of thc cavity is exposcd, as positioncd on the subsLrate. This cnd face of the
caYi~y rorrns an apcrlure to the extcnt that rcactant gases can pcnctrate into space J'o~ned
by thc cnd face. A tapercd stmcture can be formcd on thc substrate surfacc, such as~ ror
3D cxample, a tapcrcd optical coupler.
, .,
