Note: Descriptions are shown in the official language in which they were submitted.
6~9L
.
1 BACKGROUWD OF THE INVENTION
2 1. Field of the Inven~ion
3 The invention is related to diode lasers3 and more
4 particularly, to separating diode lasers from wafers.
2. Discussion of che Prior Art
6 Coherent light-emitting diodes having a GaAs-(Al,Ga)As
7 double heterostructure, such as described in "Semiconductor
8 Lasers and Heterojunction LED's" by H. Kressel and J~ Ka
9 Butler, Academic Press, New York, 1977, are known to be
efficient light sources for optical communications systems.
11 As is well-known, such diode lasers comprise
12 layers of GaAs and (Al,Ga)As on an n-&aAs substra-te. The
13 final layer is a cap layer of p-GaAsO Metallized stripes,
14 parallel to the intended direction of lasing, are deposi~ed
on the p-side of the waferO Gold contacc pads, somewha-t
16 smaller in area than the Ln~ended size of ~he diode laser, are
17 deposited on the n side of the wafer. The stripes and pads
18 are for subsequent connection to an external electrical source~
19 The wafer is then cut in two mutually orthogonal
~O directions to form the individual diodes. First, the wafer
21 is cuc perpendicular to the intended lasing ~ace~s in~o bars
22 of diodes. Then the bars of diodes, following ~assivation
23 o lasLng facets, are cu~ into individual diodes.
24 Cutting of the waer into bars is generally accom-
plished by cleaving the ~afer through the substrate side,
26 using an instrument such as a razor blade, knife, scalpel
27 blade or the like. Control over length of -~he diode laser
28 is consequently poor, and variation of diode laser length is
29 great, wi~h the resul~ thac longi,udinal mode discribution
- 2 - ~ ~4~ 6~
1 and threshold curren-~ vary considerably from one diode
2 laser to the next. Furcher, the gold contact pads must
3 be kept thin in order ~o permit xeasonably clean cleaving.
4 Also, the thickness of the substraLe is constrained in
order to promote better cleaving. This of~en limits useful
6 wafer thicknesses to about 3 to 5 mils. Ye~., such ,hin
7 wafers are suscep~ible to breaking during handling. Finally,
8 striations genera-~ed by the mechanical cleaving, if across
9 the active lasing region, affec~ device yield, since such
devices are consequen~ly prone ~o degradationO
11 SUMM~RY OF THE INVENTION
12 In accordance with ~he invention, a wafer comiris-
13 ing a semiconductor substrate, at leas~ a portion of one
14 surface of which is me~allized, and a plurality of semicon-
duc~or layers deposited on a~ leas~ a portion of the opposite
16 surface, at least one of which layers when appropriately
17 biased generates coherent electromagnecic radiation, is
18 cleaved into bars of diodes by a process which comprises
19 (a) etchLng into the substrate wi~h an anisotropic e~chan~ to
form V~grooves in the wafer and (b) mechanically cleaving
21 in~o bars of diodes.
22 For separating thin wafers (about 3 to 5 mils thick)~
23 prior to the anisotropic etchingJ the wafers are processed
24 by forming an array of exposed lines on ~he metallized sub-
strate by photolithography to define lasing ends of the
26 diodes and etching through the exposed metallized portions to
27 expose portions of the underlying substrate. The exposed
28 portions of the wafer are then aniso~ropically etched to a
29 distance of about 1 to 2 mils Less chan che total thickness
of the wafer.
31 For separating thicker wafers (about 6 to 10 mils
32 thick~, prior tO the anisocropic etching, the wafers are
33 processed by forming channels of subsLantially parallel
34 sidewalls to about 1 to 4 mils deep in the surface of che
n-substrate. The wafer is then aniso~ropically ecched to a
36 depth sufficient to form V-grooves in the botto~ of the channelsO
1 1 4(~
1 As a consequence of the process of the invention,
2 good cleavage con~rol, substantially damage-free facets along
3 the plane of cleaving and substantially uniform definition of
4 diode laser length are ob-tained. Further, cleaving in accord-
ance with the invention increases device yield by at leas~
6 50%, as compared with prior art techniques.
7 BRIEF DESCRIPTION OF THE DRAW~G
8 FIGS. la and lb, in perspective, depict a portion
9 of a thin wafer following etching of V-grooves in accordance
with the invention prior ~o final mechanical cleaving;
11 FIGS. 2a` and 2b are pho~omicrographs of lasing
12 facets of, respec~ively, a diode laser formed by cleaving in
13 accordance with prior art procedures, showing striations
14 (damage) resultinO rom clea~age, and a diode laser formed
by cleaving a thin wafer in accordance with the inventionJ
16 showing substantial absence of stria~ions;
17 FIGS. 3a and 3b, in perspective, depict a portion of
18 a ~hick wafer following forma~ion of channels and e~ching of
19 V-grooves in accordance with the invention prior to
mechanical cleaving into diode bars, and
21 FIGS. 4a and 4b depict a portion of a thick wafer
22 in cross-section, following formation of channels and V-
23 grooves, respectively.
24 DETAILED DESCRIPTION OF THE INVENTION
The description that follows is given generally
26 in terms of double heterostructure (DH) (Al,Ga)As diode
27 lasers having a s~ripe geometry. However, it will be
28 appreciated tha~ other configurations and other geometries
29 of both gallium arsenide diode lasers, as well as other semi-
conductor diode lasers, may also be beneficially processed
31 following the teachings herein. Specific configurations of
32 devices may generate coherent electromagnetic radia~ion in
33 the W, visible or IR regions.
34 FIGS. la, lb, 3a and 3b depict a porcion of a wafer,
considerably enlarged for purposes of illustra~ion, from
36 which a pluralicy of DH diode lasers are to be fahrica~ed.
- 4 ~
1 FIGS. la and 3a show the wafer n-side down, while FIGS.
2 lb and 3b show the wafer p-si~e downO The wafer includes
3 an n--~ype GaAs substrate 10, on at least a portion of which
4 are normally grown four successive layers 11, 12J 13 and 14,
respectivelyJ of n~(Al,Ga)AsJ p-GaAsJ p-(Al,~a)As and
6 p-GaAs. Layers 11 and 12 from a p-n junction r~gion 15,
7 with central areas 16 in layer 12 providing light-emitting
8 areas. The layers are conveniently formed one over the
9 other in one run by liquid phase epicaxy, using conventional
diffusion techniques and a horizontal sliding boat apparatus
11 containing four melts, as is well-known. Mecal electrodes
12 17 in the form of stripes parallel to the intended direc~ion
13 of lasing are deposited through conventional phoLolithography
14 techniques onto top layer 14 and provide means for external
lS contact. A metal layer 18 is deposi~ed on at least a por~ion
16 of the bottom of the substrate 10. Gold pads 19, somewhat
17 smaller in area than the intended device~ are formed on layer
18 18, and provide means for external co~act. When cleaved
19 nto individual devices, as shown by dotted lines 20, planar
mirror facets are formed along (110) planesO When current
21 above a threshold value from a battery 21 is sent through a
22 selected electrode 17, light L is emitted from the facet on
23 the p-n junction 16, such p-n junction lying in a plane that is
24 perpendicular to Lhe dire~tion of current flow from electrode
17 to ,electrode 18. That is, ~he cavity of ~he laser struccure
26 is bounded by the two cleaved facets, and the laser light ls
27 emitted from the facets in a direction approximately per-
28 pendicular to the direction of current flow. The necessary
29 reflectivity at the cavicy facets is provided by the dis-
continuity of the index of refrac~ion between the semicon-
31 ducting materials and air~
32 In the typical fabrication of DH (Al,Ga)As diode
33 lasers, the wafer comprises a substrate 10 of n-GaAs, typi-
34 cally about 3 to 5 mi}s thick and having a carrier concentra-
.ion ranging from abouL 1 co 3 x 1013 cm~3, usually doped
36 with silicon.
_ 5 ~ 6~
1 Alternatively, the wafer comprises a substrate 10a of
2 n-GaAs at least about 6 mils thick9 and preferably 6 ~o
3 10 mils thick, having the indicated carrier concentration.
4 Layers 11 and 13 of n-(Al,Ga)As and p-(Al,Ga)As,
S respectively, are typically about 0.75 to 2~ m thic~ with
6 both layers having a value of x (AlxGal xAs) of about 0.30
7 to 0.35. Layer 11 is typically d~ped with tin, while layer
8 13 is typically doped with germanium. Active layer 12, of
9 either p-GaAs or p-(Al,Ga)As, is typically about 0.1 to
0.3 ~ m thick and is undoped. If layer 12 is p-(Al,Ga)As,
11 then the value of y (AlyGal yAs) ranges from about 0.05 to
12 0.10. Cap layer 14 of p-GaAs is typically about 0.2 to
13 0.5 ~ m thick and provides a layer to which ohmic contact
14 may be made. The carrier ooncentration of layer 14, provided
by germanium, is typically about 1 to 3 x 1019 cm~3. Metallic
16 ohmic contacts 17 in stripe form are deposited onto layer 14
17 by conventional photolithographic techniques employing elec-
18 troless nickel plating having a thickness ranging from about
19 0.05 to 0.07~m~ followed by about 1000 A of electroplated
gold. Ohmic contact 18 is formed by evaporation of, e.g.,
21 3% silver/97% tin alloy onto the bottom of substrate 10 and
22 typically has a ~hickness ranging from about 0.18 to 0.20~m.
23 Gold pads 19, formed by electroplating through a photoresist
24 mask, typically are about 2 to 3~m thick.
Following the ~oregoing procedure, the wafer is
26 first cleaved into baxs of diodes by cleaving the wafer
27 through the substrate side, perpendicular to lasing facets,
28 along lines 20, which are between gold pads 19. However,
29 the regions covered by the gold pads are locally strained,
and cleavage is unpredictable, with the consequence that
3L prior art mechanical cleaving techniques such as a knife,
32 razor blade or other instrument, result in diode bars of
33 uneven length. Variations in diode laser length affect longi-
34 tudinal mode distribution and threshold current, with the
resul~ that these values can differ considerably for diode
36 lasers taken from different locations on the same wafer.
- 6 ~ 6 ~ ~
1 Further~ the diode bars are subsequently placed in a
2 fixture for evaporation of a film of Alz03 of about
3 1200 ~ in thickness to passivate lasing facents. If .he
4 diode bars are too long (as measured between lines 20 in
FIGS. la and 3a), then the diode bars canno~ be placed in the
6 fixture. If too short, then, due ~o a shadowing effect,
7 the lasing facets are not properly passivated.
8 Another consequence of prior art mechanical
9 cleaving is thac the gold pads must be kept thin, as must
che substrate, in order to maximize yield of diode lasers.
11 Yet, thin gold pads are bonded to only with difficulty
12 when connecting one end of an external lead, and chin
13 subs~rates render handling of the wafer difficult. Fu~ther,
14 such mechanical cleavage often generaces s-triacions (damage),
which, whe~ formed across the active region, can lead co
16 increased degrada~ion of the devicesJ with consequen~ low
17 device yield. Such striations are shown in FIG. 2a, which is
18 a photomicrograph of a facet cleaved in accordance with prior
19 art techniques.
In accordance with one aspect of the inventionJ
21 variation in diode laser length in chin wafers (e.g.J 3 to
22 5 mils thick) is minimized by the followlng procedure. An
23 array of exposed lines on the n-side of the wafer is formed
24 by conventional photolithographic techniques. The lines or
channels expose n-side metallized contact layer 18. The
26 exposed portions of the m~allized layer are then etched
27 through with an etchant which selec~ively etches the metal
28 without etching the semiconductor material. For exampleJ
29 for a contacting layer 18 of 3% Ag-97% Sn having a thickness
of about 0.18 to 0.20 ~ m, etching is conveniently performed
31 in about 10 minutes employing concencrated H~l. Grooves
32 are ~hen etched into the exposed portions of che substra.e
33 with a preferential etchant that forms V-grooves 22. Where
34 the substrate is gallium arsenide, an example of such an
etchant comprises a solution of H2S04, H22 and ~2- The
36 exact details of a successful etchant for producing a V-groove
:`
~ 7 ~ 6~
1 22 as shown in FIGS. la and lb are described in a paper
2 entitled "Selective Etching of Gallium Arsenide Crystals
3 in H2S04-H20z-H20 Systems" by S. Iida et al in Volume 118J
4 Electrochemical Society Journal, pages 768-771 (1971) and
forms no part of this invention. An example of an e~chant
6 that produces a V-groove is lH2S04-8H202-lH20J in which the
7 concentration of H2S04 is a 98% solution by weigh~ and
8 the concentration of H202 is a 30% solurion by weight,
9 whereas the formula concentration is by volume. The 1-8-1
solution, at a temperature of 25C, is able co etch through
11 the GaAs layer at a rate of about 4 ~ m/min. The e~chant
12 in this concentration produces a Y-shaped channel in GaAs
13 with sidewalls having an angle of 5444' with respect to
14 the plane of the wafer when ~he ecch is performed on the
(0~1) surface along the <011> direction which gives V-grooves.
16 The etching solution is quenched as soon as the desired
17 amoun~ of etching has taken place. O~her etchants, whecher
18 chemical or gaseous, which also give rise to similar
19 V-grooves, may also be employed. A rela~ively steep sidewall,
such as 5444', is preferred to shallower sidewalls of,
21 say, less than 45, in order ~o conserve substrate ma~erial
22 on the etched side of the wafer.
23 Of course, the rate of etching can be increased ~y
24 increaslng the temperature of the etchant. The etchant is
2~ selective according to the cryscal orientation o~ the materialJ
26 as described above. Thus, the orientation of .he wafer
27 should ~e such tha. a V-groove configuration is obtained,
28 rather than a round bottom configura.ion. The reason for
29 chis is that the bottom of the V-groove provides a precise
loca~ion for initiation of cleaving, which in turn results
31 in fabrica~ion of individual diode lasers of precise length.
32 The etching is carried ouc to a depth of abouc 1
33 to 2 mils less than the total thickness of the wafer. If
34 the etching is not deep enough, then cleavage is more dif-
ficult, since a cleavage plane is not well-defined and
36 cleavage s.riations are more likely .o occur across 'asing
- 8 ~ 4~
1 facets, as wi h prior ar~ techniques. If the ecching is
2 too deep, then cleaving is initiated beyond the substrate and
3 in the region of the epitaxial layers, and will not resul~
4 in a mirror face~
Following etching~ which, as shown in FIGS. la
6 and lb produces a V-groove 22, the wafer is mechanically
7 cleaved by rolling or other means so as to produce cleavages
8 along lines 20. A knife, razor blade or other sharp instru-
9 ment may be used from the n-side for cleaving, resulting in
diode bars of prescribed uniform length with good cleaved
11 surfaces. Alternatively, a convenient technique is to
12 mountthe wafer~ p-side up, on a flexible adhesive tape and
13 roll the assembly over a small radius toolJ such as dis-
14 closed in U.S. Paten~ 3,497,948. Most preferred is ~o simply
cleave from ~he p-side by pressing down over the V-grooves
16 with a blunt instrument, such as a tweezer edge. This
17 method is fast and accurate. The combination of etching
18 V-grooves in the substra~e to the specified depth range,
19 followed by mechanical cleavage, xesults in substantially
striation-free facets, as shown in FIG. 2b.
21 While thicker substrates are desirable, the
22 typical thicknesses of 3 to 5 mils for substrates are the
23 resul~ of the constraints posed by prior art techniques of
24 cleaving wafers. Such thin wafersJ however, are very fragile
and of~en break during handlingO The inventive ~echnique
26 discussed in further detail below is particularly useful
27 for thicker wafers, such as on che order to 6 to 10 mils and
28 thicker, after fabrication to form the oh~ic contacts as
29 described above.
Thus, in accordance with another aspect of the
31 invention, diode lasers are fabricated from relatively thick
32 wafers (e.g., 6 to 10 mils thick) by the following procedureO
33 A channel 24 of substantially parallel sidewalls is formed
34 in the exposed surface of the n-substrate, as shown in cross
section Ln FIG 4a. The channels are about 1 to 4 mils
36 deepO If the channels are no~ deep enough, then the
- 9
1 subsequenL etching step, described below, results in
2 considerable loss in surace area of the subs~race. If
3 the channels are too deep, ,hen ~he V-groove formed by
4 etching in ~he subsequen~ step will no~ be fully formed,
and thus cleaving is initia~ed beyond ~he substrate and in
6 the region of che epitaxial layers, and will no~ result in
7 a mirror facet.
8 The channels are convenienLly formed using a diamond
9 circular saw blade about 1.5 co 2 mils in thickness. While
other techniques may be used, the diamond circular saw
11 blade, which is ex~ensively usPd in semiconductor processing
12 techniques for oLher purposes, advancageously forms channels
13 having substantially parallel sidewalls. Due ~o the
14 increased thickness of the wafer, no dislocations are gen-
erated in the active region, which is a problem that generally
16 accompanies use of diamond saw blades with thinner wafers.
17 Grooves are chen etched into the bottoms of the
18 channels with an anisotropic etchant that forms V-grooves 22a,
19 as shown in cross section in FIG. 4b. When the substrate
is gallium arsenide, an example of such an etchant comprises
21 a solution of H2S04, H202 and H20. The description above
22 in reference to forming V-grooves in ~hinner wafers with this
23 etchant is applicable to thicker wafers as well.
24 The etching is carried ou~ to a depth sufficient
to form a V-groove. If the etching is noc deep enough to
26 form the V-groove, then cleaving is more difficult, since
27 ~he cleavage plane is not well-defined and cleavage
28 stria~ions are more likely to occur across lasing facets,
29 as with prior art techniques. If the ecching is too deep,
then cleaving is initiated beyond the substrate and in the
31 region of the epitaxial layers, and will not result in a
32 mirror facet. The V-groove in the bottom of the channel
33 is generally weil-formed about 1 to 3 mils deeper than ~he
34 initial channel, and etching may be cerminated at that pointO
Following etching, which, as shown in FIGS. 3a,
36 3b and 4b produces a V-groove 22a in che bottom of channel 24
- 10 - ~4~'~6~
1 ~he wafer is mechanically cleaved by rolling or other means
2 so as to produce cleavages along lines 20, as described
3 above in connection with thinner wafers.
4 Followins cleaving in~o diode bars and passivation
of lasing facets, individual diodes are formed by scribing
6 the bars, as with a diamond scribe, usually on che n-sideJ
7 along lines 23 (the wafer having previously been il~dexed by
8 well-known means to locate stripes 17). The scribed bars
9 are then mechanically cleaved by rolling a tool of small
radius over the bars, as is customary in the ar~0
11 The foregoing methods result in good cleavage con-
1~ trol and uniform deinition o diode laser lengch. Conse-
13 quently, longitudinal mode distribution and threshold curren-t
14 e~hibi~ tle variation for devices ~aken from differen,
loca~ions in ~he wafer. Cleavage co prescribed leng~hs re-
16 sul~s in easier processability for lasing facet passiva~ion
17 and in improved device yields. Yields improved by a~ leas~
18 50% are realized using the me-hod of the invention. Cleaving
19 through thin GaAs (from the bottom of ~he V-groove to the p-
side) appears LO reduce cleavage st~iations on ~he lasing
21 facet, as shown in ~he comparison between FIGS. 2a and 2b,
22 ~which are pho~omicrographs o cleaved faceLs, magnified llOOx
23 the former produced by a prior art method as discussed above
24 and the latter produced in accordance wi~h the inven~ion. An
additional benefit of he invention is that gold contac~ pads
26 19 may be made thicker wi~hout affecting the quali~y of the
27 clèaved surface. Such thicker contacts permiL better ease of
~8 con~acting to an ex.ernal power sourceO Also, a thicker sub-
29 strate may be employed than heretofore possible, thereby in-
creasing ease of handling. The method of the inven~ion for
31 thicker wafers is especially eicacious for processing
32 thicker substrates than heretobefore possible, thereby increas-
33 ing ease of handling and ease of fabricacion of shor~er diode
34 (cavi~y) lengchs. Such shorter diode la~er length, on the
order o abouL 6 mils, permi~ lowering of ~he ~hreshold
36 curren, over thac customarily found in the ar~
66~
D?LES
_
2 ~xamPle l, Thin Wafe
3 A processed piece of GaAs material (average thickness
4 about 4.0 + 0.5 mils) was divided into two pieces. One
piece was held for conventional prior art cleaving, whereas
6 the second piece was further processed for etch cleaving in
7 accordance with the invencion. The ~wo pieces were then
8 cleaved a. the same time by che same operator using che following
9 method for both pieces: pressing with tweezers in ,he direccion
of the desired cleave. The experimenc was then repeated in the
11 same way with another operacor.
12 The etch cleaving was done as follows: A pattern of
13 parallel s~rips 10 ~ m wide and 10 mils apart (center to
14 center) were formed by exposure of photoresist through a suic-
able phocoresist mask. The exposed photoresist portions were
16 removed by dissolving in developer to expose portions of a
17 cop gold layer. The exposed gold portions were removed in a
18 ~I-base gold etchant in about 1 minute at 50C to expose
19 portions of an underlying 3% Ag/97% Sn layer. The exposed
portions were removed using concentrated HCl for 10 minutes
21 at room ~emperature to expose porcions of underlying n-GaAs
22 substrate. The exposed portions were etched wi~h a V-groove
23 etchant comprising lH2SO4-8H2O2-lH2O (by volume) for 15
24 minutes at room temperature~ The etched V-grooves were
formed to an average depth of abou~ 1.5 mils less than the
26 thickness of the wafer.
27 The cleavage yields were measured in terms of ~he
28 number of useful bars obtained expressed as a percentage of
29 the total amount of macerial. The cleavage striation densi-
.ies were measured by Nomarski optical examinacion of 5 mm
31 lengchs of samples representative of the two methods.
32 The results are shown in the Table below:
33 Prior Art Etch Cleaving
34 Cleavin~ of the Inven~ion
% Yield for Operator 1 20 100
36 % Yield for Operacor 2 10 100
37 Cleavage Striatio~ Density, 300 6
38 mm~
- 12 _
1 It can be seen ~hat yields were improved by a
2 fac~or of 5 to 10 and cleavage striacion densi~y was reduced
3 by a factor of 50 employing the inventive techniqueO
4 FIGS. 2a and 2b are photomicrographs of the facets
cleaved by ~he ~wo methods, enlarged by a factor of llOOx.
6 The reduction in cleavage striation densi~y afforded by the
7 e,ch-cleave method of the invention is clearly visible.
8 Example 2. Thick Wafer
9 Processed wafers were lapped on ,he n-side to 8
mils. A coating of 3% Ag/97% Sn was evaporn~ed on ~he lapped
11 side to a thickness of l900 A. A nickel film of 2000 A ~ m
12 was electroless pla~ed on the Ag-Sn coating, followed by a
13 gold film of 500 ~. A ~hin layer of chromium (700 A), followed
14 by a chin layer of gold (1000 ~), was evaporaced on ~he opposi~e
side.
16 The wafer was sawe~ from the n-side to a depch of 2
l~ mils using a diamond blade of 1.5 ~o 2.0 mil ~hickness.
18 After sawing, ~he sample was etched in a V-groove etchan~
19 comprising IH2S04-8~2 4-lH20 (by volume) for 10 min. at room
cemperature ,o a depth of 4.3 mils rom ~he substra~e sur-
21 face (2.3 mils deeper than the ini-~al channel). Cleaving
2~ was done by pressing with tweezers in ~he direction of the
23 desired cleaveO
24 The cleavage yields were measured in ~erms of che
number of useful bars obtained expressed as a percen~age of
26 the total amounL of ma~erial. The cleavage striaLion densi-
27 ~ies were measured by Nomarski optical examina~ion of 5 mm
28 lengths o samples representative of Lhe cleaving. No com-
29 parison was made using prior art techniques with waers about
8 mils Lhick, since such wafers canno~ be controllably
31 cleaved by scribing and mechanically cleaving. A comparison
32 could be made, however, with wafers abou~ 4 mils chick
33 cleaved by conven~ional prior ar. techniques. The results
34 are shown in ~he Table below.
- 13 ~
; 1 Prior Arc Cleaving in Accord-
2 Cleavin~_ ance with Invention
3 % Yield 10j20 100
4 Cleavage Striation
S Density, mm~l 300 8
6 It can be seen tha~ yields were improved by a
7 factor of 5 ~o 10 and cleavage scriation densi~y was reduced
8 by a factor of nearly 40 employing the inventive technique.
9 The comparison presumably would be even grea~er if ,hlck
wafers could be cleaved by prior art techniques.
