Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CRO.~S-l~EFERENCES T() E'~ET ATED PATENT
Canadian Patent ~u~ber 1,029,475, issue~ April 11, 1978,
entitled '~semiconductor Device" in which the inventors are T. Matsushita,
H. Hayashi, T. Aoki, H. Yamamoto and Y. Kawana assigned to the
same assignee of the present invention discloses a polycrystalline silicon
layer as a passivation layer formed on a semiconductor singly crystal
layer in a semiconductor device.
BACKGROUND OF THE INVENTION
Field of the Invention: -
This invention relates-to a solid state image sensor,
especially employing a charge coupled device (CCD). The CCD device
.- includes a surface CCD, a bulk CCD or a bucket brigate device (BBD).
BRIEF DESCRIPIION OF THE DRAWINGS
.: , ,
Figure 1 illustrates a CCD device of the prior art,
..
: ~ Figure 2 is a plan view illustrating.a frametransfer device
` of the prior art, .
Figure 3 illustrates an inter-line transfer system according
to the prior art,
Figure 4 illustrates a CCD device of the prior art,
Figure 5 illustrates a first embodiment of the present
. invention,
Figure 6 illustrates a modified form of the invention,
- Figure 7 is a plot of oxygen content versus resistivity
characteristic,
Figure 8 illustrates apparatus used to manufacture devices
',J' according to the invention,.
Figure 9 is a graph showing the oxygen concentration versus
the band gap energy characteristic,
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Figure lO is a plan view of a modified form of the
invention,
Figure 11 is a sectional view taken on line A-A in
Figure 10, and
Figure 12 is a sectional view of the apparatus shown
in Figure 10 taken on sectional line B-B.
Description of the Prior Art:
Figure 1 illustrates a conventional three phase CCD
having an N-type silicon substrate 1 upon which a silicon
lO dioxide layer 2 is formed and with transfer electrodes 3 formed
on top of the silicon dioxide layer 2. Clock pulses 01' 02 and
03 are applied to the gate electrodes 3.
Two phase surface CCD devices are also known in which
the thickness of the silicon dioxide layer 2 under each elec-
trode 3 is varied in the charge transferring direction to form
a step-like potential well for carriers.
: A frame transfer system or an inter-line transfer
system is employed in the solid state image sensors of the prior
~ art.
: 20 A frame transfer system is illustrated in Figure 2
which has an image area 5 and a storage area 6 and includes
a shift register 7. Charges generated by incident light in
; the image area 5 are simultaneously transferred to the storage
area 6 and are sequentially read out from the shift register 7.
I The inter-line transfer system is illustrated in
Y~ Figure 3 and has vertical image areas 8, vertical shift regis-
`~ ters 9 and a horizontal shift register lO. Charges generated
;~ by the light in the image areas 8 are simultaneously trans-
ferred to the vertical shift registers 9 and are sequentially
~- 30 transferred to the horizontal shift register lO. :
In Figure l charges are generated by light energy which
passes through the silicon dioxide layer 2 between the
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electrodes 3. Such devices have good light sensitivity but
have an unstable passivation when the silicon dioxide layer 2
is exposed because of contaminating positive charges in the
silicon dioxide layer. These positive charges cause a memory
effect which is undesirable.
- Figure 4 illustrates another known CCD device in which
pure polycrystalline silicon 11 is deposited completely over
the silicon dioxide layer 2 and impurities are selectively
diffused into the silicon layer 11 to form low resistivity
regions lla. Aluminum is deposited on the pure polycrystalline
layer 11 to form transfer electrodes 3. Such device has good
passivation but the sensitivity of shorter wave length light
is lowered because of the existence of the silicon layer 11.
Also, the resistivity of the silicon layer 11 is not high
enough to prevent leakage currents from occurring between the
, electrodes 3. -
~ SUMMARY OF THE INVENTION
:,'
~' The object of the present invention is to provide an
improved CCD image sensor in which oxygen doped polycrystalline
silicon is between the electrodes.
The present device has good passivation as compared
~, with devices using only a silicon dioxide layer and has low
leakage current between the electrodes and a very high sensiti-
.~
vity to shorter wave length light (blue) as compared with
devices using pure polycrystalline layer.
The polycrystalline silicon in this invention may
include amorphous silicon.
In accordance with the foregoing objects, there is
.J provided:-
an image sensor comprising:
, a semiconductor substrate;
an insulating layer on said substrate;
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transfer electrodes on said insulating
layer; and
an oxygen doped poly-crystalline silicon
layer on said insulating layer.
Other objects, features and advantages of the inven-
tion will be readily apparent from the following description
of certain preferred embodiments thereof taken in conjunction
with the accompanying drawings although variations and modifi-
cations may be effected without departing from the spirit and
scope of the novel concepts of the disclosure.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Figure 5 illustrates a first embodiment of the present
invention in which a silicon dioxide layer 2 is formed on a
N-type silicon substrate 1. An oxygen doped polycrystalline
silicon layer 13 is formed all over the silicon dioxide layer 2.
Impurities are selectively doped into the oxygen doped poly-
`~ crystalline silicon layer 13 by diffusion or ion implantation
to form low resistivity regions 13a. Aluminum electrodes 14
are deposited on the regions 13a to form transfer electrodes
3. Photosensitive portions 15 exist between the electrodes 3.
', Figure 6 illustrates a modified form of the invention
~ in which an oxygen doped polycrystalline silicon layer 13 is
-~ deposited over the silicon dioxide layer 2 on the substrate 1
~ and aluminum electrodes 3 are formed over the polycrystalline
, .
' silicon layer 13. In either of the embodiments illustrated
1~ in Figures 5 and 6 the photogeneration portion between the
,, electrodes 3 is covered by the silicon dioxide layer and the
oxygen doped polycrystalline layer. The oxygen concentration
in the polycrystalline silicon layer 13 is maintained in the
range 10 to 50 atomic %.
The oxygen concentration in the polycrystalline
` silicon is plotted versus the resistivity in Figure 7. The
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mean silicon grain size is 200 to 300 A. It is to be noted
that the resistivity increases as
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the oxygen concentration increas~s.
The polycrystalline layer 13 has a mean grain size in the
range betweell 50 to 1000 A. If the grain size is less than 50 A the
properties of the polycrystalline silicon will approach that of silicon
dioxidc and the memory effect will appear which is undesirable. Also,
it requires a low reaction temperature and, thus, the growth rate will
be small. If the grain size is more than 1000 A the leakage current
increases which is undesirable.
The oxygen doped polycrystalline silicon layer 13 is obtained
by chemical vapor deposition (CVD) as shown in Figure 8. A reactor
21 contains the substrate 1 which is heated to the range of 600 to 750 C
for example to a temperature of 650 C. A carrier gas source (N2)
22, a silicon source (SiH4) 23 and an oxygen source (N2O,NO or NO2)
24 supply input to the reactor 21 through the valves 25, 26 and 27. SiH4
is used because it produces the desired polycrystalline silicon at a
relatively low reaction temperature. If SiC14 is used it will require a
higher reaction temperature such as 900 C which will result in larger
grain size and larger leakage currents. The oxygen concentration is
controlled by the flow ratio of the N2O and the SiH4.
As the semi-insulating silicon layer 13 covers the SiO2
layer 2 the surface state of the silicon substrate 1 is stabilized because
charges in the silicon layer 13 neutralize the charges in the SiO2 layer
2.
As the oxygen doped silicon layer 13 covers the photosensitive
portions 15 of the substrate the sensitivity to the shorter wave length
light (blue) increases as compared to devices u6ing pure polycrystalline
silicon because the band gap energy in the oxygen doped polycrystalline
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silicon is larger than that of the pure polycrystalline silicon.
The oxygen concentration in atomic ~, is plotted in Figure 9
versus the band gap energy distribution. Polycrystalline silicon containing
36 to 49 atomic % oxygen has a wider band gap by 0,2 to 0. 4 electron
volts than that of pure silicon.
The existence of the layer 13 improves the insulation between
the electrodes 3 and the substrate 1 even when they are pin holes in the
silicon dioxide layer 2.
Figures 10, 11 and 12 illustrate a second embodiment of the
invention applied to a two phase CCD device employing the inter-line
transfer system. FigurelO is a plan view illustrating the substrate 1
upon which are formed a silicon dioxide layer 2 as illustrated in ~igures
11 and 12 which are respectively sectional vie~staken on llne A-A in
Figure 10 and line B-B in Figure 10. The illustrated embodiment is
applied to a two phase CCD device employing inter-line transfer system
using image areas 8 and shift registers 9. Oxygen dopes polycrystalline
layer 13 illustrated in Figure 11 and 10 is formed over the silicon
dioxide layer 2. Impurity doped polycrystalline layer electrodes 3 on a
silicon dioxide layer 2 are formed, and an oxygen doped polycrystalline
silicon layer 13 is formed over the silicon layer 3. Photogenerated
carriers originate beneath a photosensitive portion 15 and are transferred
beneath the electrodes 3 in the A-A direction. There are provided
step-like potentials beneath the electrodes 3 to prevent the reverse flow
of the charges. Channel stoppers 33 illustrated in Figure 11 are also
provided. The charges are sequentially transferred in vertical shift
registers 9 in the B-B direction. Aluminum electrode layers 34 may
cover the oxygen doped polycrystalline silicon layer 13 except over the
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photosensitive regions 15.
It is seen that this inventioll provides a new and novel CCD
device and altllough it has been described with respect to preferred
embodiments it is not to be so limited as changes and modifications may
be made which are within the full intended scope as defined by the
appended claims.
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