CA2091055A1 - Acoustic charge transport imager - Google Patents

Abstract

ACOUSTIC CHARGE TRANSPORT IMAGER

ABSTRACT

An acoustic charge transport imager, suitable for use as a High Definition Television (HDTV) camera element, is disclosed in which an array of amorphous hydrogenated silicon based avalanche photdiodes ( 102) are combined with acoustic charge transport channels (108) in a GaAs substrate, to achieve very high speed read out of photgenerated charge. High speed read out allows the fabrication of detector arrays large enough to meet the resolution requirements of HDTV while ensuring operation within the timing constraints of the HDTV frame rate.

Description

Wt~ ~2/0~734 2 ~ Pcr/us91/06422 ACOUS~C C~RGE T~ANSPO~T IMAGE:E~

TECIINICAL~CKGROIJND.
T.ll;s invcntion relatcs to a sin~lc chip, low cost camera elcment for Migh Definibon Tc]cvision (HDTV). ~forc spccifically the invcntion rclates to the use of acoustic chargc transport ;n a ~allium arscnide (GaAs) substrate to achievc high speed read out of photogcnerated c]cctric charge.
BACKGROUND ART

Existing HDTV cameras utilize a vacuum ~ube as the detecting element. One example of such a tube is the I-I~RPICON H4318 15 manufactured by Hitachi. Although the HARPICON H4318 camera tube provides ]ow no;se, h;gh sensit;vity perfonnance for HDTV app]ications, a camera using th;s tubc is relatively large, fragile and heavy. Therefore, it is not well su;ted for usc in light-weight app]ications such as hand-he]d HDTV
broadcast carneras, camcorders, or spacc app]ications. For thesc app]ications 20 a compact, light-wei~ht, comp]cte]y solid state dcvicc is idea].

The conventional choice for a solid-statc camera chip is onc which uses a charge coupled device ~CCD) to read out the image chargc.
However, there is presently only onc CCD camera for I-IDl`V which can 25 operate at thc required video framc rate. This camcra, manufactured by Toshiba, is very costly and there~ore unsuitab]e for consumer applications.

The HARPICON H4318 achieves high sensitivity and reso]ution 30 by ava]anche multip]ication induccd gain within the dctcction stagc, whicl1 is made from amorphous selenium (a-Se). Howevcr) the use of an a-Sc detector stage means that holc transport and collcction arc rcquircd bccausc WO ~2/04734 PCr/US9~/06422 the hole ionization rate is substantially highcr than thc elcctron ionization rate in a-Se. I-Iolc transport is not compatiblc witll high spcecl inte~rated circuit operation.

Thercfore, thcrc is a nccd for a small, low-cost, lo~v voltage, ea~sy to manufacture, rcliablc, high rcso]ution HDTV camera elcment, capablc of read out specds compatiblc with HDTV framc rate requirements.

DISCI,OSIJRE OF T~IE TNVENTION
Accordingly, thc general object of the invention is to provide a mecms for detecting light in the visib]e spectrum with an imager array sufficiently large to achicvc ~ TV resolution and to couple the imager array to a means for read out which operatcs at sufficiently high speeds so as to 15 achieve compatibi]ity with HDTV framc ratcs. The devicc of the present invention wou]d a]so bc suited for many spccial typcs of fast imaging app]ications, such as high reso]ution super-slow-motion or fast processing app]ications, such as on-chip ima~c enhancement.

It is an object of the present invcntion to coup]c a ]ow-noisc, high-gain photodctector element to a means.capab]c of high speed charge read out.

It is anothcr object of the present invention to monolithically combine heterostructure acoustic charge transport (ACI) devices and amorphous hydrogenated silicon avalanche photodiodes (APD).

It is a further object of thc present invention to achievc an ima~er architecturc which is both acoustically and optically efficien~.

O 92/1~1734 2 ~ 5 PCI/US~1/06422 It is a furthcr object of the prcsent invention to provide a so,'ld-state camcra chip ~or HDTY applications.

It is a ~urthcr objcct o~ this invcntion to providc such a chip .~ wllicll is small ancl cfricicnt enou~h to bc uscd in camcordcrs and spacc applic<ltions.

It is yet a further object of this invention to provide such a chip made from an ~CT structure in a GaAs substrate and ~m APD constructed 10 from amorphous silicon to providc an cfficient .md integrated system for use with HDTV equipment.

It is still a further object of this invention to provide such a chip which globa]ly optimizcs pcrÇormance of the heterostructure by balancing the 15 photodetector and ACI' rcad out structllre design parameters.

The invention comprises four main components: (1) a photodetector which may be an amorphous si]icon avalanchc photodiode (APD), or a super]attice APD, (2) an npn bipolar junction transistor (BJT) 2d to provide char~,e stora~,c and clamping action, (3) an acoustic charge transport (ACT) structure in which char~e is rcad out for imagc reconstruction, and (4) a metal-insulator-semicorlductor field effect transistor(MISFET) structure to controllably transfer, that is gate, the stored charge into the ACr channel. These four componcnts arc integrated in a compact, 25 comp]etely solid-state device suitablc for use in light-weight applications such as hand-hcld HDTV broadcast cameras, camcorders or space appl;cations.
This structural combination of the prcsent invention is novel.

Several materials choiccs arc available for both thc 30 photodetector and ACI structures. The photodetector may be silicon bascd or fabricatcd from a compound semiconductor matcrial system. Althou h a W~ 92/04734 ~ PC~r/US91/06422 C~aAs photodetector has ccrtain theoretical advantages, such as improved gain, ovcr a silicon photodctector, thc GaAs photodctector has a number of problcms re~ated to rnanufactulabi1;ty. Thc substratc in which the ACT
structurc rcsidc~ may bc G~u~s/~]GaAs or somc othcr snatcrial systcm such 5 as silicon Wit]1 ~n associated piczoclectric laycr. ~ny substratc materia]
system, or combination, that supports acoustic charge transport can be used.

In the preferred embodiment, the ~PD is made from an amorphous silicon. This type of APD uses electrons as thc initiating carriers, 10 allowing it to yield low noise pcrformance while still being compatible with the ACI~ component. The particular ad~antage of amorphous si]icon is its reproducibility, cost effectiveness, uniformity, and potential ]ow noise perforrnance.

~5 The charge from thc APD is co]]ected in thc emitter region of a bipolar junction transistor (BJT) and is then gated into an ACT channe].
The ACI componcnt to which the APD is coup]ed in the prcferred embodiment uti]i~es a GaAs/AlGa~s structurc which is capab]c of very high speed, high efficiency charge transfcr, thus enab]ing higher frame rates than 20 have previously been possib]e with charge coup~ed devices (CCDs). Thc speed of the ACT can be attributed to high electron mobi]ity and a potential which is propagated with a surface acoustic wave ~SAW3 instead of a potential imposed by clock pu]ses, as can be found in previously existing CCDs.
BI~rEF I~SCRIPIION OF T~IE DRAWINGS

Fig. 1 is a circuit schematic representing one imagcr ce]l showing the interconnections betwcen thc APD, the BJT, thc ~ISFET
30 transfer gate and the ACT channe].

WO ~2/04734 PCT/US91/06422 Fig. 2 is a cross sectional view of one imager cell illustrating thc spatial relabonship of the clements comprising the ce]l.

F;g. 3 is a block dia~r~m showing a two dimcnsiona~ imager 5 array and a hori~ontal ACI ch~r~c rcad out rc,gistcr.

l~EST MODE OF CA~YING OU'I' TI-IE INVENTION

The preferred embodiment of the invention is now described 10 with referencc to tlle drawings.

1. Electn'cal,_CQnncctivi,t,v Referring now to Fig. 1, there is shown a schematic 15 representation of one cell of the acoustic charge transport imager in accordance with the present invention. The cell includes an avalanche photodiode (APD) ~02, an npn bipolar junction transistor (BJT) 104, a MISI;ET 106, and an acoustic charge transport channcl represcntcd here as a capacitor 10~S. The APD 102 has its p region connected to a volt~gc sourcc 20 100 which is negative with respec~ to ground. The emitter 110 of BJT 104 forms a common node with the n region of APD 102. The base 112, of BJT
104 is connected to ground. The collector 114 of BJT 104 is connected to the positive voltage supp]y 116 of the substrate. The common node of BJT
104 and AI'D 1û2 is couplecl to the ACT channel 108 through the MISFET
25 1(~6. A control signal is connected to the MISFET gate 11~ terrnina].

2. Phv~ical Structure The four main elements of the imager ce]l, the APD, the BJT, 30 the MISFET and the J4C~ channel, are compatible with each other and suitable for monolithic integration. Fig. 2 shows a cross section of thc WO 92/04734 ~ pcr/uss]/oG422 physical architecture of thc cell. In this view, the ACI channcl runs perpendicul~rly through the plane of the figurc.

Thc ~PD of thc prcfcrrcd embodimcnt is cornprised of a p typc S rc~ion 21)'1, an intrinsic region 206, and an n type rcgion 20S. The p typc region 204 is rnadc ~om ~unorphous hydrogenated silicon carbide (a-Si, ,~C~:(H) whcre O<x<1). Thc intrinsic re~ion is made from an amorphous hydrogenated silicon (a-Si:(:EI)). The n type region is made from polysilicon.
In the art to vhich this învention pertains, polycrystalline silicon is referred 10 to as polysilicon.

~ n electrode 202 which is transparcnt to some portion of thc electromagnetic spectrum forms the top surface of thc APD. In the preferred embodiment of the prescnt invcntion this electrode ~02 is 1~ tr~nsparent to the visible portion of the spectrum and is comprised of an indium-tin-o.cide (ITO) layer. This ITO layer of the preferrcd embodiment does not require a passivating topside layer. The ability to fabricate this structure without a topside layer simplifies opb'cal and acoustic design considerations.
Whcn photons pass through the transparent e]ectrodc 202 and ineo the reverse biased APD, a current is generated. This currerlt passes out of the diode through an n-doped po]ysilicon plate 208 and an n-doped polysilicon pillar 214 into the n~ emitter 226 cf a vertical npn BJT. The flat, 25 rectangu~ar polysilicon plate 208 may be fabricated in alternatiYe shapes, such as circular and/or non-planar. The preferrcd embodimcnt utilizes a polysilicon region having two major surfaccs.

~ channel stop 218, comprising a p~ region, abuts thc I3JI` and 30 MISFET structures. In the preferred embodiment thesc p+ rcgions are forrned by Be doping. Ion implantation of Be is thc prcferrcd mcthod of wo 92/1)4731 Q ~ T CI`/US9l/OG422 achieving said doping. rhe channel stops serve to electncally isolatc thc BJT
and MISFET structures from other electrical elements and signals which mc-ly be present in or on the ima~cr chip.

The channel stops 218 of thc prcf~rrcd cmbodimcnt arc dcsi~ncd as long stlips which run through thc imagcr array, with thc ground contacts formed nt both ends. Thc shect resistancc and ]cngth of the implantecl channel stop strips must bc considered carcfully during the design stages in order to avoid fonvard biasing. Any currcnts passing through the channel stop str;ps wi]l act to dcvclop somc voltagc (V=IR). If thc voltagc in some part of the channcl stop strip becomes greater than the diode turn-on voltage then unwanted conduction will takc placc. One result of this unwanted conduction is corruption of thc stored ima~e data.

A ]ight shield 210 having two major surfaccs, scrves to isolate the underlying BJT, MISF~T and ACT channel from pcrformanc~ degrading photons. In the preferred embodimcnt the ]ight shicld is formcd from a layer of molybdcnum.

If the light shie]d is formcd from a conductivc material, such as molybdenum, then it must be electrically isolated from thc photodctector. In the preferred embodimcnt an insu]ating material is disposcd between the photodetector (including the conductivc pil]ar~ and thc light shie]d.
Additiona]ly, a conductive light shie]d should be grounded in ordcr to eliminate the possibility of trapped charge in the light shic]d. Charge trapped in the li~ht shield may advcrsely afrcct the performance of the underlying MISFET structurc. Such effects might be manifested in shifts of MISFET thicl~ fic]d thresho~d voltage.

The li,ht shie]d 210 is physically and clectricallY separated from the underlying BJT, MISFET and ACT ch~mncl by a die]ectric laycr 212. In w~ 92/04734 ~ PCI/US91/06422 the preferred embodiment phospho-silicate-glass (PSG) is uscd as the dielectric layer although other dic]ectric materia~s, such as boro-phospho-silicate glass (BPSG), ZnO or BaTiO, may a]so be suitablc.

Both ZnO and BaTiO, can bc used to crcatc an insulating ]aycr which is also a piezoelectric film. If a material is chosen ~vhich is more piezoelcetric than the Ga~s substrate, a surfacc acoustic wave can be generated havin~ a deeper potentia] well than could be obtained in GaAs alone. That is, the voltage potentia] developed by the SAW would be larger.
The MISFET structure consists of a gate e]ectrode 222, a die}ectric 224, and scmiconducting material 230. In the present invention thc gate electrode is separated from the semiconductor material by thc dielectric.
Said dielectric may be either Si3N,, or undoped G~s. As can be seen in Fig.
2, the gatc 2?2 physically spans a region in which an ~iCI channel e.~ists, as well as a }atera}ly adjoining p re~ion and a portion of the n+ emitter 226 region.

The BJT portion of the present invention is fabricatcd using the n substrate 220 as the collector. A p type basc 228 region is then fonned vertica]]y above the col]ector. The corresponding n~ emitter 226 is fonned above the base, as shown in Fig. 2, so that taken together there is an npn stack.

The ACI' device operation requires confinement of the charge to be transported, within a thin ]ayer 234 of GaAs sandwichcd bctween two relatively tnicker laycrs of AlGaAs 230, 236. The GaAs quantum well 234 is undoped while the ]ower AlGaAs layer 230 is p type dopcd and the top ]ayer 236 is lightly n dopcd. In the preferrcd cmbodimcnt, Bc is thc dop~nt used to achieve p type doping in the lower AlGaAs ]ayer.

WO 92/Q47~4 ~ ; Pc~r/us91/06422 3. Ima~er Operation Light from an object to bc ;magcd is focused upon an array of photodetcctor clcmcnts. Thc prcfcrrcd cmbodimcnt utili~es a two-5 dimcnsionn1 array. The numbcr of photons incidcnt at any particular pointon the array is a function of the object being imaged. Thc photodctcctors generatc an electrical charge proportional to thc number of incident photons.
This plloto~eneratcd chargc, which e]ectrically rcpresents an image of the object, is temporarily stored. A]ternatively, this process could be described as10 converting electroma~netic imagc inforrnation into a two dimensional array of discrete analog charge packets and thcn storing those packets.

In order to transfcr thc stored image information from a largc numbcr of photodetector elcments it is necessary to usc a seria] read out 15 technique. Howcvcr, in order to mect HDTV vidco framc rate requirements, the serial read out must be fast.

Higll speed read out, or accessin~, in the prescnt invention~ is achieved by gating the stored charge into channe]s where an e]ectric 20 potential, resulting from a surface acoustic wave, swecps the charge to othcr chip circuitry for further processin~.

The imager cells 302 are arranged in a two dimensiona] array 304 of rows and co]umns as shown in Fig. 3. During the read out operation, 25 a paraliel trans~er of all the charge packets in a row of thc imager array toan intermediate storage row 30G is perforrncd by acoustic charge transport.
Each charge packet in the intermediate storage row ~6 is then scria~]y transferred out, by means of acoustic charge transport, for further processing.

The novel cornbination of a silicon bc~sed photodetector with an ACI read out structurc in GaAs can be used to form thc basis of a~tcrnativc WO 92Jo4734 2 ~ pcr/us9l/oG422 read out schemes such as frame transfer. In a framc transfcr all the charge packcts in the irnager array arc transferred en massc, rathcr than row by row, to an cqually si~cd storagc array. This stora~c array is typical]y a non-imagin,~ array.

~ dctailed description of the opcration of the im~gercomponcnts is included below.

By reverse biasing the APD, a depletion region is created within 10 the photodiodc whcrein current multiplication through impact ionization rnay take place. Photons incident upon the imager generate the charge which initiates current multiplication. This mult]plicd photo-initiated charge passes to the emitter 110 of BJT 10~ and is stored therein.

Thc ~PD must be reverse-biased in ordcr to operate. Becausc of this requirement, a transparent electrodc 202 is used to make e]ectrical contact to the p sidc of thc APD.

C~arge rom the ~PD is stored primari]y by the capacitancc of 20 the emitter-basc junction. Although the emitter nodc of a BJT is uscd for charge storage in thc prefcrred embodiment, other means for storing chargc, such as a diodc or parallcl plate capacitor, may be uscd. A means for charge storage is physical]y and clcctrically associatcd ~vith the photodetector e]ements.
There are three possible mechanisrns by which this storcd charge may movc. One mechanism is by way oi reverse-biased diode leakage currents into the b~se and channe] stop regions. A second mechanism is thc forward-biased diodc current which f]ows whcn tlle cmitter nodc bccomcs 30 negativcly biased with respcct to thc basc node bv morc than a diodc drop.

Wo 92/0~1734 ~ PCr/US9~/0()422 A third mechanism is the creation of a conductive pathway from thc charge storagc nodc to thc ACT channcl by way of MISFET action.

Rcvcrsc-biascd cliode leakagc currcnt acts ovcr timc to corrupt 5 thc storcd ima~c data. This puts ~a practical limit on thc sizc of thc imager array sincc thc ima~c data is read out esscntially serially. Scrial rcad out mcans that a fin;te timc is required to access all the storage nodes in the irnager ~ay. As the array becomcs larger the acccss cycle timc becomes longer. The longer a storage nodc must wait to be accessed, the greater will 10 be the image data corruption due to rcverse biased diode leakage currcnt.

When a large number of electrons collect in the emitter of BJT
104, said emitter will become sufficiently negativc with respect to the grounded base, so that the base-emitter diode will become forward biased.
15 Fo~ward biasing will result in current flow until thc numbcr of electrons ;s sufficiently reduced so that the diodc turns off. The result of this action is essentially to limit the amount of chargc availablc for rcad out. That is, the magnitude o the stored charge is c]amped. This limits the imagcr dynarnic range but minimizes blooming, which has becn a problcm for imagcrs of 20 older technology.

The usc of the vcrtical BJT structurc to remove excessive charge is a particular]y useful feature of the preferred embodiment. Without the use of the BJT, excess charge wou]d pass into the channel stop. Current 25 in the channel stop creates a voltage, the magnitude of which is a function of the current and the channel stop sheet resistivity. The vo]tage dcvc]opcd could possib]y forwarci bias some channel stop-emitter junction resulting in ima,6e data corruption. The BJT of the present invention conducts the excess char~e into the substratc.

wO 92/04734 ~ ~3 ~ ~ 3 ~ 5 PCr/US91/06422 Chargc transfer from storage node to ACT channel occurs whcn a conductivc pathway between them is created by action of thc h~lSFET
structure. Thc MISIET structurc providcs a mcans of controllab]y transfcrring chargc from thc storagc nodcs to thc Acr channcls.

Oncc thc chargc rcprescnting thc image data has bccn transfcrrecl to thc ACr channc] ;t is swcpt along by thc potcntial resu]ting frorn a sur~ace acoustic wave. The SAW forces the charge a]ong the channe]
to that part of the chip where further processing may take p]ace.
It is to be understood that the forms of the invention described and illustrated herein are to be taken as preferred e.l~amples of the same and that various changes in the arrangcmcnt of components or type of matcria]s may be made without departin~ from the spirit of the invention or scope of 15 the claims.

Claims (24)

WO 92/04734 -13- PCT/US9]/06422 AMENDED CLAIMS
[received by the International Bureau on 25 February 1992 (25.02.92);
original claims 1,3,6, and 21 amended; other claims unchanged (4 pages)]

1. An imager comprising:
a) an array of photodetector elements for producing photogenerated charge and b) a plurality of acoustic charge transport channels for read out of said photogenerated charge, coupled to said photodetector elements.

2. The imager of claim 1, wherein said photodetector comprises:
a) a photodiode; and b) a means for applying a reverse bias voltage to said photodiode.

3. The imager of claim 2, wherein said photodiode comprises a silicon based diode having an n type region and a p type region.

4. The imager of claim 1 further comprising an electrically insulating layer disposed between said photodetector elements and said acoustic charge transport channels.

5. The imager of claim 1 wherein said insulating layer is a material selected from the group consisting of phospho-silicate-glass, boro-phospho-silicate glass, ZnO
and BaTiO3.

6. The imager of claim 1, wherein said photodetector comprises:
a) an hydrogenated amorphous silicon based avalanche photodiode having a p type region, an intrinsic region and an n type region, said n type region having two major surfaces, the first major surface of which is in electrical contact with said intrinsic region;

b) a transparent electrode in electrical contact with said p type region; and c) a light shield having two major surfaces, one surface of which faces the second major surface of said n type region.

7. The imager of claim 6, wherein said transparent electrode comprises a layer of indium-tin-oxide.

8. The imager of claim 6, wherein said light shield is comprised of a layer of molybdenum.

9. The imager of claim 6, wherein said light shield has an opening to permit electrical connection to be made to said polycrystalline silicon region.

10. The imager of claim 6, wherein said p type region of said photodiode comprises a-Si1-xCx:(H) where O<x<1.

11. The imager of claim 6, wherein said intrinsic region of said photodiode comprises a-Si:(H).

12. The imager of claim 6, wherein said n type region of said photodiode comprises n type polysilicon.

13. The imager of claim 1, wherein said array of photodetector elements is a two dimensional array.

14. The imager of claim 1, wherein said photodetector elements are responsive to electromagnetic radiation in the visible region of the spectrum.

15. The imager of claim 1, further comprising a means for storing charge, which is physically and electrically associated with each of said photodetector elements.

16. An imager as claimed 15, wherein said means for storing charge is comprised of an emitter node of an npn BJT.

17. The imager of claim 15 further comprising a means for controllably transferring charge coupled between said ACT
channel and said means for storing charge.

18. The imager of claim 17, wherein said means for controllably transferring charge is a MISFET structure.

19. The imaqer of claim 16, wherein said BJT acts to clamp the amount of charge stored at said emitter node.

20. The imager of claim 1, wherein said acoustic charge transport channel comprises a GaAs region surrounded by an AlGaAs region.

21. A semiconductor heterostructure camera element cell comprising:
a) an n type GaAs substrate;
b) a three terminal npn bipolar junction transistor having an emitter, base and collector wherein said GaAs substrate forms the collector;
c) an amorphous silicon avalanche photodiode having a p type region, an intrinsic region, and an n type region, said n type region comprising a polycrystalline silicon plate electrically connected to the intrinsic region of said photodiode;
d) a polycrystalline silicon pillar disposed between and electrically connecting said polycrystalline silicon plate and said emitter of said bipolar junction transistor;
e) an acoustic charge transport channel in said GaAs substrate;

f) a MISFET structure having a gate electrode, an insulating layer and a semiconducting layer, for transferring charge stored in said emitter to said acoustic charge transport channel; and g) a p+ channel stop abutting said bipolar junction transistor and electrically connected to said base.

22. The semiconductor heterostructure camera element cell of claim 21, wherein said insulating layer of said MISFET
structure is a material selected from the group consisting of Si3N4 and undoped GaAs.

23. A method of generating a serialized stream of analog charge packets suitable for MDTV image construction comprising the steps of:
a) converting electromagnetic image information into a two dimensional array of discrete analog charge packets;
b) storing said two dimensional array of analog charge packets;
c) clamping the magnitude of said charge packets;
and d) accessing said charge packets by means of acoustic charge transport.

24. The method of claim 23, wherein the step of accessing further comprises the steps of:
a) performing a parallel transfer of all charge packets in a row, by means of acoustic charge transport, to an intermediate storage row; and b) transferring serially each charge packet in said intermediate storage row by means of acoustic charge transport to circuitry for further processing.