A novel junction surface acoustic wave detector in silicon

Sensors and Actuators, Atl-A23

(1990) 675-678

675

A Novel Junction Surface Acoustic Wave Detector J C HAARTSEN

in Silicon

and A VENEMA

Electneal Engmeermg Department, Delft Untverstty of Technology, Deft (The Netherlands)

Abstract

A new, surface acoustic wave detector m slhcon is presented Its basic operation consists of the modulation of a lateral inJectIon current flowmg between two p+ Junctions The AC behavlour of the detector can be analysed unth an eqmvalent clrcmt model, conslstmg of lumped network elements The active detector has been reahzed m a ZnO-SlO,-Si layered structure Expenments at 80 MHz demonstrate Its operation and agree well with theoretlcal results

As Hnth conventlonal SAW devices on plezoelecmc crystals, the acoustoelectncal conversion process m tis layered configuration can be accomplished with mterdlgtal tranducers (IDTs), consistmg of metal patterns placed on top of the ZnO layer or at the Interface between ZnO and SIO~ layer, see Fig 1 Metal plates below or above the mterd@al pattern may be added to enhance the couphng strength [1] However, m tis layered structure another type of transducer can be created by usmg conductive, mused or implanted electrodes m the mhcon, resultmg m Junction transducers

1. Introduction

2. Junction Detectors

The mtegratlon of surface-acoustic-wave (SAW) components m senuconductors IS qmte challengmg, because of the extended signal processmg capabihtles It offers To generate and detect acoustic waves in silicon, plezoelectnc overlays are used In this paper a ZnO-90,-Sl layered structure IS consIdered

Ddfused or Implanted patterns m the senuconductor have some major advantages over metal structures Thepnction pattern results m a smooth surface Thus wdl improve the growth of the ZnO layer, and reflections are reduced since there 1s hardly any mechamcal loadmg of the surface and topologcal dlscontmmties are avoided Unul now, only passive transducers have been considered Their basic operation consists of the mductton of charge at the conductive parts of the IDT Because of reclprmty these passive devices can both detect and generate waves In layered structures mvolvmg sermconductors active detectors can be made, m whch there is an interaction between the SAW and the mohle charge carriers 111the senuconductor Because the basic operation consists of a modulation of the earner flow by the SAW potential, these devices can only act as detectors, not as generators As a consequence, no regeneration effects occur which w111 cause dlstortmg reflections

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2 I Passwe Junction Detectors The general configuration of a passive Junction detector ISdepicted m Fig 2(a) The oxide layer IS used to pasavate the s&con surface but may be onutted To avoid any metal obstructlons m the SAW propagation path, the output contacts of the detector are placed at the ends of the electrodes 0 Elsevler Sequola/Prmted m The Netherlands

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W Fig 2 The (a) passive and (b) acuve Junction detector

For p = 112 where 1 IS the acoustic wavelength, alternate p+ regons have opposite polarity, which 1s smnlar to the configuration of Fig l(a) For optimal coupling, the regon around the p+ electrodes should be completely depleted, which can be accomphshed d the n- region consists of an epilayer [2] Another type of Junction detector results if p = Iz In this case the p+ electrodes form one side of the detector, while the n- region forms the other side The operation frequency 1s twce as large as m the previously discussed detector for the same hthographlcal constramts [3,4] The high conversion losses found m these devices were attnbuted to the resistance of the p+ electrodes and the large, inactive capacitances to ground 22

When the reversed dram voltage 1s increased, the dram depletion reBon will expand and eventually it will reach the source depletion regon For low doping levels of the n- regon and small gap lengths L, no breakdown will occur For voltages larger than the reach-through voltage V,, the gap repon IS completely depleted, and source and dram are separated by a potential barrier & as 1s shown m Fig 3 A lateral mJe&on current will flow from source to dram, conslstmg of thermally excited holes that can cross the bamer [5]

Active Junction Detector

In the active detectors there 1s a &rect mteractlon between the electnc fields of the SAW and the mobile charge tamers m the semiconductor Because the surface wave 1s concentrated at the surface, FET-like components are self-evident The new, active Junction detector 1s shown m Fig 2(b) The p+ electrodes are separated by a small gap L and alternately form the source and dram terminals of the detector

m which A* IS the Kchardson constant Due to the absence of a gate structure, the electnc fields of the travelhng SAW can penetrate mto the gap thereby modlfymg the potential datnbution As a consequence, the height of the potential bamer 1s modulated by the SAW potential 4sAW, which results m a modulation of the thermionic-injection current [6] Tl’us is analogous to the change of the emitter current m a bipolar transistor by the modulation of the enutter-base voltage, and indeed close slmllamles exist between the behavior of this detector and a bipolar translstor An effective transconductance can be dmved usmg eqn (1) a1 &=a=

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The sensltlvlty of the detection operation is hnearly proportional to the has current I& m fact, g,,, corresponds to the small signal conductance l/r, of a bipolar transistor To analyze the behavior of the barner-modulated detector (BMD) theoretically, an equivalent clrcmt model conslstmg of lumped network elements was developed Because the metal contacts to the detector are placed outside the propagation path, the source and dram electrodes have to be considered as dlstnbuted RC networks, see Fig 4 The resistor values are determmed by the sheet reslstlvlty of the p+ regions, while the capacitor values are determined by the depletion capacltance of the p+n- Junctions The small-signal current sources represent the current modulauon by the SAW potential 1 =g&,,w The feedback resistance rd m this small-signal eqmvalent model represents the dram-induced bamer modulation and 1s m first order inversely proportional to the bias current I,, [6]

az (rcl)-’ =-&=gmav,, Fig 3 Potential chstnbution at the BO,-SI source and dram after reach-through

(1)

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current as IS suggested by eqn (2) Using the clrcmt stimulation program SPICE, this model can be applied to predict the detector’s a c behavlour 3. Experhnent 3 I Design The basic test configuration consists of a delay hne using two conventional IDTs of the type shown m Fig l(a), placed 10 mm apart To obtam low conversion losses a thick oxide of 2 pm 1s present beneath these IDTs The penod of the IDTs was 40 pm, whuzh corresponds to a resonance frequency of about 80 MHz In the propagation path between the IDTs, single BMDs were placed, consisting of a smgle source and a single dram electrode To acheve a strong field penetration of the SAW rnto the depleted gap, a thm oxide layer of 0 I pm was used above these detectors The aperture of the mdlvldual detectors is 1 mm, while the gap length 1s 6 pm The Hrldth of the source and dram electrodes was made 40 ,um to prevent any passive detection 3 2 Technology The substrate matenal IS a p-type (100) wafer of 20 ohm cm On this wafer a 10 pm thick n-type epltaxlal layer of 10 ohm cm was grown After the epilayer a thick oxide of 2 pm was thermally grown In the area destined for the active detectors, the thick oxide was etched away and a thm oxide of 0 1 pm was thermally grown Through ths oxide source and dram electrodes were implanted using BF2+ ions at an energy of 150 keV and a dose of 5 x 10” m* After an anneal step of 1050 “C for 30 mm the resultmg p+ sheet resistance was about 30 ohm/ 0 Finally the metal interconnections and IDT patterns were made wth a 0 3 /rn thick alummum layer As a last process step the ZnO layer was fabncated Several layer thicknesses between 10 and 20 pm were fabncated

4. Results

In ths Section the a c measurement results of a single BMD conslstmg of one source and one dram electrode are presented For comparison the theoretical predictions obtained Hrlth the eqmvalent SPICE model are also gven Optimal coupling for the conventional metal IDTs at the 2 pm oxide IS theoretically achieved with 18 pm thick ZnO However, expenments demonstrated high losses due to the poor quality of these thck ZnO layers Better results were obtained wth the 10 pm thick ZnO layers Delay hnes unth these layers had an untuned (2, = 50 ohm) insetion loss of about 40 dB around 80 MHz Although 40 Junction electrodes were present between the IDTs formmg a SAW reflection gratmg, the delay hne response was quite smooth, which demonstrated the low reflection level of the Junction detectors The active detection operation was mvestigated by deternumng the untuned insertion loss at 80 mHz between a transrmttmg IDT and a single BMD as a fimction of the bias current I,, Usmg the conversion loss (CL) of the IDT, which resulted from the delay hne measurements, the CL of the BMD could be denved, see Fig 5(a) This data was normahzed and hneanzed to obtain the output signal, see Fig 5(b) For small bias currents, there 1s a steep nse of the output signal At larger current levels, the feedback resistance r, accounts for the reduced nse of the output signal At high current levels, space-charge effects occur the barner height & 1snot only determmed by the rmpunty level of the epdayer but also by the space charge of the i)ected holes The space-charge effect, whch can be snnulated by a resistance m senes Hrlthr, [7’J,explams the constant output nse for large currents Varymg the transnuttmg power between - 13 dBm and + 7 dBm revealed no sign&ant change m the CL of the active detector, whtch demonstrates the lmeanty between the SAW amphtude and detector output In ad&tion, no sign&ant dtierence was found when the opposite IDT was applied as transnutter Hence, the

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to mhomogeneltles m the high-ohnuc epilayer causing a non-umform V,, and to dispersion Although the parallel structures were designed for thick ZnO layers, thm layers of about 10 pm were eventually used to avoid the propagation losses For this layer thckness the SAW velocity above the thick and thm oxide parts &ffered by 5% As a result, the penods of the transnuttmg IDTs and the recelvmg parallel Junctions &d not match For the single BMDs tlus effect was unharmful, due to their wde bandwdth Although only p+ electrodes m an n--type layer have been discussed, it IS also possible to use n+ electrodes m a p--type layer However, m this case one has to take mto account the energy-band bending at the BO,-p- interface due to interface traps and tied oxide charge This band bending will cause an mverslon layer and will prevent the reach-through operation m the gap completely An extra boron implantation m the gap regon can compensate for the band-bending effects [B]

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(W Fig 5 (a) Conversion loss and (b) the normahzed output signal vs bms current for a smgle BMD opcratmg at 80 MHz and loaded with 50 ohm

senntlvlty 1s mdependent of the direction of the travelhng SAW and legitimates the parallel structure of Fig 2(b) The passive operation of the detector was mvestlgated by applymg the active detector as a transmitter and an IDT as a receiver The measured CL decreased with increasing I,-,due to the shunting operation of rd At I0 = 5 mA the CL of the passive operation was about 30 dB higher than the CL of the active operation 5.

Discussion

The IDT configuration of Fig l(a) in comblnation wth Junction detectors unavoidably mtroduces a step dlscontmmty m the oxide Thts step ~11 cause reflections and the generation of bulk waves By using the structure of Fig l(b) the step can be avoided In this case a metal field plate must be placed on top of the ZnO above the active detectors m order to maxlfnze the SAW potential at the silicon surface In the test structures Just discussed, parallel versions conslstmg of four single BMDs were included The mtergap spacing was 160 pm, wbch corresponds to four wavelengths Their operation was not very successful, which was attnbuted

6. c0nc1nsions

A new, active Junction detector for SAWS m slhcon has been presented Its operation IS based on the modulation of a thernuomc-inJection current by the SAW potential The expenmental results agree well with the theoretical predictions obtained with an equvalent cmxut model The construction of the detector results m a smooth surface, and IS fully IC compatible Due to the active operation of the detector, the conversion loss 1s lower than m passive Junction detectors and reflections due to regeneration are avoided References 1 A Venema and J J M Dekkers, Enhauccment of surfaceacoustic-wave piezoelectr~ couphng m three-layer substrates, IEEE Trans Mcrowave Theory Tech, 23 (1975) 765-761 2 A Venema and E A Wolsheuner, A SAW Juntion mterdlg1ta1 transducer m sd~con, Proc IEEE UItrasomcs Symp, 198.2,pp 46-465 3 H C Lm, N A Papamcolaou, J Acevedo and W Tansh, Senuconductor Junction surface wave transducer, Proc IEEE ulfraronlcs symp, 1973, pp 548-551 4 B T Khun-Yakub, The apphcatlon of ZnO on s&on to SAW devaxa, Ph D Drpsertatmn, Stanford, CA, 1975 5 J L Chu, G Pcrsky and S M Sac, Thernuomc mJcctlon and spacecharge-hm&d current m reach-through p+np+ structures, J Appl P/rys, 43(1972) 3510-3515 6 J C Haartsen and A Venema, The bamer-modulated tap a new SAW dctect~on method m sdlcon, Proc IEEE Cllfrasome Symp, 1988, pp 159-163 7 S M SIX., Phystcs of Semtconductor Lkvrces, Waley, New York, 2nd edn , 1981 8 J T Walton and F S Gouldmg, S&on radiation detectors with oxide charge state compsnsatlon, IEEE Tram Nucl

SIX , NS-34 (1987) 396-400