Laser-recrystallized polysilicon resistors for sensing and integrated circuits applications

Sensors

and Actuators,

4 (1983)

527 - 536

527

LASER-RECRYSTALLIZED POLYSILICON RESISTORS AND INTEGRATED CIRCUITS APPLICATIONS* J BINDER Stemens

FOR SENSING

and W HENNING

AG-UBB

Munchen,

Balanstr

73, D-8000

Muntch

90 (F R G )

E OBERMEIER Fraunhofer

Znstrtut

fur Festkorpertechnologze,

Paul Gerhard-Allee

42, D-8000

Munrch

60

(FRG) H SCHABER Stemens

AG-ZT

and D CUTTER Munchen,

Otto-Hahn-Ring

6, D 8000

Munzch

83 (F R G )

Abstract

The influence of gram boundaries on the electrical properties of laserrecrystalhzed polycrystallme silicon films with typical gram size 1 X 20 pm2 and dopmg concentration between lOI cmm3and 10” cmm3has been mvestlgated Sheet resistance and its temperature dependence are found to be very close to the value expected for the single crystal material over a wide range of doping concentrations Deviations occur at low doping and are related to the average gram size perpendicular to the current flow Standard deviation and annealing power dependence of sheet resistance R, are also strongly correlated to the influence of gram boundaries as measured by the devlatlon of R, from the ideal, single crystal value At dopmg concentrations >I0 ‘s cmP3, extremely linear resistors are reproducibly obtained Resistance ratios can be controlled to better than 1% and temperature coefflclents S10-4 per degree can be achieved by proper choice of doping concentration The pressure dependence of these polycrystalhne s&con films was also measured Very large gauge factors of about 55 for p-type samples are found The temperature coefficient of the gauge factors at doping concentrations of about 1018 cmm3 1s near the single crystal value of about -1 8 X 10e3 per degree As an apphcatlon example, a pressure sensor using laser-recrystalhzed polycrystallme slhcon as plezoreslstlve resistors on a thm slhcon diaphragm 1s introduced At a doping concentration of about 10” cmW3a temperaturecompensated output signal of about 2 X 10m4per degree can be achieved

*Based on a Paper presented May 31 June 3,1983 0250-6874/83/$3

00

at Sohd-State

Transducers

83, Delft,

0 Elsevler Sequola/Prmted

The Netherlands,

m The Netherlands

528

1 Introduction Smce the ploneermg work of Gat and Gibbons [ 11, recrystalllzatlon of polycrystallme s&con (poly-Sl) layers has been widely mvestlgated [ 2] As shown m many publlcatlons, sheet resistance, temperature coefflclent [ 3 - 5] and stram dependence [ 6, 7 ] can be controlled by varying the doping concentration or the annealing condltlons Thus, these recrystalhzed polycrystalline slhcon fdms have proved to be suitable for sensing and integrated circuits applications The laser recrystalllzatlon method has the advantage of very locahzed heatmg and results m polycrystallme layers with gram sizes of typically lOpm The question arises, to what extent are the propertles relevant for sensmg and integrated clrcults apphcatlons affected by the remammg gram boundaries m laser recrystalhzed poly-S1 resistors’ We mvestlgated this problem, usmg a c w argon laser for recrystalhzatlon of the layers The followmg quantltles have been measured (1) absolute value of sheet resistance (compared to its Ideal, single crystal value), its standard devlatlon, reproduclblllty and dependence on annealing condltlons, (11) temperature coefflclent of sheet resistance, (HI) strain dependence of the resistance, characterized by the gauge factors The measurements of sheet resistance and temperature coeffrclent were performed over a dopxng concentration range between 2 X 1016 cmw3 and 102’ cm-’ for n-type material, the gauge factors were measured for n-type as well as for p-type poly-S1 layers over a more limited range of doping concentrations

2. ExperImental Samples were prepared on 3 m, (lOO)-orlented slhcon substrates, which were thermaly oxidized A 0 5 pm thick polyslhcon layer was then deposited by a standard LPCVD process, doped by Ion lmplantatlon and photollthographlcally structured Prior to recrystalhzatlon, the polyslhcon was covered by a passlvatmg layer of S102 or S13N4, which also served as an antireflection coatmg and encapsulation during the recrystalllzatlon process Recrystalhzatlon of the poly-Sl was performed using a c w argon Ion laser [4] The typical gram structure of the recrystallized poly-SI 1s shown m Fig 1 The typlcal gram size m the recrystalhzed layers IS seen to be about 1 pm X 20 E.tm The electrical measurements were performed with resistor geometries with different length to width ratios For the gauge factor measurements, we used cantilever structures to evaluate the longltudmal and transverse plezoreslstlve effects

529

Fig 1 (X1400)

Gram

structure

of the recrystallized

layers

Laser scm

dn-ectlon left to right

We studled resistors which were perpendicular and parallel to a given laser scannmg dlrectlon The influence of laser scan direction could thus be mvestlgated with otherwise identical annealing parameters 3. Results and dlscusslon (A) Sheet reststance

The over-all behavlour of the sheet resistance of the laser recrystallized poly& layers as a function of doping concentration N 1s shown m Fig 2 for arsenic and phosphorus doped layers Included for comparison are some data for unlrradlated, furnace annealed poly-Sl obtamed from the same wafers, as well as published data [9] for fme gramed poly-Sl and the ideal curve expected for a single crystal film /lOI The sheet resistance RII of samples laser-scanned parallel to the dlrectlon of current flow closely follows the ideal mono-S1 curve (for phosphorus doping) over the whole range of doping concentrations The sheet resistance RI of perpendlculary scanned samples, however, 1s considerably higher than RIIat dopmg levels N 5 lOi cm-j It was venfled by Hall-effect measurements that the carrier concentrations m the paraIle1 and perpendicularly scanned samples were equal and close to the value calculated from the known implant dose and sample thickness The reason for the resistance arnsotropy 1s clearly the elongated shape of the poly-S1 grams In samples where the laser scannmg dlrectlon 1s perpendicular to the current flow, the density of gram boundarles IS about 20 times higher than m samples laser-scanned parallel to the dlrectlon of current flow In terms of the gram boundary trapping model [S, 11, 121, the reslstlvlty contrlbutlon due to the potential barmers depends on the gram size and thus on the density of gram boundarles along the current flow Due to the

1

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-

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mono-S I101 finegralnedpoly-51[g

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14L 12 -

Y

,' -1

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:

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Fig 2 Sheet crystal sAcon lmes mdlcate cients Fig

3

Standard

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1CP

I

10'3

I

10x

1

,&I

j

10"

1

I

10’5

N/Cfl

10’6

10'7

lo'*

10'9

10'0

N,cm ’

resistance us dopmg concentration for n-type samples Values for single and fme gramed poly-SI are gwen for comparison Dotted and dash dot mean values of RI{ and RI later used to calculate the temperature coeffl deviation

of sheet resistance

us dopmg

concentration

lowermg of the potential barriers with mcreasmg doping concentration, both curves RII(N) and RI(N) (Fig 2) become identical at iV > 10lg cm-3 We next discuss certain quantities of immediate technological interest, which are plotted m Frg 3 In this Figure standard devlatlons of the R values over wafers u-radiated at three different laser powers, PL = 5W, 6W and 7W, are plotted At low doping concentrations the laser power dependence and standard deviation of RII are relatively high, which may be explained by the increased influence of gram boundaries The laser power incident on the sample can easily be controlled to +O 3W m our system Therefore an overall reproduclblllty (within one wafer and from wafer to wafer) of 1 to 2% can be anbclpated for RII at the higher dopmg concentration, as seen m Fig 3 In summary, sheet resistance values of the laser recrystalhzed polysillcon layers closely approach the ideal, single crystal value Deviations from thxs value, standard deviation and dependence on laser power all increase as the influence of gram boundarles IS increased, either by reducmg the gram size (RI) or by lowermg the doping concentration (B) Temperature dependence The temperature dependence for all types of samples between

of the sheet resistance has been measured 20 “C and 120 “C and for some selected

531 I

i

i

i

i

i

i

i

i

i

i

i

r

1

n-type

I

I

I

n-type

I

I

-mono-St

1131

Laser - retrystalked

1 poly-SL

calcuIatlon (f=Ol) Rl

\__

02 2

_I

0

RI, -02 -04

6 1&m

3

-06 -08

O’!!

-40

! 0

!!!! 40

Fig 4 Temperature

!!! 80

120

! T/Y

dependence

!

1

-10

1

10'6

I

lon

I

w

I

I

10'9

10zo N/cme3

of RI1 and RI for three typlcal dopmg concentrations

Fig 5 Measured and calculated temperature coefflclents for n-type films as a function of dopmg concentration Measured data for arsenic and phosphorus doplng agree wlthm experimental error Values for single crystal sillcon are given for comparison

samples also between -40 ‘C and +120 ‘C usmg a temperature stabilized measuring cell Figure 4 shows the R,(T) curves between -40 “C and +120 “C for three different dopmg concentrations At the highest doping level (N = 2 X 1019 cmm3), RI(T) and RII(T) are almost parallel to each other In contrast, the temperature dependences of RI and RI! are different m the lower doped samples, and eventually RI has a negative slop while RII mcreases with increasing temperature above room temperature We define a temperature coefficient at room temperature for our samples as TCR = [R(4O “C) - R(20 “C)] /R(20 “C) The experimental data for n-type samples are plotted m Fig 5, together with the correspondmg single crystal values taken from the work of Bulbs et al [ 131 The most stnkmg feature of Fig 5 IS the close agreement of measured TCRs for the parallel scanned samples with the TCR of single crystal slhcon Only at N < 5 X 1017 cmV3 1s a slgmflcant devlatlon of the measured TCR from the single crystal curve observed due to the negative temperature dependence of the gram boundary resistance For perpendicularly scanned samples, the TCR 1s always negative at N < 1Ol9 cmm3 The TCR of nonrecrystallized samples with still smaller gram size IS always negative m the doping range mvestlgated here The large negative TCRs observed for RI at low concentrations are strongly dependent on the laser power used and thus on the gram size achieved In contrast, the TCRs show a reasonably low scatter and P,-dependence m all cases where they approach the single crystal values

532

From the apphcatlons point of view, it 1s most important that temperature coefflclents of the recrystalhzed poly-Sl layers can be varied over a relatively wide range and that Z’CRs close to zero can be achieved with good reproduclblhty

(C) Stram

dependence

(gauge factors)

We investigated the longltudmal and transverse gauge factors of these laser recrystalhzed poly-Sl resistors by a standard cantilever experiment for n-type and p-type samples Since p-type samples have been more thoroughly investigated m the literature than n-type samples, a comparison of measured gauge factors with single crystallme values IS shown m Fig 6 for p-type samples The single crystallme curve m Fig 6 1s based on data from ref 14 Data of fine gramed, furnace annealed poly-Sl are also included [ 151 Here we discuss only the longltudmal plezoreslstlve effect, since we have observed experlmentally that the longltudmal gauge factors are about 3 times higher than the transverse gauge factors over the whole dopmg range Due to the large gram sizes, relatively large gauge factors between 45 and 55 are achieved The average values of these gauge factors are about a factor of 1 5 larger than the values of fme gramed poly-Sl While a slight increase of gauge factor with decreasing doping concentration IS observed for RII, RI shows a decrease of gauge factor m the xnvestlgated dopmg range

Longitudmal Gauge FaCtor AR/R F

I

’

I

Ptvpa

mono SI [ill] Laserrecrystallrzed polySI -

Longltudmal Gauge Factor JRIR E

I

/

n iwe Laser-recrystallned potySI

- 50 -

=

R,,

-o--R, -o-R,

-40 -

phosphorus arsenic

+ -30 50 -20 -

- 10

01

10'7

70'8

10'9

N/Cd

+/

I

OL

4 10'7

1018

10'9 N cm 3

Fig 6 Measured gauge factors us doping concentration for p-type samples Dotted and dash-dot hnes lndlcate the mean values of RIIand RI Values for smgle crystal slhcon and fme gramed

poly-SI

are gwen

for comparison

Fig 7 Measured gauge factors us doping concentration dash-dot lmes mdlcate the mean values of RII and RI

for n-type

samples

Dotted

and

533

which 1s opposite to that of monocrystallme slhcon This decrease 1s caused by a lowering of the cmer mobility with mcreasmg influence of the potential barriers m the range of low dopmg This behavlour was observed even more drastically m n-type samples, as shown m Fig 7 In this Figure the longltudmal gauge factors of parallel lrradlated resistors (dotted lme) and perpendicular lrradlated resistors (dash-dotted lme) are plotted As m the case of single crystallme slhcon, the gauge factor of n-type poly-Sl 1s negative over the whole doping range Perpendicular lrradlated samples show a strong decrease of gauge factor below N = 10 I9 cmm3, which 1s exactly correlated with the devlatlon of the sheet resistance RI from the single crystallme curve, as shown m Fig 2 4. Apphcatlons Based on the experlmental results, a pressure sensor utlhzmg laser recrystalhzed poly-S1 resistors was developed The sensor design conslsted of four poly-S1 plezoreslstors arranged m a Wheatstone bridge, which 1s located on a thin rectangular diaphragm (see insert of Fig 8) The resistors were arranged on the diaphragm so that the longltudmal plezoreslstlve effect predominated The resistors were boron doped and recrystalhzed parallel to the given laser scannmg dlrectlon These condltlons were chosen m order to obtam the highest possible sensltlvlty, as expected from the gauge factor data presented The dlmenslons of the diaphragm were chosen for a pressure range up to 500 mbar Figure 8 shows the output voltage of this pressure sensor as a function of absolute pressure (vacuum reference) at 25 “C and 125 “C The dopmg concentration of the poly-Sl plezoreslstors was N = 2 X 10” cms3, the measured gauge factor at this doping concentration was about 50 The dashed hne m Fig 8 shows the calculated pressure sensltlvlty of a standard pressure sensor with implanted resrstors The same geometric design was used m both cases The calculation of the sensltlvlty of implanted resistors assumed a gauge factor of 85 for [lOO]-mono-&, non-hnearlty induced by the mechanics of the diaphragm was not considered In the linear region of the characterlstlc curves, the ratlo of the sensetlvltles 1s approximately equal to the ratio of the gauge factors of mono-S1 and laser recrystallized poly-S1 Up to 500 mbar, the non-hneanty of the characterlstlc curve 1s about 1% (relative to the output voltage at 500 mbar), thus this value 1s approximately equal to the non-hnearky of a standard mono-& pressure sensor In the nearly linear range of the characterlstlc curve, the temperature coefficient of the sensitivity 1s about -2 0 X lop3 per degree, which 1s equal to the value of mono-% Figure 9 shows an example of how a pressure sensor can be temperature compensated by suitable choice of the dopmg concentration At a doping concentration of N = lo*’ cm- 3, the temperature coefficient of the sensetlvlty 1s only -2 0 X low4 per degree

Output

Voltage Output Voltage

-

Laser recrystallned

---

mono SI [IN]

poly SI -

(calculated)

Laser recrystahzed

poly SI

60 -

a) Design (schematically)

”

200 Absolute

Pressure

(mbar)

600 400 Absolute Pressure (mbar)

800

1000

Fig 8 Characterlstlc curve5 (output voltage us absolute pressure) of a pressure sensor with poly-S: plezoreslstors measured at 25 “C and 125 “C The dopmg concentration of curve of a standard sensor the p type plezoreslstors IS N = 2 x lOI* cm -3 The calculated with implanted resistors 1s given for comparison (dotted line) Fig 9 Characterlstlc curves of a pressure 25 “C and 125 “C The dopmg concentration

sensor with poly-S1 plezoreslstors measured at of the p-type plezoreslstors IS N = 1020cm-3

Hence startmg with a design that has been optimized for maximum sensltlvlty, this modlflcatlon of doping concentration can lead to a reduction m the temperature dependence by a factor of 10, while reducing the sensitlvlty by a factor of only 1 6 5. Conclusions Despite the existence of gram boundaries m laser-recrystallized PolYslhcon films, resistors with essentially single crystal properties can be formed m those layers The temperature coefficient of resxstance can be chosen within certain hmlts and very low TCRs can be reahzed at medium doping levels of about a few times 1018 cma3 Devlatlons from single crystal values m the sheet resistance itself and its temperature coefficient are both caused by gram boundaries Undesvable properties such as high scatter m the electrical properties, strong annealing power dependence and non-lmearlty are also due to the remammg gram bouudarles m the films However, these effects can be made neglqqbly small for practical apphcatlons if the average gram size m the dlrectlon of current flow 1s large enough Srgnlflcant devlatlons from single crystal behavlour

535

have been detected m our samples for N 5 2 X 10 I’ cmv3 at an average gram size dG of about 20 E.trnand for N 5 5 X 10 l8 cmm3at dG = 1 pm In the sheet resistance range between 20 4-Zand at least 1 ka, however, precisely controlled resistors are readily obtained The resistors are well suited for mtegratlon and the fabrication steps are compatible with conventlonal MOS processmg In [ 161 a novel MOS electrically read-only memory (PROM) using a highly reslstlve polycrystalhne silicon resistor as a memory element 1s proposed In insulated-gate held-effect transistor integrated cmxuts, deposited polycrystallme slhcon 1s used as the gate electrode 1171 Low pressure CVD polyslhcon films have been used for making reststors, lateral diodes and bipolar poly-mono transistors [ 181 Control of the resistance ratio to better than 1% 1s achieved and makes these resistors especially attractive for sensing apphcatlons and analog integrated clrcults Electrical lsolatlon from the substrate IS pruvlded by a dlelectrlc layer rather than by the pn-Junction lsolatlon used for conventional integrated resistors Substrate leakage currents are thus negligible even m high temperature apphcatlons Due to the large gauge factors of laser recrystallized poly-Sl, pressure sensors with plezoreslstlve poly-S1 resistors can be fabncated By a proper choice of doping concentration a nearly complete compensation of the temperature dependence can be achieved Such sensors could also be used over a wider temperature range than conventional sensors, due to the lack of pnJunctions This opens the posslblhty of a new generatlon of low cost sensors whxh are characterized by a simple technology, and are suitable for the consumer market References 1 A Gat and J F Gibbons, A laser scannmg apparatus for annealing of ionimplantation damage m semiconductor, Appl Phys Lett , 32 (1978) 142 2 J F Gibbons, m J F Gibbons, L D Hess and T W Slgmon (eds ), Laser and New York, Electron Beam Sohd Interacttons and Materials Processrng, North-Holland, 1981,p 449 3 E Obermeler and H Relchl, Polykrxtallme Slhzlumschlchten als Basismaterial fur Sensoren, NTG-Fachbenchte, Bd 79 (1982) 49 4 H Schaber, D Cutter, J Binder and E Obermeler, Laser-recrystallized polycrystallme slhcon resistors for integrated circuit apphcatlons, J Appl Phys , 54 (8) (1983) 4693 5 Y Onuma and K Seklya, Plezoreslstlve propertles of polycrystallme shicon thin film,

Jap J Appl Phys, II (1) (1972) 20 6 J Y W Seto, Plezoreslstlve properties of polycrystalhne &con, J Appl Phys , 47 (11) (1976) 4780 7 W Germer and W Todt, Low cost pressure/force transducers with slhcon thin film 4 (1983) 183 strain gauges, Sensors and Actuators, 8 G Baccaram, B Rlcco and G Spaduu, Transport properties of polycrystallme &con films, J Appl Phys , 49 (1978) 5565 9 M E Roulet, M Dutolt, W Luthy and K Affolter, Electrical propertles of CW-laserannealed ion-implanted polycrystallme slhcon, Appl Phys Lett , 37 (1980) 737 10 See, e g , S M Sze, Physws of Semrconductor Devices, Wiley, New York, 1969, p 43

536 J Y W Seto, The electrical propertles of polycrystaIhne &con films, J Appl Phys , 46 (1975) 5247 C Y Lu and J D Memdl, Modellmg and optlmlzation of 1.2 N C C Lu, L Gerzberg, monohthtc polycrystallme slllcon resistors, IEEE Tram Electron Dev , ED 28 (1981) 818 13 W M Bulbs, F H Brewer, C D Koldstad and L J Swartzendruber, Temperature coefflclent of reslstlvity of slhcon and germanium near room temperature, Solid State Electronrcs, I1 (1968) 639 14 I-I W Keller, Plezoreslstlve Druckaufnehmer, Messen Prufen, 89 (1974) 15 E Obermeler, Polyslllcon thm films for sensor apphcatlons, ESSDERC 82, Mumch 1982 16 M Tammoto, J Murota, Y Ohmorl and N Jeda, A novel MOS PROM usmg a highly resistive poly-Sl reslstor, IEEE Tram Electron Rev, ED 27 (3) (1980) 517 and T Klem, Slhcon gate technology, Solrd State Electronm, 13 (1970) 17 F Fuggm 1125 Polycrystalhne devices m bipolar IC-technology, 18 H C de Graff and J G de Groot, IEDM 1980 11