New materials for large-area position-sensitive detectors

b;

ELSEVIER

Sensors and ActuatorsA 68 ( 1998) 244-248

PHYSICAL

New materials for large-area position-sensitive detectors Elvira Fortunate * , Rodrigo Martins Department of Materials Science, FCT- UNL rind Centre of Excellence for Microelectronic and Optoelectronic Processes, UNINOVA, Quinta da Ton-e, P-2825 Monte de Caparica, Portuga:al

Abstract Large-areathin-film position-sensitive detectors(TF’PSDs)using the hydrogenatedamorphoussilicon (a-Si:H) technology arepresented. The detection accuracyof thesedevices (lengths of about 80 mm) is betterthan + 0.5% of the value of the full scaleof the sensor,the spatial resolution is better than f 20 &m, the non-linearities measuredare below -i: 2% and the frequency responseis in the rangeof a few kilohertz, compatible with the sampling frequency of most electromechanicalassembling/control systems. The obtained results are quite promising 0 1998 Elsevier Science S.A. All rights regarding the application of these sensors to a wide variety of optical inspection systems. reserved. Keywords: Position sensors; Thin films;

Amorphoussilicon;Large-area devices

1. Introduction The use of optical detectors to determine the position or alig;nment of objects has increased since they are able to

perform measurements in a non-contact manner, with a high precision. This characteristic makes them the core of development in several fields, since they are simple and convenient to use and so are preferred to charge-coupled devices in applications where the device response frequency is limited to a few kilohertz and measurements are performed continuously. These conditions apply to a wide variety of systems, like: machine-tool alignment and control; angle measuring; rotation monitoring; surface profiling; medical instrumentation; targeting; remote optical alignment; guidance systems; etc., for which automated inspection control is needed. Recently, it was suggested that p-i-n a-Si:H devices could be used to produce thin-film position-sensitive detectors (TFPSDs) [ l] for such applications, since they combine the possibility of being produced in large areas, at low cost and with the required precision. In this work we report the reason why a-Si:H is used in this application, referring in particular to results achieved in large-area one-dimensional TFPSDs with an excellent non-linearity and a high spatial resolution. 2. Why a-Si:H? Nowadays the majority of the currently developed largesize optical sensors use a-Si:H as the thin-film photo* Corresponding author. Tel.: +351-l-294-8564; Fax: +351-l-295-7810; E-mail:[email protected] 0924-4247/98/$19.00 0 1998EisevierScienceS.A. Al1 rightsreserved. PII 50924-4247(

98)00012-O

conductor [2] in applications such as solar cells, facsimile machines, displays and, more recently, medical X-ray systems [ 31. The growing interest in a-Si:H is because it is a semiconductor which can be deposited onto large-area glass or flexible substrates at low cost, and patterned into complex electronic devices. This material is often discussed in the popular press in terms of its potential for being spun on, into rug-like soft display screens. The a-Si:H base device also has a unique property: it does not require an electrically common substrate, as is the case, for instance, with crystalline silicon (c-Si) . In simplified terms, we can paint the ‘stuff onto large surface areas. To understand the distinctive role of a-Si:H, a comparison is made with other candidates for use as thin-film photoconductors in Table 1 [4]. This comparison is guided by the requirements for large-area optical sensors. Although Table 1 lists only some of the most important properties, the superiority of a-Si:K is clearly seen. The main arguments in favour of a-Si:H are the simplicity of manufacturing very uniform thin films in large areas, the relatively low darkcurrent properties, the high sensitivity and its suitable response speed (not high due to the low electron mobility, but sufficient for these applications). The uniformity of the material is an essential requirement for application on large-area optical sensors and it is ensured by the a-Si:H technology { 51. Heterogeneities, which occur in polycrystalline material, for example, may be avoided by using proper deposition parameters and systems. An overview of the significant advantages and also some of the present problems of a-Si:H is given in Table 2 161.

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Table 1 Comparison of thin-film photoconductors Chalcogenides

CdS-CdSe

Poly-Si

a-Si:H

Photosensitivity

good (field dependent) possible slow ( - ms)
low (limited by grain boundary) maximum in the red

high (kr-

Sensitive to visible light Response speed Carrier diffusion (crosstalk) Non-injecting contacts Homogeneity Simplicity of manufacture (low-cost substrate) Compatible with crystalline semiconductor technology

good (controlled by grain boundary) yes slow (5 ms) -lp,m

yes granular (100 nm) no (deposition at 630°C)

yes amorphous

yes

possible granular ( - km) no (annealing at 500°C)

no

no

yes

yes

possible amorphous

100 nm (crystallite site)

10m6cm* V-‘)

yes fast ( - ps) <1 (*m

yes

Table 2 Advantages and disadvantages of a-Si:H Advantages

Disadvantages

l Large-area fabrication 0 Low-temperature process 0 Multilayer structures l Compatible with microelectronics technology 0 Photoconduction (similar to c-Si) 0 Dopability (similar to c-Si) l Interfaces blocking or ohmic (similar to c-Si) l Light sensitivity in the visible region l Optical absorption greater than with c-Si l Lateral carrier diffusion smaller than c-Si 0 Good reproducibility

Low deposition rate Small drift mobility Schubweg (drift length) much smaller than the diffusion length of c-Si Light degradation that conditions the absolute values measured response time conditioned by the size of the sensor + the mobility of a-Si:H

The great interest in a-Si:H is due to its unique combination of a single fabrication process (the elementscan be mixed andmatchedin arbitrary proportions, and different layerscan be depositedon eachother without the constraintsof a crystalline structure) with an almost complete set of the most important semiconductor properties. The deposition (and doping, if necessary) of a-Si:H thin films is performed by a single-stepprocessat a temperaturebetween200 and 250°C in a plasmareactor.No subsequenttreatmentsuchasannealing or implanting is required.

3. Fabrication

gas-phaseconcentrationand a depositionrate of 2 A s- ‘. The i-layer was grown using pure silane with a deposition rate of 1.7 A s- ‘. The overall thickness of the final device ranges from 400 to 800 nm, wherethe dopedn-and p-layers areonly 10 and 100 nm thick, respectively. The ohmic contacts are formed by the indium tin oxide layer, underneaththe p-layer, and by a thin resistive aluminium layer on top of the n-layer that form the required device equipotentials for carrier collection.

steps of TFPSDs

The reportedTFPSDshave sizesup 80 mm X 2.5 mm and a p-i-n structure producedby the plasma-enhancedchemical vapour deposition (PECVD) technique [ 71, deposited on glass substratescovered with indium tin oxide with a sheet resistanceof 20 Wsq. and a transmittanceof 80%. During the deposition of the p-i-n structurethe temperaturewasfixed to 210°C and the deposition pressurekept at 600 mtorr [ 81. For the p-layer we have useddiboraneasthe doping gaswith a 0.6% gas-phaseconcentration and a deposition rate of 1.8 A s- ’ . For the n-layer we have used phosphine with 1.6%

Top view

electrode

~..~~~~~~:~~~~~~~

Fig. 1, Structure of the TFPSD, showing top and sectional views.

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Finally, a co-evaporationof Al and Ag is realized to produce the edge contacts,located at both endsof the tin oxide layer, as shown in Fig. 1. For the fabrication of the TFPSD mentionedaboveit is necessaryto usethreephotolithography patterning steps,

4. Results and discussion In Fig. 2 we show the dependenceof the short-circuit photocurrent on the light illumination intensity. The slope of the power-law dependenceon the light intensity and thephotocurrent was found to be 0.988. This value refers to monomolecular recombination kinetics and it is usually found in intrinsic a-Si:H sampleswith the Fermi level nearthe midgap 191.

The data depicted in Fig. 2 show that for light intensities below 2.5 X 10B6W cm-*, the detectedphotocurrent starts being limited by the backgroundreversedark current,responsible for the non-linearity behaviour observedand allowing the noise equivalent power (NEP) for this specific device to

lo-3 ’

detection

limited by NEP

and Actuators A 68 (1998) 244-248

be determined. From this result, we observe that the dark current is a very important featurein the device performance since it influences the sensoroperation in static and dynamic ranges. Fig. 3 shows a typical spectralresponse( Rh) for thin and thick i-layer p-i-n devices,respectively. The data reveal that both devicesmatch well with the spectralrangeof the visible light, exhibiting a maximum at h = 610 nm that almost correspondsto the maximum quantumefficiency of the i-layer. Therefore, good sensitivity and thus signal-to-noise ratio are achievedif a light-emitting diode or laser emitting at a wavelength close the one reportedaboveis used as a light source; Overall, the data show that RA decreasesas h increases beyond 610 nm, due to an enhancementof the optical losses in the red region of the spectrum.On the other hand, as A decreasestowards the blue region of the spectrum, the decreasein RA canbe attributedto: reflection lossesandsmall absorptionon the transparentconductiveoxide (TCO) layer; absorption at the p-layer and possibleback diffusion of carriers generatednear the top of the i-layer; electron-hole pairs generatedat the i-layer that are not collected. Fig. 4 shows the linearity measurementsperformed in a TFPSD (L = 80 mm and W= 2.5 mm) with the back metal contactfloating andtheoutputsignaltakenfrom the two metal electrodeslocatedat the deviceedges,using a light beamspot diameterof about 1mm.The datashow a good fitting between the recordeddata and the spatialposition of the light spot on the device surface,agreeingwell with the model proposedby Martins and Fortunato [ lo]. r

-

y = 2.57 * ~“10.988331 -

y = 4. I688e.00

10

R= 0.99699

F

/I

Nonlinearity = 2%

l*

+ 2.755 I x R= 0.99704

Fig. 2. Photocurrent of a p-i-n TFFSD as a function of light intensity. The equations for the power-law (solid line) and linear (dashed line) dependencies are also indicated. -

0.4

.

-10 0 10 20 30 40 ~ Scanned position (mm) Fig. 4. An example of the measurement results of 1D large-area TFFSD non-linearity, using an HeNe laser beam with a diameter of 1 mm. The linear fitting obtained from the experimental points is also shown. -40

0.35

$

y = -0,40X9 +0,3707x R= 0,99933

II

0.3

-30

-20

Table 3 Position detection resolution

400

450

500

550 600 650 700 750 Wavelength (nm) Fig. 3. Spectral response of two TFFSDs with different i-layer thicknesses. All the other parameters have been kept constant.

SteP

Distance scanned

Standard deviation

Non-linearity

1pm1

1I*@

[@I

[%I

80 40 20 10

4000 2000 1000 500

0.297 0.173 0.144 1.249

0.98 0.98 2.60 15.01

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Table 4 Characteristics presented by a-Si:H TFPSDs and c-Si PSDs Characteristics

Amorphous silicon TFPSD

Crystalline PSD ( S 1869 Hamamatsu)

Detecting range [cm] Detecting accuracy Non-linearity Spatial resolution (mm) Wavelength at peak sensitivity [ nm] Sensitivity [A W- ‘1 Frequency response [kHz] Transparency

10x10 i 0.5% F.S. +2% +20 550 0.3 >l more than 50%

1x1 +0.5% F.S. &-0.5% to &75% * 300 to 300 850 0.6 500 non transparent

Fig. 5. Photograph of some a-Si:H 1D and 2D duolateral TFPSDs with different areas, produced at FCT-UNL/CEMOP-UNINOVA, Portugal.

The device non-linearity (position detection error, a), is obtained taking into account that 1:111 6 = 2alF, where (Tis the r.m.s. (root meansquare) deviation from the regressionline datathat fit the experimentalpoints andF is the measured full scale. On the other hand, the position resolution is the minimum light-spot displacementthat can be detected,i.e., the limit of detection expressedby the distanceon the sensitive surface.In general,it dependsdirectly on the device area and noise of the measuring system and, inversely, on the detectedoutput signal. To determine the position resolution, several measurementswere performed using different steps and keeping the size of the light spot impinging on the device surfaceconstant.Table 3 showsthe different resultsobtained as a function of the stepsusedand of the scanneddistance. The data in Fig. 4 show that good position resolutions are obtained (6= &2.6%) for up to 20 p+rnsteps,above which the detection error increasesand a non-linear correlation is achievedbetween the light-spot position and the output signal. The non-linearity observed below 20 km steps (6> +3%) can be attributed either to limitations of the device or to the noise introduced by the hardwareusedin the measuring system. Here, it is important to notice that the diameter of the light spot used is about 1 mm, showing that the sensoris able to discriminate between two consecutive positions within the spot area. This result means that the

sensoroutput signal is mainly dependenton the position of the centroid of the light spot, showing also the advantageof the analoguedetection processwhen comparedwith the digital detection process,as is the casewhen CCDs are used. That is, TFPSDs are able to perform optical detection in an almost continuous way, while CCDs make it by a discrete process.This is really the relevancy andsuperiority presented by TFPSDsor other similar devices,which makethem quite suitable for use directly in inspection control systems,simplifying the required optical componentsand allowing a control in real time. For illustration, Fig. 5 show a collection of someexperimental a-Si:H 1D and 2D TFPSDs fabricated in the course of this development at FCT-UNL/CEMOP-UNINOVA, Portugal. 5. Conclusions

The devices developed by this group exhibit excellent linearity up to device sizesof 80 mm, opening a new field of applications where large-areaTFPSDs are required for unmanned control processes.This makesthis type of TFPSD competitive when comparedwith the conventional c-Si PSD. In Table 4, we presentsomeof the main characteristicsexhibited by a-Si:H TFPSD and c-Si PSD devices used in optical position-sensingapplications. In conclusion, we are extremely encouragedby the promising large-areaTFPSD associatedwith a low-cost technology due to the fact that this type of sensor could have a profound impact on the development of several industrial applications. References [ 1] E. Fortunato, G. Lavareda, M. Vieira, R. Martins, Thin film position sensitive detector based on amorphous silicon p-i-n diode, Rev. Sci. Instrum. 66 (1994) 2927. [2] R. Street, Semiconductor of distinction, Physics World 54 (Apr.) (1993). [3] M. Hoheisel, Amorphous silicon X-ray detectors, J. Non-Cryst. Solids ( 1997) in press.

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[41 K. Kempter, Large area electronics based on amorphous silicon, Adv. Solid State Phys. (Festkt)rperprobleme) 27 ( 1987) 279. [51 R. Martins, A. MaGarico, M. Vieira, I. Ferreira, E. Fortunato, Role of the deposition parameters on the uniformity of films produced by PECVD technique, Philos. Mag. B 76 (1997) 249. [61 K. Kempter, a-Si:H image sensor: some aspects of physics and performances, SPIE Proc. 617 ( 1986) 120. [71 R. Martins, I. Ferreira, N. Carvalho, L. Guimaraes, Engineering of plasma deposition systems used for producing large area a-Si:H devices, J. Non-Cryst. Solids 137-138 (1991) 757. [81 E. Fortunato, M. Vieira, G. Lavareda, L. Ferreira, R. Martins, Material properties, project design and performances of single and dual a-Si:H large area position sensitive detectors, J. Non-Cryst. Solids 164-166 ( 1993) 797. [91 H. Dersch, L. Schweitzer, J. Stuke, Recombination processes in a-Si:H: spin dependent photoconductivity, Phys. Rev. B 28 (1983) 4678. [lOI R. Martins, E. Fortunato, Interpretation of the static and dynamic characteristics of thin film position sensitive detectors based on a-Si:H p-i-n diodes, IEEE Trans. Electron Devices 43 ( 1996) 2143. 1111A. Kawasaki, M. Goto, On the position response of a position sensitive detector irradiated with multiple light beams, Sensors and Actuators AZl-A23 (1990) 534.

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Biographies

Ehim Fortunate wasborn in Portugalin 1964.Shegraduated in materials sciencefrom the New University of Lisbon. She receivedthe Ph.D. degreefrom the New University of Lisbon in 199.5,wheresheis adocentattheMaterials ScienceDepartment. Her researchinterestsdeal with the physics and technology of hydrogenated amorphoussilicon thin films and their application to optical and chemical sensors. Rodrigo Martins received a Ph.D. degree in amorphous semiconductorsin 1982.He is associateprofessorwith aggregation at the New University of Lisbon, and head of the Materials ScienceDepartmentof the New University of Lisbon and the Centre for Microefectronic and Optoelectronic Processes.He has worked in the field of amorphoussilicon, germanium and their alloys since 1975, initiating his career in this field at the University of Dundee, working with ProfessorsW. Spearand P.G.LeComber,the inventors of amorphous silicon material as a truly electronic material.