“Large area position sensitive detector based on amorphous silicon technology”

Large area position sensitive detector based on amorphous silicon technology

Conference Paper · January 1993

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Carlos NUNES Carvalho

New University of Lisbon

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LARGE AREA POSITION SENSITIVE DETECTOR BASED ON

AMORPHOUS SILICON TECHNOLOGY

E. Fortunato, M. Vieira, L.Ferreira", C. N. Carvalho, G. Lavareda and R. Martins

Dept. Science Materiais, Faculty of Science and Technology, New University of Lisbon

2825 Monte de Caparica, Portugal

*EID, Optoelectronic Dept., Quinta dos Medronheiros, 2825 Monte de Caparica, Portugal

ABSTRACT

We have developed a rectangular dual-axis large area Position Sensitive Detector (PSD), with

5 em x 5 em detection arca, based on PIN hydrogenated amorphous silicon (a-Si:H)

technology, produced by Plasma Enhanced Chemical Vapor Deposition (PECVO). The metal

.eontacts are located in the four edges of the detected area, two of them located on the back side

of the ITO/PIN/Al structure and the others two loeated in the front side. The key factors of the

detectors resolution and linearity are the thickness uniformity of the different layers, the

geometry and the contacts location. Besides that, edge effects on the sensor's comer disturb the

linearity of the detector. In this paper we present results concerning the linearity of the detector

as well as its optoelectronic characteristics and the role of the i-Iayer thickness on the final sensor performances.

INTRODUCTION

Up to now, crystalline silicon have been used to produce PSDs. Nevertheless its detection area

is small (around 1 x 1 crn-'), whieh implies the need of expensive and eomplicated optical

magnification systems for supporting their field of applications towards large area inspection

systems. Amorphous silicon based devices is now well established as a viable low-cost

technology for a variety of large area applications, such as solar eells [I], image sensors [2], flat

panel displays [3], ete .. These deviees take advantages of certain amorphous silieon film

properties such as: low temperature proeessing capability; high photosensitivity; short response

time; therrnal stability and high production yield. The application of a-Si.H to PSDs have already

been used for a digitezer, an image transfer system [4] and for a telephone terminal [5].

In this paper we report results eoneerning the application of a-Si:H to rectangular dual-axis

position sensitive detectors and its correlation with film properties. The obtained results are quite

promising making possible the applieation of these sensors to a wide variety of optical inspection

systcms such as: machine too I alignrnent and control; angle rneasuring; rotation monitoring;

'surface profiling; medical instrumentation; targeting; remote optical alignment; guidance systems;

etc., to which inspection autornated control is needed.

OPERATING PRINCIPLE

The PSO is an optoelectronic sensor that provides continuos position data of a light spot

traveling over its surface. Compared to the discrete element detectors such as ecos (Charge

Coupled Devices), the PSD features high position resolution, fast response speed, and simple

operating circuits. Thus, when a light spot falls on the PSD, an electric charge proportional to

the light energy is generated at the incident position. This e1ectric charge is driven through the

resistive layer and the photocurrent collected by an electrode is inversely proportional to the

distance between lhe incident position and the electrode. So, it is possible to obtain the following

formulas for the photocurrents Ilx, 12xand IIy' 12 collected by the four e1ectrodes, as a function

ofthe electrodes interdistanee (2L) and of the to ta) photocurrent related to each direction (IOx and

loy), as it is shown in figure 1.

DEVICE FABRICATION

The main steps used for the fabrication of large area PSDs are summarized in figure 2, and they

consist basieally on three deposition steps (production of the ITO layer and metal contacts, by

evaporation techniques and, the semiconductor structure, by PECVD), associated with 4 photolithographic patterning processes.

L I I I I I I I ~I yB 12)' =/0)'-2L 11 I xB I x •••,Ii(E--- ....~••••, I I I I I , I Photocurrent :

~'""".,.,.

electroderesistance layer semiconductor layer

When the end of the PSD is set at the original point

:

... 2L-xB IIx = /0%---2L xB 12%

=

10%-2L . 2L - yB I.y =lo}·--' --2L L Photocurrent IOx

=

Ilx+ 12x

Figure 1 - Cross section of a one dirnensional PSD view.

Deposition

Substrate Cleaning

•••P_h_o_to_li_to_gr_a_p_h.•.y•••.••••_E_t_c_hi_ng__

II

ü_·f1_-0_ff__ Figure 2 - Technological processes for the fabrication of large area PSD.

The cross sectional structure of the a-Si:H PSD used in these work is shown in figure 3. The

déposition procedures are as follows. First, a transparent electrode is made using indium tin

oxide (ITO) as the bottom electrode on a glass substrate, by reactive thermal evaporation [6].

Second, the p (a-SiC:H)-type, i-type and n-type semiconductor layers are continuously

depositedby Plasma Enhanced Chemical Vapor Deposition (PECVD) technique [7].Then, a thin

layer of aluminum is evaporated by electron gun, as the upper electrode. Finally a co-evaporation

of aluminum and silver at both sides of the ITO and aluminum electrodes have been deposited.

In order to see the influence of the i-Iayer thickness we have produced two series of sensors, one

with 4700

A

and the other 7000

A

thickness, respectively.

OPTOELECTRONIC CHARACTERISTICS

The I-V characteristics for the position sensitive detectors are shownin Figures 5 and 6 for different white light intensities (G). These measurements have been carried out using an ELH

dichroic lamp and the light intensity was changed from 20 lux to 500 luxoWe would like to point

out that even for illumination levels below 20 lux the PSD still can be used. The dependence of the photocurrent on the light intensity, for the two PSDs with different i-layer thicknesses are

sbown in figure 6. There, we observe an almost linear dependence (Iph cc GO.98) for the device

with the high thickness while the thinner one shows a sublinear dependence (Iph oc GO,81). Such

behavior can be explained by the increase of the optical and recombination losses [8] on the

--thinner device, leading to the conclusion that optimized PSD devices should used thick i-layers in arder to enhance the optical absorbance, allowing so a better device sensibility.

electrode glass

ITO layer

~~,

~

,

,~

~

.

,

~

,

,~,~,

~

.

,

~.

'~'~!~

'

~~i'

.!It-

conductive layer

CL '~ semiconductor layer

Figure 3 - Cross sectional structure of large area a-Si:H PSD.

1

.

1

1

.

1

1.1

,.

1.1

'Ó'Ó»

'-1.1

Figure 4. Optical photograph of the fabricated large area PSD,

40 35 500lux 30 ~_::t 25 300lux '-' E 20 1) t: 15

a

10 200lux 5 100 lux

o

0.0 0.1 0.2 0.3 Voltage (V) 0.4 0.5

Figure 5 - IV characteristics of the a-Si:H PSD in the

forward bias region at different illumination

intensities. 10.4r---,--...,...,,..,.,...,=--:-:-:~===-=_{:::::::::: :!::::::::: ..._.._.....::::::::.:.... ::::::::::::::~:::::;:::;:::~:~::::::} :::::::::::::::::::,: i-layer ,:n:;:::::::::::~::::::t:::;:::::::,:r:i:;: T~,),~:::::::::::r:::::~:::::):. .::~:r1 T 4700 À ~~., ~... ..: ; :.;.. :.:. • 7000 ~;

H+

·

.

.

1 ;

+.

.

:

.

H

.

H

.

102 Intensuy (lux)

Figure 6 - Photoresponse of a-Si.H PSD as a

function of light intensity for two PSD with

The photo-to dark I-V PSD characteristics are shown in figure 7, where we observe a ratio

higher than 103between the photo to dark current ratio, for the highest photon flux used, for the

PSD with the thicker i-Iayer. The S/N ratio was measured under reverse bias, and we have

obtained a value between 60 dB and 53 dB for a reverse bias of -0.5 and -1.0 V respectively.

This change in the S/N ratio indicates a non optimize shunt resistance of the device developed,

attributed tothe quality of the P/I and I/N junctions and to the resistance of thedoped layers.

10-1 !O-.~ ...10-5

.

<

E

10-7 ~

810

-

9 IDhn'" ~o-

.

0

~o-o- '0-0 0-0·6o·o-o-o-L'-Q I

.

I

•

•

Dark

•

._

...

_'.

••••

.. 1OJ.1 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 Voltage (V)

Figure 7 - Reverse bias charucteristicx 01"a-Si.H PSD, using ani-Iayer 7000 Â thickness. 0.4 ,.---,---, ~ '.; 0.3

:

~

OVo ~ 0.2 .... CI) i-layer __ 70(KIÃ --=-~7(XI Ã ~ t>_{~ 0.1} CI) o.o~~~-'-~~'-'-~~-'--'~~~ 300 400 500 60() Wavelenght (nm) 70()

Figure ~- Spectrul sensitivity of lhe PSD for two different i-layer thickness. Allthe others paramett:rs of lhe device structure have been kept constant.

The PSD spectral sensitivity was measured in the range frorn 300 to 700 nm light wavelength,

À. for athin and athick i-layer devices, as shown in figure 8. The light intensity was measured

by a calibrated sensor. The data show, that both devices have a maximum for À = 610 nm,

which corresponds almost to the maximum quantum efficiency of the i-layer (the thin device has

a maximum slightly shift towards lower wavelength), with different sensitivities, which agree

with the behavior also observed in figure 6). Overall, we observe that the spectral sensitivity

decreases for increasing À beyond 610 nm, due to an enhancement of the optical losses in the

red region of the spectrum. On the other hand, as we decrease À towards the blue region of the

spectrum, the decrease on the spectral sensitivity can be attributed to: reflection losses and some

small absorption in the ITO layer; some absorption at the p-layer and possible some back

diffusion of carriers generated near the top of the i-layer and electron hole pairs generated at the i-layer, which are not collected. Similar results on pin photodiodes have been reported elsewhere

[9].

DEVICE LINEARITY

The experimental setup for measuring the position response of the PSD consists of a 5 m W

He-Ne laser (632.8 nm), and a xy table. The ali set-up is mounted on a heavy table for improving

the systems stability. The light is beamed to the sensor mounted on a xy table, using a mirror, in

such a way that the contacts strips are perpendicular to the axis.

In figure 9 is depicted the relationship between the total photocurrent and the position on both

axis (XX, YY). In general, we observe the following relationship: Iph

=

168.81 +22.74X and

Iph

=

168.76 + 18.23Y. The observed changes in the slopes are due to.the differences in the

resistances of the thin AI and ITO layers.

In figure 10 we show the spatial dependence of the photocurrent (!-lA) on both directions. FTOm

the obtained results we observe alinearization better than ~ 2.0%, in the XX axis direction and

better than :t3.0% in YY axis direction, for all sensitive area, excIuding the edges. These

deviations are related to films non uniformity, not only due to the layers in which the PIN

structure is based but also due to changes inthickness overall surface, as it is c1early indicate by

resolution of the device that is better than

5

0

11m(dimensions of the laser spot), indicating the

possibility to use such type of detectors in highly accurate inspection tools.

300 280

--

-

260 ~ ::t 240 '-'

c

_~ 220 •... •... ::::l U 200 o Õ .c 180 ~ 160 140 : : ~ :

:

: . ~ _{. .} ' -···i···~···

6

-.

:.

.

:

:

.

. .

.

. . ._._.- _..~ ~..•..,.:_ . ••··0·-••-•••••••• ~•••••••••• -•••••••••• ~... °.00.0••••••-1 o••••••• ~••••••••••• -•••••••• _ ; _ _ ~ _..

o

1 2 3 4 5 Distance (em)

Figure: 9 - Relationship between detected phorocurrent and real distance along lhe:surface of a large area PSD.

Figure: 10 - Spatial dependence on lhe:phorocurrent inhoth directions.

CONCLUSIONS

We have developed a large area position sensitive detectar with a detection are a of 5 x 5 em, able to detect, in a continuos manner, the optical signal supplied by a laser beam, with the maximum

of resolution (Iaser beam spot dimensions). As the PSD does not use integrated cells the

resolution can be very high since it is not limited by cell size as in the case of image detector or

CCDs. The output currents are a linear functions of the incident light distribution even for low

ilIumination levels.

ACKNOWLEDGMENTS

The authors want to tank ali those who have participated in the development of the large area

PSD at New University of Lisbon, CEMOP-UNINOVA, EID, SMP and SISTEL and specially

to: M. Sanematsu, F. Soares, J. Fidalgo, U. Sousa, A. Campos, S. Soutinho and F. Godinho

for their technical help. Part of this work have been supported by Programa PRODEP/SEAD,

REFERENCES

[1] Y. Hamakawa, "Recent Progress of Amorphous Silicon Solar Cell Technology", Proc. 6~

IPVSEC (1992) pp. 3-10.

[2] K. Kempter, "Large-Area Electronics Based on Amorphous Silicon", Festkorperprobleme

27 (1987) pp. 279-305.

[3] K. Suzuki, "Arnorphous and Microcrystalline Semiconductor Devices", Capo 3, Artech

House (1991) pp. 77-140.

[4] M. Yamaguchi, S. Murakami, S. Todo and Y.Tawada, Mal. Res. Soe. Symp. Proc. VoI.

149 (1988) pp. 631-641.

[5] T. Takeda, "Amorphous anel Microcrystalline Semiconductor Devices", Capo 9, Artech

House (1991) pp. 331-343.

[6] C. Carvalho,

o.

Figueiredo, E. Fortunato, M. Vieira, A. Maçarico e L. Guimarães, Proc. 8~

PVSEC (1988) pp. 801-805.

[7] R. Martins, I. Ferreira, N. Carvalho e L. Guimarães, J. Non-Crystalline Solids VoI.

137&138 (1991) pp. 757-760.

[8] M. Vieira, E. Fortunato, G. Lavareda, C. N. Carvalho and R. Martins, presented at the

same conference.

[9] M. J. Powell, I. D. French. J. R. Hughes, N. C. Bird, O. S. Davies, C. Glasse and J. E.

Curran, Mal. Res. Soco Symp. Proc. VoI. 258 (1992), pp. 1127-1137.

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