Amorphous silicon nitride optical grating beam splitters

Journal of Non-Crystalline Solids 115 (1989) 165-167 North-Holland

165

AMORPHOUS SILICON NYrRIDE OPTICAL GRATING BEAM SPLrlq'ERS M.R. TAGHIZADEH, J.I.B. WILSON

Department of Physics, Heriot-Watt University, Edinburgh, U.K. J. TURUNEN, A. VASARA and J. WESTERHOLM

Department of Technical Physics, Helsinki University of Technology, Finland Two-dimensional arrays of regularly spaced, equal intensity beams have been generated from a single laser beam by a Dammann grating fabricated from plasma deposited silicon nitride. The properties of this material which make it suitable for such application and the process of converting the thin-films into gratings is described, as well as the performance of the completed devices.

5.00 1. INTRODUCTION Silicon oxide is a well-used optical material, and amorphous silicon oxynitride thin-films have been used for two-dimensional planar waveguides 1 since this material is transparent, durable, stable and can be prepared with a range of refractive indices. Here we discuss the application of plasma deposited amorphous silicon nitride (a-Si:N:H) in the holographic optical elements known as Dammarm gratings2.

> a. < 4.00 O Z < m , 3.00 < b13.. O 2.00

These can provide one- or two-dimensional arrays of optical images and/or beams, which are required in optical circuits

I

0

for image processing or in optical digital computing. 2. DEPOSITION AND PROPERTIES OF a-Si:N:H We prepare the amorphous thin-films on Coming 7059

E t.t~ 3.20 2.80

following conditions: 0.1-0.2 torr total pressure, ammonia

_Z w > 2.40

parallel plate electrodes), substrate temperature 200-350°C 3. A mixture of 30 standard cm 3 min "~(sccm) NH 3 and 3 sccm SiI-L from mass flow controllers deposits an a-Si:N:H alloy onto a 270°C substrate with an optical gap of 4.7 eV and a refractive index at 633 nm of 1.8. The deposition rate for

I

I

I

I

I

I

I

x

glass by ff decomposition of silane and ammonia with the gas proportion 0 to 0.90, rf power 10 W (100 mm diameter,

I

0.20 0.40 0.60 0.80 1.00 AMMONIA GAS CONTENT (NH 3/NH 3 +Sill ,= )

< tr 2.00 u. w rr

1

i° •

I

I

I

i It

I

2.00 3.00 4.00 5.00 OPTICAL BANDGAP [eV]

these conditions is - 0.4 lain hr ~. The index and optical gap are related as shown in Fig. 1. The deposition rate increases and the index falls slightly with reduction in substrate temperature, but the films are then more porous (and contain more hydrogen according to infrared spectrometry). 0022-3093/89/$03.50 O Elsevier Science Publishers B.V. (North-Holland)

Fig. 1. Refractive index and optical bandgapof amorphous silicon nitride alloys.

M.R. Taghizadeh et al./ Amorphous silicon nitride optical grating beam splitters

166

array of beams, although larger arrays have been produced.

3. DAMMANN GRATING FABRICATION The Dammann grating is a periodic, binary-phase, computer-generated hologram, with a complex-amplitude transmission function as shown in Fig. 2. This will convert

Yj 1

a unit amplitude plane wave into a one- or two-dimensional block of equally intense plane waves propagating into discrete directions. The calculation of the geometry of the required

holographic element4 is

fairly rapid

for

one-dimensional arrays of up to 100 beams whereas solutions for up to 1000 beams can take ~ one week on a SUN 3/50 workstation.

Two dimensional grating patterns are

constructed directly from the one dimensional results2"47or by using a more general type of Dammann gratings. Both the required reconstruction error (<2%) and a high diffraction efficiency into the desired orders (60-70%) may be obtained with this procedure.

It(x)l

0 U

1

X

Fig. 3. Structure of one period of a two-dimensional binary grating beamsplitter generating 15 x 15 equally

t(x) = t ( x ) e x p [ i 0 (x)]

intense beams. The procedure for the silicon nitride patterning is shown

-LS-L

in Fig. 4: it is basedon positive photoresist and nitride etching in hot phosphoric acid.

o(x)

x Z

7t/2+ 0

The nitride thickness required for the phase delay in the grating pattern is L/2(n-1), where we have selected a refractive index, n, of 1.8 and the operating wavelength, ~1, is one of several laser emissions. For instance, for the 488 nm

x

Ar÷ laser output, the thickness is 0.305 lain, and since this is deposited in about 45 mins, the precision in thickness is

rt/2- 0

readily achieved if all deposition parameters remain constant. a 1 b 1 a2 b2

Fig. 2. Complex-amplitude

aI bI

Improved accuracy would result from in situ optical

aL bL 1

monitoring (e.g. ellipsometry) to replace our present scheme transmission

function

of calibration depositions and subsequent timed deposition.

t(x) = It(x)l exp[i0(x)] of a Dammann grating period. Length of the period is normalised to unity, at = 0 and a ~ t = 1. The calculated pattern has then to be transferred to the silicon nitride f'tlm by photolithography. The intermediate

4. RESULTS The overall efficiency of a grating which generated a 15 x 15 array of 488 nm beams was 65% (compared with a calculated value of 68%). The power in each beam of the

step is the production of a chromium on glass photomask of

array was'measured with a Si detector and a pinhole, and was

a few periods of the grating by a pattern generator (CGA

uniform in all quadrants of the array to + 6%, with about 15%

3600), followed by photoreduction by a factor of five, and

of the total power in the central spot due to a slight error in

multiplication by a wafer stepper. This produces a minimum

the thickness of the nitride. Greater resolution in mask

feature size of 0.8 lamandaresolutionofatleast0.3 lain. Fig. 3 shows oneperiodofthepattern generator mask fora 15 x 15

generation (using electron-beam lithography) and pattern construction, as well as optical monitoring of nitride

M.I~. Taghizadeh et al./ Amorphous silicon nitride optical grating beam splitters deposition should improve this. The low absorption and scattering of these films allow the gratings to be used at high optical power densities at visible or near infrared wavelengths) which would damage other materials. Optical damage thresholds greater than 1 kW cm2 at 488 nm and 514 nm have been achieved. In optical parallel processing applications large densities of arrays of logic elements are simultaneously operated using a single high power laser and so it is important that the grating can handle high powers. (In some schemes it is possible to use reflecting rather than transmitting optics, which would make heat dissipation more easily solvable). Currently we are designing gratings to produce arrays of 200 x 200 beams using e-beam lithography, which will require plasma etching to maintain resolution of features to less than 1 ~m.

167

5. CONCLUSION Although uniform arrays of beams have been generated by these Dammann grating holographic beam splitters, using stable, transparent, damage-resistant plasma silicon nitride, there is the possibility of designing and fabricating more complex optical elements, perhaps using other amorphous silicon alloys in addition to nitrides. ACKNOWLEDGEMENTS We are grateful to M. Miller, B. Robertson and A. Salin for their experimental assistance. Partial financial support from the Academy of Finland, the British Council, the Royal Society and Jenny and Antti Wihuri Foundation is gratefully acknowledged. REFERENCES

.l~//////////////////,//////////////I ~

PHOTORESIST SILICON NITRIDE GLASS SUBSTRATE

,~ PATTERNINGBY CONTACT PRINTING

/

~ ETCHINGIN PHOSPHORICACID

~ REMOVINGTHE PHOTORESIST

PHASE ONLY BINARY GRATING BEAMSPLITTER Fig. 4. Schematic of the steps involved in fabrication of phase-only binary gratings in amorphous silicon nitride.

1. D.E. Bossi, J.M. Hammer and J.M. Shaw, Appl. Opt. 26 (1987) 609. 2. H. Dammann and K. G6rtler, Opt. Commun. 3 (1971) 312-315. 3. A. Qayyum, J.I.B. Wilson, K. Ibrahim and S.K. A1-Sabbagh, Proceedings of the E-MRS Meeting XVII, Les Editions de Physique (1987) 463-468. 4. J. Turunen, A. Vasara, J. Westerholm, G. Jin and A. Salin, J. Phys. D: Appl. Phys. 21 (1988) S I02-S 105. 5. H. Dammann and E. Klotz, Opt. Acta 24 (1976) 505-515. 6. W.H. Lee, Appl. Opt. 18 (1979) 2152-2158 7. U. Killat, G. Rabe and W. Rave, Fiber and Interf. Opt. 4 (1982) 159-167. 8. J. Turunen, A. Vasara and J. Westerholm, Technical Digest of the Optical Society of America 1988 Annual Meeting, Santa Clara, 126.