Lift-off deposition of a 300A thick aluminium film over the antireflection coating ... Indexing term: Aluminium gallium arsenide, Gallium arsenide,. Electro-optical ...
To avoid the vertical parasitic structure, several methods are possible.
Simple technique for fabricating limited coupler gratings by holographic method using standard thick photoresist D. Pascal, R. Orobtchouk, S . Lava1 and A. Koster Indexing term: Holographic gratings
The fabrication of photoresist gratings is considered. A solution to the problem of ‘transversal standing waves’ is given and appears simple and versatile. Introduction: Periodic corrugations have many applications in integrated optics such as Bragg reflectors, DFB lasers, grating couplers, filters etc. A simple technique for fabricating submicrometre period gratings is reported here, which involves an antireflection coating and optical interference. It has been developed on a silicon-on-insulator/SIMOX wafer, but the principle can be easily generalised to other substrates. Fabrication process and results: Holographic lithography is a
widely used technique for exposing submicrometre periodic gratings over large areas [l], but only a few papers report the use of an antireflection coating. A photoresist layer coated on a substrate is exposed to the interference pattern of two laser beams coming from a single laser and exhibits, after development, a periodic structure which can be managed in several ways. Many authors have shown that the process is complicated by a two-dimensional structure [2, 31. The period A of the desired grating depends on the wavelength k of the laser, and the incidence angle 8 according to the formula A = U(2n sin e) where n is the index of the photoresist and e is supposed to be measured in the photoresist. If the reflection coefficient of the substrate is nonzero, there is an extra vertical interference pattern which causes the photoresist to develop unevenly. The spatial period of the vertical pattern is then P = k(2n cos e). For each point in the photoresist layer, the light intensity can be calculated [4], taking into account the reflection coefficients at each interface, multiple reflection (i.e. Fabry-Perot effect) and optical losses in the medium (imaginary part of the indices). Examples of constant intensity contours are shown in Fig. I. 0
-0.1
f
5 -0.2 -0.3
(i) Reduction of photoresist layer thickness: Typically, the photoresist layer must be thinner than P/2, but the phase of the reflection coefficient must not be too close to a, to avoid an intensity minimum at the resist-substrate interface, leading to a resist profile unsuitable for lift-off and undesirable for anisotropic dry etching. Moreover, the low thickness of the resist does not ‘cover’ the possible steps of the substrate and limits the modulation depth of the grating. We can show that if the thickness is not precisely chosen, the nonuniformity is enhanced by the Fabry-Perot effect of the layer. (ii) Addition of two processing steps: Cr (or Al) oblique deposition and oxygen plasma etching may be used, hut this increases the process complexity [5, 61. (iii) Reduction of refection coefficient: Reducing the reflection coefficient to < I%, by using an antireflection coating, allows the use of a standard photoresist ( 0 . 4 ~ thick AZ 1400-17) under standard conditions. This technique is consistent with many variants such as lift-off, plasma etching and parallelogrammic-shaped gratings. Moreover, it is easy to bound gratings within welldefined areas, by spatially limiting the ‘antireflection’coating. Si
0.195 prn
buried SiOz 0.45prn
m
SI substrate 5 0 0 p Fig. 2 Structure of material
The process has been completed on SIMOX material whose structure is defined in Fig. 2. It is, of course, adaptable to other materials. The first operation to be performed is to accurately measure the reflection coefficient at the wavelength used to expose the photoresist (here 0 . 4 5 7 9 ~ argon laser line). A phase and module characterisation would be useful, but it is rather difficult to obtain the phase information. The difficulty can be avoided by measuring the module alone, for different incident angles, and then, by means of a proper model, we can fit the parameters of the SIMOX according to the measured reflection coefficient. The fitted model allows us to design an antireflection coating consisting of two layers, Si,N, and SO,, respectively, which will be deposited hy plasma enhanced chemical vapour deposition (PECVD) on the SJMOX substrate. The thicknesses of the two layers are not critical. Fig. 3 shows that typical thicknesses are 500 f SOA for Si,N, and 800 t 200A for SO,. Any values within these ranges lead to a reflection coefficient < 1%. reflection coefficient .7.
-0.4
0
0-2
0.4
0.6
0.8 0.15
E m
L
IJm Fig. 1 Constant intensity contours in photoresist layer (intended refection coefficient = 4%)
M: maximum m: nnnimum rm.: relative minimum Assuming that the two beam intensities are equal, the minimum at x = N 2 f mA where m is an integer, is zero. The higher the reflection coefficient on the subtrate, the lower the relative minimum at x = mA. The development of the photoresist (dissolution in the developer) stops on a constant intensity contour depending on the insolation energy. If the reflection coefficient is high, the relative minimum is low, and the exposure needed to reach the substrate for a positive photoresist (i.e. to pass through the relative minimum) leaves a very thin strip of insoluble resist. A theoretical strip width larger than N 2 requires a reflection coefficient less than 2.9%. in fact, a small overexposure (to be sure that the relative minimum is passed through) leads to a still thinner strip. 1% seems to be a practical maximum.
914
Si&. pm Fig. 3 Constant refection coefficient confours for fwo layer untirefection coating
Lift-off deposition of a 300A thick aluminium film over the antireflection coating permits us to define high reflection regions and to bound the grating within well-defined areas. Instead of alu-
ELECTRONICS LETTERS 25th May 1995 -
Vol. 31
No. I 1
minium, an SO, layer deposited prior to the antireflection coating can also be used successfully. A 900A thick layer ensures a 30% local reflection coefficient. Some results are given as examples in Fig. 4.
GaAs/AIGaAs electro-optic modulator with bandwidth 5 40GHz R. Spickermann, N. Dagli and M.G. Peters
Indexing term: Aluminium gallium arsenide, Gallium arsenide, Electro-optical modulation
A high speed GaAdAlGaAs travelling wave Mach-Zehnder electro-optic modulator has been fabricated. The device uses a novel slow wave electrode design to achieve p h a x velocity matching and has a measured electrical bandwidth > 40GHz.
a
b
Fig. 4 SEMphotographs of resist profiles
~
mzi
~ photoresist on a 3% reflecting substrate with 390nm a 0 . 4 thick period gratin showing the vertical standing wave due to nonzero reflection coekcient b 0.4 pm thick photoresist with 380nm period grating where left part is antireflection coated and right part includes an extra Si0,layer (R = 30%) Conclusion: To obtain deeply modulated gratings in thick photoresist, control of the reflection coefficient of the substrate is important. Two dielectric layers (Si,N, and SO,) are required to obtain a reflection coefficient smaller than 1% on SIMOX material. It is worth noting that in this case, control of the phase is not required. Limited gratings are obtained by deposition of an extra Si02 layer spatially limited. After etching, the photoresist grating is transferred onto the antireflection coating layer. 0 IEE 1995
Introduction: High speed optical modulators are essential components for the transmission of microwave and millimetre wave analogue and digital signals over optical fibres. The main limitation on the electrical bandwidth is the capacitance of the electrode used to apply the electrical signal to the modulator. If the device is driven as a lumped circuit element, the bandwidth is limited by the RC time constant of the electrode. It is possible to reduce the electrode capacitance by making the device very compact. Multiquantum well electroabsorption modulators are such devices [I]. Although bandwidths up to 40GHz have been achieved using such designs, they suffer from chirp, high optical insertion loss and limitations on the maximum optical power that can be handled. An alternative approach is the travelling wave technique in which the electrode is designed as a transmission line distributing the electrode capacitance [2 41. Travelling wave modulators have most commonly been fabricated in GaAdAlGaAs [3] and LiNbO, [4] material systems. Of these two, only GaAs/AlGaAs allows the monolithic integration of lasers, detectors and microwave electronic circuitry on the same substrate. Our goal is to realise an integrable small signal optical modulator with a 3dB bandwidth of 100GHz. Here we report a device with a measured electrical bandwidth in excess of 40GHz.
6 April 1995
In the travelling wave configuration, the modulating microwave and modulated optical signals travel colinearly along the device. It is well known that phase velocity matching of the microwave and optical signals is necessary to achieve maximum bandwidth [2]. If phase velocity matching is achieved, the bandwidth of the modulator is limited by the loss of the microwave electrodes. The frequency at which the total electrode loss becomes 6.34dB determines the 3dB electrical bandwidth of the modulator.
+‘cm
__f
ligtt in
light out
Electronics Letters Online No: 19950623
D. Pascal, R. Orobtchouk, S. Lava1 and A. Koster (Instirut dElectronique Fondamentale CNRS URA 22 Bdt. 220, Universite Paris-Sud 9140s Orsay Cedex, France)
I
Referenees and KATZIR, A.: ‘Photoresist gratings on reflecting surfaces’, J. Appl. Phys., 1982,53, (3), pp. 1387-1390
1
KAFQN, E.,
DILL,
I
O Au Schottky electrodes
wdfer
edge
Fig. 1 Schematic top view of modulator
-
F.H.: ‘Optical lithography’, IEEE Trans., 1975, ED-22, pp.
44&444 JOHNSON, L.F., KAMMLOTT, G.w., and INGERSOLL,
K.A.:‘Generation of periodic surface corugations’, Appl. Opt., 1978, 17, (8), pp. 1 165-1 181 NEUREUTHER, A.R.,and OLDHAM, W.G.in ENGEL, W.L. (Ed.): ‘Process and device modelling’ (North Holland, Amsterdam, 1986), pp. 71106 HORWITZ, C.M., and SMITH, H.1.: ‘Holographic lithography with thick photoresist’, Appl. Phys. Lett., 1983, 45, (9), pp. 874875 LI, M., LIN, J.C.H., CHERRILL, M.J., and SHEARD, S.I.: ‘Fabrication Of submicrometre parallelogrammic-shaped gratings in SiO,’, Electron. Lett., 1994, 30,(25), pp. 212622128
Schottky A u 1 urn
x : O . 4 AlGaAs 0.92 urn
ANDERSON, E.H.,
ELECTRONICS LE7TERS
25th May 1995
Vol. 31
substrate
Fig. 2 Schematic cross-section of modulator illustrating push-pull vertical electric fields
No. 7 1
915
