aspects of the processing of semiconductor optoelectronic devices

, Vacuum/volume 40/number 4lpages 363 to 36711990 Printe> in Great Britain

0042-207x/90$3.00+.00 0 1990 Pergamon Press plc

Aspects of the processing of semiconductor optoelectronic devices S J Clements, STC Technology Ltd, London Road, Harlow, Essex CM17 9NA, UK

Some of the problems are reviewed which currently inhibit the progress of optoelectronic integration. The importance of wet etching is highlighted in device fabrication whilst demonstrating its limits as a consequence of the general orientation dependence. The development of dry etching techniques for InP based compounds is reviewed. Processing methods for the fabrication of distributed feedback lasers, which are of importance in integrated schemes. are discussed. Finally an example of an integrated receiver device is given as a demonstration of state-of-the-art integration technology.

1. Introduction

Since the first developments of semiconductor light emitting devices and the proposal for fibre-optic communications nearly 20 yr ago a rapidly growing industry has appeared. Success in this area is such that there are already proposals to bring fibre to the home and the use of optoelectronics has rapidly become commonplace. The fabrication technology has kept pace with these requirements for increasingly sophisticated devices, and a large research effort has been devoted to the development of optoelectronic devices based on III-V materials which are uniquely suited to light generation and detection as well as integration.

The first semiconductor optoelectronic devices were based on the GaAs system with emission at wavelengths of around 0.85 pm, but the developments in silica optical fibre with transmission windows at I.3 and I.5 pm led to increased interest in longer wavelength emitting materials based on InP compounds for use in fibre-optic communication systems. Recent research has con- centrated on these materials and it is the processing of these which is discussed here.

The technology is maturing from discrete devices towards more sophisticated integrated components and the rate of progress is affected in no small way by the limitations which are imposed by current processing techniques. Some of the key areas are examined here in addition to some recent solutions which illustrate state-of-the-art optoelectronic integration.

2. Semiconductor lasers

Because of their size, coherence, brightness and directional emission, semiconductor lasers are the chief candidates for use in current and future optical telecommunication systems. The ridge waveguide (RWG) laser is an example of a laser which is currently in production. Whilst simple in construction, it exemplifies the fabrication technology which is employed. Electrically, the laser chip is simply a p-n diode formed and contacted between top and bottom surfaces of the wafer. Optically, the laser is a light guiding structure with light emission and gain in the plane of the wafer surface, confined by a stripe structure, and partially reflected by end facets.

The fabrication sequence for the RWG laser is illustrated in Figure I. The initial semiconductor layer heterostructure. pro-

/,;:P; layer(lnGaAsP) ,

I Active layer A . T.52~m

j-7-substrate(lnP)

Grow planar double heterostructure

SiO mask

I 1

Etch ridge structure through cap layer and p_lnP

Metal contact

SiO,insulation

j LMetal contact

Apply stripecontact lo ridge and metallize n-side

Figure 1. Ridge laser processing sequence.

duced in this case by either liquid phase epitaxy (LPE) or metal- organic chemical vapour deposition (MOCVD), is the predominant feature of optoelectronic devices. The multilayered structure has meant in turn that selective etching of one layer over another is most important as a processing technique. Selec- tive wet etching is used to define the double channel which forms the light guiding ridge with suitable width and vertical walls. The lack of a readily grown native oxide, in common with GaAs, means that contact isolation is usually achieved by deposition of dielectric layers, normally silicon oxide or nitride, by plasma enhanced chemical vapour deposition. The stringent requirements of this process are exacerbated by the need for low stress, since stress has a stronger effect on optical properties than the electrical properties for which the technique was originally developed. Following metallisation and thinning, which are carried out conventionally by electron beam evaporation and combined mechanical and chemical polishing respectively, the slice must be diced. Although often a problem during processing, the

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SJ Clemenfs: Semiconductor optoelectronic devices

extreme brittleness of InP means that the wafer is readily cleaved, particularly when it is thin, and it is easy to form good quality

<facets which have a reflectivity of around 30%. This is ideal for laser operation without further processing.

The simple structure of the RWG laser is excellent for production manufacture and has resulted in a very reliable laser which has been qualified for the 25 yr service life demanded by submarine cable applications’. Many of the processing techniques, however, are not compatible with the demands made by integration. Dicing of slices to form facets, for example, is clearly only acceptable for discrete devices. Current work is focused on the development of techniques which overcome these problems.

3. Semiconductor etching

3.1. Wet etching. The zincblende crystal structure of indium phosphide dominates its etching characteristics which are quite different from those of silicon. The polar nature of the bonding of the two different elements leads to a reduction of symmetry in the crystal lattice, which produces anisotropic etching behaviour from a variation of etch rate in different crystal orientations. In particular the { 11 I) planes consist either of all indium atoms (designated by convention {I I I) A) or all phosphorus atoms (designated { 11 I} B). Most wet etchants are based on a com- bination of an oxidising agent and a solvating agent and the chemical behaviour of these are quite different on the group III planes compared to group V. This difference in etch rate explains the prevalence of { 11 I} planes revealed by many etchants, exemplified by bromine-methanol (Figure 2). For the commonly used (100) slice orientation this results in corresponding overcut and undercut profiles for stripes etched in the orthogonal a and /3 directions respectively (equivalent to [Oil] and [OiT] directions). It is possible for particular orientations to reveal a wide variety of planes with various etchants but this places severe restrictions on the orientation of the ultimate device. In the case of the ridge laser, for example, it is possible to preferentially wet etch a vertical wall corresponding to the (011) plane when the ridge is oriented in the fi direction but a sloping wall (a (I I 1) A plane) is revealed if etched in the t( direction. Clearly, if wet etching is used, ridge lasers must always be aligned with the ridge in the fl direction which is no real problem when producing discrete devices, but it leaves little flexibility for integrating devices. It is likely, for example, that it would be desirable to etch the laser facet in some integration schemes which is necessarily orthogonal to the laser stripe.

The most beneficial aspect of the anisotropic nature of wet etching is its common ability to selectively etch different III-V compounds. In the case of the ridge laser this selectivity can be used to advantage, when large enough, to provide an etch stop at the base of the ridge. This is critical for laser operation since

(06) - toii)

Figure 2. Etch planes revealed by Br,-CH,OH on InP.

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it is the change in refractive index between the InP of the ridge and the air in the channels which provides the confinement for the lasing action.

As the demand appears for guiding light round curved waveguides, through couplers or demultiplexers, the orientation dependence of wet etching becomes an increasingly severe fabrication limitation and as a result dry etching is being established as the alternative with the additional advantages of more precise control and lack of undercut.

3.2. Dry etching. The development of techniques for the dry etching of indium phosphide and its compounds has benefited from the prior art on other semiconductors, and that of GaAs in particular. The main difference in the etch characteristics of indium phosphide and its related materials is the lower volatility of indium compounds compared to those of gallium. In etching semiconductors by chemical dominated processes, such as reactive ion etching (RIE) it is common to employ halogen based chemistry, with emphasis on fluorine and chlorine. It can be seen from a table of the boiling points of indium compounds (Table I), however, that the fluoride is essentially involatile and etching with fluorine based etchants is not possible. Chlorine chemistry has therefore tended to be the predominant technique in the past’-‘. Even so, the lower volatility of indium chlorides compared to gallium means that it is usually necessary to heat the substrate to get acceptably smooth etched surfaces. It also suffers from a tendency to produce overcut profiles and the selectivity in etch rate over common masking materials is poor, both of which limits its applicability.

As with all RIE-type processes, it is important to balance chemical and physical etching mechanisms. Increasing the physical component in chlorine based systems, for example by reduc- ing the pressure, tends to improve the etch profile but only by compromising the roughness of the etched surface. As an alternative, physical sputtering has been increased by the introduction of argon with chlorine compounds to produce smooth surfaces at low temperatures.

Ion-beam etching (IBE) or reactive ion beam etching (RIBE), in which physical processes predominate, has proved successful with GaAs but again tends to produce rough surfaces in InP6. The introduction of chemical components, in this case oxygen6.’

Table 1. Boiling points of indium and gallium halides (atmospheric pressure and inert atmosphere)

Compound Boiling point (“C)

IIIF, > 1200

InCl, 600 (volatilises) InCl2 550-570 InCl 608

InBr, (sublimes, mpt. 436) InBrz 662 InBr 632

InI, 210 InI, 212 In1 351

GaF, 800 (sublimes)

GaCl , 201

GaBr, 279

GaI, 345 (sublimes)

(Data from CRC Hnndhook of Pl1ysic.r and Chemistry).

S J CIaments: Semiconductor optoelectronic devices

or iodine’, has increased the quality of the etched features with acceptable selectivity over mask materials.

A major concern with all dry etching techniques, however, is the degree of damage that physical bombardment may introduce. This is of particular concern in optoelectronic devices when actually etching, or etching close to, active layers in lasers or detectors, but it may also affect the overgrowth of etched layers or the reflectivity of mirrors. It has been shown that the degree of damage, as measured by photoluminescence or the ideality factor of Schottky diodes for example, shows some dependence on the amount of ion-bombardment a surface receives’. The damage is likely to extend to between 10 and 100 nm below the etch surface and diffusion of defects may extend the damage effects further.

For lowest overall damage effects, RIE techniques are to be preferred over those which are ion beam based. This has led to a persistence in trying to find a chemical system which produces adequate etch characteristics. It was known that hydrogen, in common with many other etches, preferentially removes the phosphorus, leaving an indium rich surface ; but more recently the addition of methane at around l&20% has been demonstrated to produce a very successful etch for InP and its related compounds”. The etch mechanism is still not fully understood but it has been suggested that organometallic type species are generated such as (CH,),In which are extremely volatile, such that etching is possible even at 0°C. The overall etch rate is not high, generally 50-100 nm min-‘, but unlike other systems it is faster for InP than GaAs.

The overall etch characteristic is exemplified in Figure 3. The etch surfaces are extremely smooth, the edge profiles are close to vertical and the indications are that the overall damage to the substrate is low. A notable property of the etch is the build-up of deposit on masked areas. whether metal. dielectric or resist.

which protects the mask and provides a high effective selectivity. The rate of production of the deposit must be controlled, however, since it can prevent etching in the unmasked areas, if extreme, and even at lower rates can build up on the edges of the mask and cause roughness on the sidewall. The deposit is readily removed in an oxygen plasma.

In terms of usefulness for future optoelectronic integration, the significant property of dry etching, exemplified by the methane/hydrogen based process, is its absence of crystallographic selectivity. Near-vertical profiles are obtained for all crystallographic orientations, which makes possible the fabrication of curved waveguides and y-couplers for example, as well as orientation independence for the fabrication of RWG and other lasers. The methane-hydrogen process does, however, have the drawback of a comparatively low selectivity between compounds, which is currently insufficient for etch stops.

4. Distributed feedback lasers

A basic limitation to the integration of discrete devices involving laser devices is the requirement for mirror facets at each end of the laser cavity. This is commonly achieved by cleaving, which necessarily has tended to separate the laser source from integration schemes and has dictated a hybrid approach. One technique for overcoming this problem is the etching of the facet, and this has been demonstrated using both wet and dry techniques’ ‘. This has allowed integration of, for example, lasers and detectors but a high degree of control is demanded, particularly in efficiently coupling the light from the laser facet.

Systems into which lasers are likely to be integrated, however, have further system based requirements. Systems requiring mul- tiplexing based on separate wavelength channels demand well- defined lasing wavelengths closely separated, at maybe 1 nm intervals. Coherent communication, using for example frequency or phase modulation of the laser in analogy with existing radio techniques, demands narrow linewidth of any particular wavelength and tunability of the wavelength. This has led to the requirement for single mode lasers, the most predominant of which is the distributed feedback (DFB) laser. The DFB laser incorporates a grating close to the active layer (Figure 4) which reflects light of a wavelength appropriate to the pitch of the grating and destructively interferes with other wavelengths. The DFB is thus single moded in comparison with the many modes of conventional lasers, with around 40 dB of suppression of

Figure 3. Double channel profile etched with methane/hydrogen RIE.

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S J Clements: Semiconductor optoelectronic devices

anywhere over a slice. A quarterwave phase shift in the middle of a grating leads to better spectral control in the laser and these can be written as desired without further complication. ’ 3 Phase shifts can be written holographically but only at the cost of additional processing steps’“‘6.

Whichever technique is used it is important to be able to control the coupling coefficient of the grating, which has wide- ranging effects on DFB laser performance. This is affected by the

Figure 4. Structure of the DFB laser.

refractive index and thickness of the lasers adjoining the grating, but also the mark : space ratio and depth of the grating which are determined by the process. Both wet”.18 and dry I9 etching of the gratings have been used. Those illustrated in Figure 5 used

the next highest mode. Such properties are ideal for system requirements.

A further advantage is also apparent however. Because the feedback of light is distributed along the length of the cavity (hence the term DFB) and not by separate facets, there is no need for the mirror facets. This makes the DFB laser an excellent choice for integration since light can be coupled directly from the laser waveguide into other elements such as a passive guide, switch or coupler.

Fabrication requirements of the DFB laser grating are demanding. For the emission wavelengths of interest the pitch of the grating is 0.2-0.23 pm, which is beyond the capability of conventional photolithography. Two different methods are currently being employed in production : holographic lithography and electron lithography.

Holographic lithography uses the impinging of two monochromatic beams to generate an interference pattern in the photo- resist coating on the surface of the slice. The technique has been developed to a high degree of sophistication with computer control of the optical arrangement to select and control the exposure conditions and the pitch of the grating. In this way it has been possible to expose five different period gratings on the same wafer using a movable mask close to the surface to mask off selected parts of the wafer12. The processing problems of holographic exposure are basically associated with the size of the features involved, and close control of temperature, development conditions and cleanliness are all required to obtain good results reproducibly. The use of monochromatic light, however, adds to the difficulties which are encountered, in particular as a result of the reflections from the large refractive index step between the resist and the semiconductor. This leads to ripples in the sidewall from the vertical standing wave pattern and also causes an inten- sity null at the base of the resist which can lead to a residue in the exposed areas after development. The first problem is normally overcome by the use of thin resist, of around 100 nm. whilst the second can be overcome by either overexposure or over- development. Both problems increase the degree of control necessary in all parts of the process to maintain well controlled quality. In spite of these difficulties the technique is routinely used to produce gratings with periods of 197 nm.

Electron lithographic gratings are generated by a scanning electron beam and the development of the pattern in electron resist. The sophistication involved in scanning the beam means the capital equipment cost is higher and the serial nature of the exposure means the exposure time is longer compared to holographic exposure. To some extent, however, this is con- trasted with an inherent higher resolution capability which leads to high quality gratings in high yield. It also has an advantage of flexibility. Computer control of the beam means that gratings can be written at varying orientations and of varying periodicity

366

>) Dry etched

Figure 5. First order DFB grating profiles.

S J G’emenrs: Semiconductor optoelectronic devices

iodic acid to form the triangular profiles whilst the methane- hydrogen RIE system was used for the dry etching. In each case the success of achieving reproducible coupling coefficients is determined by the ability to control the linewidth and hence mark : space ratio in the resist image.

5. Integrated receivers

The integration of optoelectronic devices has progressed fastest in the area of receivers and it is here that the processing technology is being exploited. Even in its most advanced form however the performance of integrated devices is still surpassed by hybrid technology. To some extent this is because of the compromises which have to be made in the processing as a result of the conflicting requirements of the individual components. In the case of a detector integrated with a field effect transistor, there are conflicting geometric and material requirements (Figure 6). The vertical difference in size makes it difficult to process the FET as a result of the non-planarity in processing, whilst the differences in semiconductor have generally meant that complex multi-epitaxial stage growth and processing have been required.

One solution to this problem exemplifies the current technology involved in optoelectronic integration”. The geometrical conflict was overcome by embedding the detector (Figure 7), and selective etching allows separate contacting to detector and FET layers all grown in a single growth step. The FET layers actually underlay the detector structure but play no part in its operation.

The structure was built up in several stages. The recess was etched by methane-hydrogen RIE to produce smooth vertical walls followed by a single step MOCVD growth step of the FET/detector layer structure. Combined RIE and selective wet etching was used to remove the detector layers everywhere except in the recess and around its perimeter. Selective wet etching was also used in all subsequent critical etch steps to form the separate device structures as shown. In a sequence which is characteristic of conventional electronic integration schemes, the structure was completed using via hole and contact formation, with all devices

PET

Figure 6. Conflicting requirements of opto (photo diode) and electronic (transistor) integration.

?.I hP substrate

Figure 7. Preferred planar receiver solution.

being metallised simultaneously. A silicon nitride layer was used which combined electrical passivation with optical antireflection coating of the detector.

In this way the detector is made without undue compromise of the FET performance. The quasi-planar structure is compatible with potential sub-micron gate lithography with conse- quent improved performance. The existing sensitivity performance of -32.7 dBm for 10e9 bit error rate approaches that of hybrids but with the additional potential of integration to extend to arrays or other more complex components.

6. Conclusions

Integration technology for optoelectronics is developing rapidly as a result of intense activity. Of these technologies it is likely that etching systems will be one of the most crucial to success. The effects of damage induced by dry etching still require further study and this should provide one of the stimuli to develop new techniques. The development of electron cyclotron resonance etching systems is one possible example of this. The development of a material selective etching capability or possibly suitable end- point detection would increase the use of dry etching over wet etching.

It is probable that increasing numbers of DFB lasers will be demanded in the future as integration is developed. As the demand grows, reproducible fabrication and control of the properties of DFB lasers will provide a severe test of the high resolution processing technology.

The development of integrated optoelectronics is currently led by receivers and is dependent on the materials and processing technology. It is clear that fabrication methods are and will continue to be a fundamental part of the adoption of optoelectronics into everyday life.

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aspects of the processing of semiconductor optoelectronic devices

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