Integration of GaAs on Si substrate

Abstract: A method of growing GaAs on a Si substrate by means of which stresses in the materials due to the difference of the thermal expansion coefficients of these materials are reduced. Microcracks are induced in the GaAs layer on locations where the operation of the device or part thereof is not hampered by use of a SiO₂ mask having at least one vertex in the cleave direction of the crystal.

Description: Field of the invention The present invention relates to the integration of GaAs on a Si substrate. The invention more specifically relates to the monolithic integration of GaAs/AlGaAs electronic or optoelectronic structures such as light emitting diodes on a Si substrate on which Si electronic circuitry such as LED-drivers (field effect transistors) can also be integrated. Description of the prior art GaAs layers or Gas structures, i.a. GaAs/AlGaAs light emitting diodes can be grown substantially free of stresses on top of a Si substrate at high temperature, i.e. 700°C. However, during the cooling-off period the different shrinkage of Gas and Si brings about stresses in the materials. These stresses may cause the wafer to bend if the thickness of the Gas layer grown on the Si substrate is rather small, e.g. when this thickness does not exceed 3 micrometer. If the GaAs layer is thicker, microcracks might appear in the epitaxial layer structure on the wafer. The presence of material stresses affects negatively the electrical and/or optical properties of the GaAs/Si devices. Material warpage hinders lithography and further processing of the wafer. The arbitrarily occuring microcracks might harm the proper operation of the electronic devices that are formed. The above issues of concern that are met when growing GaAs layers on Si substrates are described in the following publications: "Stress variations due to microcracks in GaAs grown on Si" published by B.G. Yacobi et al. in Appl. Phys. Lett. Vol. 51, No. 26 and "Stress variations and relief in patterned GaAs grown on mismatched substrates" by B.G. Yacobi et al. in Appl. Phys. Lett. Vol.52, No. 7. Few solutions are known in the art that deal with the above problems. In Appl. Phys. Lett., Vol. 53, No. 3 a paper on "The warpage of GaAs-on-Si wafers and its reduction by selective growth of GaAs through a silicon shadow mask by molecular beam epitaxy" is published. The warpage of fully covered GaAs-Si wafers and of Si wafers with selective growth of GaAs (obtained by using a Si shadow mask) have been studied. A significant reduction in warpage is observed by restricting the growth area of GaAs. It is also known to provide the back side of the silicon substrate with a film in order to counteract the stresses that are induced by the GaAs epitaxial layer. Summary of the invention It is an object of the present invention to provide a method of growing GaAs on a Si substrate by means of which a reduction of stresses in the materials resulting from the difference of the thermal expansion coefficients of these materials is obtained. A further object of the invention is to provide a method of growing GaAs on a Si substrate as a result of which plain wafers are obtained so that further lithography is not hindered. Still a further object of the present invention is to provide a method of integrating GaAs electronic or optoelectronic devices on a Si substrate so that no microcracks occur inside the active area of the devices. Other objects will become apparent from the description hereinafter. The present invention provides a method of forming microcracks on predetermined locations in a GaAs layer grown on a Si substrate characterised in that prior to the growth of said GaAs layer said Si substrate is partially covered by means of a mask, said mask being designed with at least one vertex extending approximately into a cleave direction of the crystals of said GaAs layer. The present invention further provides a method of forming an integrated electronic device or part thereof by growing GaAs on a Si substrate at a given temperature and subsequent cooling of the substrate characterised in that microcracks are induced in said GaAs layer on locations where the operation of said device or said part thereof is not hampered, said microcracks being induced by prior to the growth of GaAs covering said Si substrate partially by means of a mask that is designed with at least one vertex extending approximately into a cleave direction of the crystals of said GaAs layer. By intentionally creating microcracks in an integrated device according to the present invention stresses in the materials can be reduced and the occurence of microcracks on arbitrary locations can be avoided. To achieve the above microcracks on locations where the operation of the devices or part thereof is not hampered, part of the substrate is covered with a mask prior to the deposition of GaAs. This mask is preferably a SiO₂ mask. It has been found that by using a specific topography of the mask (as will be explained furtheron with reference to the drawings) microcracks can be induced on desired locations. By use of a mask provided with at least one vertex directed in a cleave direction of the GaAs crystal, microcracks are induced in the monocrystalline layer initiating from said vertex and propagating in said cleave direction. The mask topography, more specifically the location and the number of vertices can be selected so that microcracks can be induced on purpose on predetermined locations. A so-called vertex acts as a wedge that cleaves the GaAs crystal and hence is also called "wedge". In a first step SiO₂ is deposited for example by plasma enhanced chemical vapour deposition (PECVD) on the Si substrate. Parts of the SiO₂ layer are removed again by photolithography and wet chemical etching so as to form a mask. Next, on said partially coated Si substrate GaAs is deposited for example by means of the metal organic chemical vapour deposition technique (MOCVD) which is known in the art. According to this technique GaAs is deposited on Si in a quartzreactor. The metalorganic source trimethylgallium (CH₃)₃Ga can for example be used to transport a group III component, viz. Ga, using hydrogen as a carrying gas. Arsine can be used for transporting the group V element As. Hydrogenselenide can be used to obtain n-type doping and diethylzinc (transportation vapour hydrogen) can be used to obtain p-type doping. The above sources are heated in a quartzreactor. By the following simplified pyrolytic reaction at 700°C, GaAs is deposited on a Si substrate present in the reactor. (CH₃)₃Ga + AsH₃ → GaAs + 3CH₄ at 700°C. The morphology of the GaAs that is deposited depends among others on the characteristics of the underlying material. Monocrystalline GaAs is obtained on non-coated Si area whereas on the SiO₂ mask polycrystalline GaAs deposition is found. By using a specific mask topography according to the present invention, microcracks are introduced in the monocrystalline GaAs initiating from the vertices and propagating in the cleave direction(s). No microcracks are found on other locations. BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention and of the preferred embodiments thereof, reference is made to the following detailed description given in connection with the accompanying drawings and pictures, in which Fig. 1 schematically shows the topography of a GaAs/AlGaAs light emitting diode and a field effect transistor (MOSFET) that are monolithically integrated on a Si substrate, Fig. 2 is a reproduced photograph showing a spontaneously originated microcrack, Fig. 3 schematically illustrates the locations of intentionally induced microcracks on a substrate on which electronic or optoelectronic devices are grown, Fig. 4 illustrates the use of a SiO₂ mask and the growth of GaAs on the wafer, Fig. 5 represents a mask that can be used for obtaining induced microcracks, Fig. 6 is a reproduced photograph showing induced microcracks, Fig. 7 and 8 illustrate different kinds of masks, Fig. 9 is an enlarged representation of different shapes of vertices, Fig. 10 to 13 illustrate the results of experiments performed on crack formation. Figure 1 shows an example of a device that is composed of a first component which is a Si circuit more specifically a field effect transistor 2 and a second component that is a GaAs component, namely a light emitting diode 3. Both components are integrated on a common silicon substrate 1. Light emitting diode 3 comprises 5 layers grown on top of each other. On the silicon substrate a GaAs buffer layer is first grown. On top of said GaAs buffer layer, a AlGaAs cladding layer, an active GaAs or AlGaAs layer, a second AlGaAs cladding layer and finally a GaAs contact layer are grown. Figure 2 shows a microcrack that is originated spontaneously in a GaAs/AlGaAs layer structure deposited on a Si substrate. Unfortunately, this microcrack extends through the operational part of the GaAs light-emitting diode and makes the device useless (photograph taken by means of an electron microscope). Due to different shrinkage of GaAs and Si during the cooling off from the temperature at which GaAs is grown on the Si substrate to operational temperature (room temperature), microcracks such as illustrated by this reproduced photograph can originate on arbitrary locations and might harm the operation of the device. This problem is solved by the method of the present invention according to which microcracks are intentionally created so that material stresses are reduced and so that the exact position of the microcracks can be controlled so as to prevent generation of a microcrack within a device. Figure 3 illustrates that microcracks can be created on locations outside the active devices. Figure 3 schematically shows a series of light emitting diodes 4, 5, 6 and 7 integrated on a common substrate. The drawn lines numbered 8 to 13 represent the locations on which microcracks could for example be formed outside the devices so as not to affect the device operation. In order to obtain said microcracks at the locations on which no harm can be brought to the device operation the following procedure is followed. The silicon substrate (14) is initially covered by a mask, preferably a SiO₂ mask (15), by means of deposition of SiO₂ on the Si-substrate (fig. 4) and removal of part of the SiO₂ layer by photolithography and wet chemical etching. Next, GaAs is deposited. In figure 4 the different morphology of GaAs on different surfaces is shown. The morphology of GaAs deposited on SiO₂ is polycrystalline (16), whereas on the Si-substrate (not covered by SiO₂) it is monocrystalline (17). The following is a description of a specific procedure of forming a mask according to the present invention. The mask used in the experiments described furtheron have been obtained by using this particular procedure. The masking layers can for example be plasma-deposited on Si-wafers (p⁻, (100) +3° towards perpendicular [0 1 1] direction) using Plasma Enhanced Chemical Vapour Deposition. According to a preferred embodiment of the present invention, the wafers are cleaned in a mixture of 50% HF-solution and deionisised water DI (1part/19 parts) for 15 seconds prior to the deposition process. Then the wafers are loaded into the vacuum chamber of a PD80-system (Plasma Technology). The parameters of the deposition process leading to a deposition rate of 33 nm/min and deposition of a layer of 100 nanometer, are the following: total pressure in the chamber: 40,0 Pa (300 mTorr), temperature of the sample: 300°C, RF-power: 25W RF-frequency: 100kHz, gasflow: 10 sccm SiH₄, 120 sccm N₂O, duration: 3 min. For the patterning of the SiO₂-layers a standard photolithographic process is used. The wafers are cleaned in boiling solvents in soxhlets (15 min trichloroethylene, 15 min in aceton, 15 min in isopropylalcohol). Then the wafers are rinsed in deionised water (DI) and dried in air at a temperature of 120°C for 30 min. After drying, the samples are spincoated (3000 rpm, 40 sec) with photoresist. For these experiments AZ1350-resist (Shipley) is used. The resist is baked for 5 min in a hot-plate furnace at 90°C in a nitrogen atmosphere. The resist is then exposed to UV (high pressure Hg-lamp) in a mask-aligner through a mask such as illustrated in figure 7. (Exposure time: 34 sec) The exposed areas are developed. (developer: diluted AZ 2401 developer (Shipley), dilution: 1 part AZ 2401, 4 parts DI, developing time: 30 sec) With the photoresist as a mask, the SiO₂-layer is etched away. (etching solution: BHF from Transene, etching during 50 sec). The next step is the epitaxial growth of GaAs by means of the metal organic chemical vapour deposition method (in a Thomas Swan Epitor 04 system). The used reactor accomodates four metalorganic sources and three hydrides. As carrier gases nitrogen (inert) and Pd-purified hydrogen can be selected by bellow sealed pneumatic valves. Temperature controlled baths are included in each metalorganic line. The gas flow in each line is controlled by MKS mass flow controllers. The system is equiped with a EPIFOLD fast switching gas manifold for deposition of abrupt interfaces. The growth can be controlled manually or by computer. The reactor is a horizontal quartzreactor with a rectangular cross section and working at atmospheric pressure. During growth the substrates lay on top of a rectangular Si-susceptor, which is heated by an IR-lamp (1000W, 110V). The temperature is measured by means of a thermocouple placed in the Si-susceptor and through a PID regulator the lamppower is continuously adapted. The available sources are trimethylgallium (TMG), trimethylaluminium and diethylzinc for the metalorganics and 5% AsH₃ (arsine) in hydrogen, 200ppm SiH₄ (silane) and 200ppm H₂Se (hydrogen selenide), both diluted in hydrogen. Normal growth conditions are T gr =660°C, growth velocity=100nm/min, mol fraction (TMG)=2.10⁻⁴, mol fraction (AsH₃)= 2.10⁻³. Preferably a substrate preparation step for the growth of GaAs on Si is included. This step consists of an ex situ etch procedure of 15 sec in a HF:DI 1:19 mixture followed by rinsing under flowing DI (18,03 MOhm) for 60 sec and blowing dry in N₂ flow. (HF solution: 49%-51%) Immediately after this etching procedure the substrate is loaded into the reactor. The in situ substrate preparation consists of a bake-out procedure at 950°C under a 1 l/min H₂ flow during 1 hour. After the in situ bake-out, the substrate is cooled to 450°C and the H₂ carrier flow is raised to 4.4 l/min. When the temperature is stabilized at 450°C the growth environment is flushed with 200cm³/min AsH₃ (5% in H₂) flow during 2 min. Immediately hereafter the trimethylgallium flow is switched to the reactor and a thin GaAs nucleation layer (about 10 nanometer) is grown during 60 sec. Under a continued AsH₃ flow the set-point of the reactor heating is set at 720°C. After 5 min., the read-out value of the temperature being approximately 700°C, the set-point is gradually decreased to 660°C (normal growth temperature). After an extra 5 min, the read-out value of the temperature being stabilized at 660°C, the growth of the wanted GaAs/AlGaAs structure is started. The deposition on the quartz-reactor wall has to be removed after each growth run of GaAs on Si. In order to prevent desorption of this deposition an AsH₃ overpressure is required during the in situ bake-out step for the substrate preparation of the second growth run of GaAs on Si. At the elevated temperatures needed (950°C), the AsH₃ cracks too fast so that the dust filter at the outlet of the reactor saturates and eventually causes a pressure built-up in the growth environment. At every growth run the quartz reactor is removed from the system and replaced by a clean one. The first reactor and susceptor are then etched in aqua regia, rinsed in overflowing DI and in isopropylalcohol. After a bake-out for several hours at 100°C in vacuum furnace, the reactor-susceptor combination is replaced in the system and heated up to 850°C under a H₂ flow of 2.2 l/min for at least one hour. The etched reactor is ready for the following growth run of GaAs on Si. For the intentional creation of microcracks on specific locations (where no devices are present), the topography of the SiO₂ mask is of particular importance. For this purpose the SiO₂ mask comprises at least one vertex directed towards a cleave direction of the GaAs crystal (namely the [0 1 1] or [0 1 1 ] direction). In a particular embodiment said vertex is symmetric relative to this direction. An example of such a mask is shown in figure 5. The shaded part represents the substrate covered by a SiO₂ mask. Inside the border of the SiO₂ mask, on the uncovered substrate (also called window), active devices can be fabricated. Vertices (for example being indicated by numerals 18 and 19) in the SiO₂-mask are directed towards the cleave direction of the crystal namely towards the [0 1 1] or the [0 1 1 ] direction. As a result of said vertices in the mask, microcracks appear in the GaAs layer deposited in the opening in the mask. These microcracks extend into the cleave directions of the crystal (shown by the dashed lines in figure 5). The microcracks diminish the material stresses on the one hand and do not affect device operation since there location is well known and devices can be fabricated so that these microcracks are situated outside the devices active regions. Figure 5 represents one possible mask topography. Other configurations are possible, other examples are illustrated in figures 7 and 8. It will be clear that at least one vertex directed into the crystals cleave direction is required to obtain the induced microcracks according to the present invention. Figure 6 is a reproduced photograph showing a Si substrate partially covered by a SiO₂ mask 20. Microcracks extend from one vertex to an opposite vertex. The microcracks extend in the cleave direction of the crystal. Microcracks (very fine vertical lines on the reproduced photograph) are for example indicated by the numerals 21 and 22. Figures 7 and 8 illustrate different kinds of masks, dimensions are given in micrometers. Figure 7 a, b, and c represent masks in which the vertices have the same shape and the same location, only the width of the opening (or window) in the mask is changed. The influence of the spacing of the different vertices can also be studied by means of this mask. The distance in between the vertices is varied between 50 micrometer and 500 micrometer, the width of the window is varied between 100 and 500 micrometer. Figures 7d, figures 8 e and f illustrate masks in which vertices are located on different sides (opposite sides as well as perpendicular sides) of the window in the mask. In figure 8 e, the shape of the vertices is also changed. Figures 8 l and k represent masks having vertices on one side, the vertices are equidistant. In figure 8 l, the shape of the vertices is not identical. Figures 8 g to j represent masks in which the configuration of several vertices is formed in the central part of the opening in the mask. (cross form, linear form, rhomb-shaped). Figure 9 is an enlarged representation of different shapes of vertices. The angle is varied from approximately 20° to approximately 60°. An alternative form of a vertex is also shown. The results of experiments on crack formation are shown in reproduced photographs 10 to 13. In successive growth runs GaAs layers of different thicknesses have been deposited on p-Si substrate using a mask defined in a 100 nanometer thick SiO₂ layer. The crack formation after cooling down of the substrate from growth temperature to room temperature and unloading was studied by means of a Nomarsky interference microscope. The cracks are visible as very fine lines in the monocrystalline zones. The grown structures were also immersed in liquid nitrogen (77K) for 30 sec to look at the eventual propagation of crack formation due to thermal strain at these low temperatures. The cracks are better revealed by a very short etch in a H₂SO₄:H₂O₂:H₂O mixture. Dimensions on the reproduced photographs correspond with dimensions identified on the mask designs of figures 7 and 8. Reproduced photograph 10 shows a 6 micrometer thick GaAs structure (lower structure). The mask used is represented in figure 8 k and 8 l. Crack formation starts from the vertices and continues in the cleave direction. The cracks run parallel. Cracks not initiating from a vertex (initiating spontaneously) are not found. Reproduced photograph 11 shows another 6 micrometer thick GaAs structure. The mask used is represented in figure 8 f. Cracks originating at vertices located at the same or a parallel side of the mask run parallel while cracks originating from vertices at perpendicular sides are perpendicular to eachother. A slight constant misorientation of approximately 3° can be seen between the cracks and the mask edges. This results from a misorientation of the rectangular Si samples induced during the saw procedure. As a result hereof cracks induced from vertices located on opposite sides of the substrate do not meet each other. Reproduced photographs 12 shows cracks formed on a 3 micrometer thick GaAs structure, whereas reproduced photograph 13 shows cracks formed on a 6 micrometer structure. The same mask topography (cfr. figure 8 e) has been used for both structures. The cracks show the same behaviour as hereinbefore described. The number of the cracks crossing the whole monocrystalline area however is higher in a 6 micrometer thick structure than in a 3 micrometer thick structure. Since the mask of figure 8 e is used to obtain the result of reproduced photographs 12 and 13, the influence of the shape of the vertex can also be studied on these reproduced photographs. No consistent relationship between the shape of the vertices and the resulting crack formation is established in these growth experiments. Experiments have also shown that no particular relationship exists between the interspacing of the vertices and the formation of microcracks. Experiments using the masks of figures 8 g to j learn that sharp mask angles in the middle of the growth zone also induce cracks in the epitaxial layer (no reproduced photographs). The masks of figure 8 g, i, and j induce microcracks in two perpendicular directions whereas by means of the mask of figure 8 h, microcracks are induced in only one cleave direction (microcracks originate from the sharp angular point). Experiments hence have shown that microcracks can be introduced in the GaAs layer on a Si substrate in a controlled way. The microcracks originate from vertices in a mask and are not found on other locations. The distance between the wedges does not show much influence. The generation of microcracks is insensitive to the shape of the vertices and the use of perpendicular wedges results in perpendicular microcracks. Variation of the layer thickness influences the density of microcracks. Also cross, linear and rhomb formations in the middle of the mask-window can generate microcracks.

Claims: A method of forming microcracks on predetermined locations in a GaAs layer grown on a Si substrate characterised in that prior to the growth of said GaAs layer said Si substrate is partially covered by means of a mask, said mask being designed with at least one vertex extending approximately into a cleave direction of the crystals of said GaAs layer. A method of forming an integrated electronic device or part thereof by growing GaAs on a Si substrate at a given temperature and subsequent cooling of the substrate characterised in that microcracks are induced in said GaAs layer on locations where the operation of said device or said part thereof is not hampered, said microcracks being induced by prior to the growth of GaAs covering said Si substrate partially by means of a mask that is designed with at least one vertex extending approximately into a cleave direction of the crystals of said GaAs layer. A method according to claim 1 or 2 wherein said designed mask is obtained by deposition of SiO₂ on said Si substrate followed by photolithography and wet chemical etching. A method according to claim 3 wherein said deposition of SiO₂ is formed on said Si substrate by plasma enhanced chemical vapour deposition. A method according to any of the preceding claims wherein said substrate is cleaned prior to said mask formation by means of HF:DI cleaning. A method according to any of the preceding claims wherein GaAs is deposited by means of the metal organic chemical vapour deposition technique. A method according to claim 6 wherein prior to said deposition of GaAs said substrate is cleaned by means of HF:DI cleaning. A method according to any of the preceding claims wherein vertices are provided on one side of an opening in said mask in which GaAs is deposited. A method according to any of the preceding claims wherein in an opening in said mask wherein GaAs is deposited a second mask configuration is provided having at least one vertex directing approximately in a cleave direction of GaAs crystals.

Inventors: ACKAERT ANN MARIE, DEMEESTER PIET MARIE, LOOTENS DIRK URBAIN, VAN DAELE PETER PAUL, ACKAERT, ANN MARIE, DEMEESTER, PIET MARIE, LOOTENS, DIRK URBAIN, VAN DAELE, PETER PAUL
Applicants: AGFA GEVAERT NV, AGFA-GEVAERT NAAMLOZE VENNOOTSCHAP
Application year: 1989
Main classification: H01L21
IPC codes: H01L27, H01L21, H01L33