WO2016201526A1

WO2016201526A1 - Silicon film and process for forming silicon film

Info

Publication number: WO2016201526A1
Application number: PCT/AU2016/050520
Authority: WO
Inventors: Chee Yee Kwok; Aron Michael
Original assignee: Chee Yee Kwok; Aron Michael
Priority date: 2015-06-19
Filing date: 2016-06-20
Publication date: 2016-12-22

Abstract

A process for forming a thick silicon film comprises depositing a thick film of silicon by ultra-high vacuum electron beam deposition.

Description

SILICON FILM AND PROCESS FOR FORMING SILICON FILM

BACKGROUND

The disclosure relates to low stress, low thermal budget, high thickness silicon films, and a process for forming the films comprising depositing by ultra-high vacuum electron beam evaporation. The film and process are described particularly in relation to use in Micro- Electro-Mechanical Systems (MEMS). However, it will be clear to a person skilled in the art that the process and resultant film can be utilised for alternative uses.

SUMMARY

Disclosed in some forms is a process for forming a thick silicon film, the process comprising depositing silicon by ultra-high vacuum electron beam deposition.

The process allows formation of thick silicon films exhibiting excellent mechanical properties. Thick, low stress films are useful for high performance inertial sensors like accelerometers and gyroscopes as they allow the formation of high aspect ratio structures. Currently, such structures are formed either from SOI (Silicon-On-Insulator) or high temperature Chemical Vapor Deposited (CVD) epi-poly [2-4] or LPCVD based HARPSS (High Aspect Ratio Polysilicon Silicon Structures). However, these technologies may require chemical mechanical polishing or high temperature deposition and annealing, which is not appealing for low thermal budget applications.

In some forms the deposition occurs at a substrate deposition temperature of less than approximately 500°C, or less than approximately 460°C. In some forms the deposition occurs at a substrate deposition temperature of between 350°C and 400°C yet achieve full crystallisation.

Silicon films can be deposited at a low thermal budget using various techniques. They include LPCVD (Low Pressure Chemical Vapor Deposition), PECVD (Plasma Enhanced Chemical Vapor Deposition), sputtering, thermal evaporation. Among these techniques, LPCVD deposited polysilicon films have been extensively investigated and used for MEMS applications. Films deposited at temperatures below 570oC are amorphous and highly compressive. Between the temperature 570oC and 6IO0C, they are semi-amorphous and highly tensile. Above 6IO0C, LPCVD films are fully crystallized and exhibit high

compressive stress. High stress gradient at a low thermal budget is also the characteristics of LPCVD deposited silicon films. High temperature annealing in an inert atmosphere has been effective in reducing the stress in LPCVD polysilicon films. However, such a technique is not compatible with applications that do not tolerate high temperature processing. As a result, other methods compatible with low thermal budget have been reported. For example, LPCVD deposition of alternate thin tensile and compressive layers to form an almost overall stress free multilayered polysilicon film. But, these methods have very low deposition rates. More than 12 hours will be required to deposit just 4μπι thick polysilicon film. Therefore, it is not suitable for thick silicon film formation. RF sputtered, PECVD deposited and thermal evaporated silicon films also suffer from the low deposition rates and difficulty in controlling residual stress, especially stress gradient.

In a second aspect, disclosed is a low stress thick polysilicon film formed by depositing silicon by ultra-high vacuum electron beam deposition.

In a third aspect, disclosed is a process for fabricating a MEMS or EMS device comprising depositing a thick polysilicon film onto an integrated circuit or a CMOS die.

BRIEF DESCRIPTION OF THE FIGURES

The disclosure will now be described in view of the Figures, in which,

Fig. la shows an X-TEM image of a film of one embodiment of the present disclosure;

Fig. lb shows an X-TEM image of a film of another embodiment of the present disclosure;

Fig. lc shows an X-TEM image of a film of another embodiment of the present disclosure;

Fig. Id shows an X-TEM image of a film of another embodiment of the present disclosure; Fig 2 shows a graphical representation of the measured average distance of early crystallization from Si/Si02 interface at various substrate temperatures;

Fig. 3(a) graphically represents the as-deposited film fraction of crystallization as a function of substrate deposition temperatures determined from Raman Spectra;

Fig. 3(b) graphically represents the as-deposited film fraction of crystallization with higher dopant concentrations at various deposition rates determined from Raman Spectra.

Fig. 4 graphically represents the as-deposited film ratio of (110)/(111) oriented grains in one embodiment of the disclosure;

Fig. 5 graphically represents the as-deposited film measured grain sizes at varied substrate temperatures;

Fig. 6 graphically represents the stress and surface roughness of 4μπι thick as- deposited silicon films for various substrate temperatures at lOOnm/min deposition rate;

Fig. 7 graphically represents the as-deposited film stress behaviour of 4μπι thick evaporated silicon as a function of deposition rate at a substrate temperature of 500oC;

Fig. 8a shows an X-TEM image of a film of another embodiment of the present disclosure;

Fig. 8b shows an X-TEM image of a film of another embodiment of the present disclosure;

Fig. 8c shows an X-TEM image of a film of another embodiment of the present disclosure;

Fig. 8d shows an X-TEM image of a film of another embodiment of the present disclosure;

Fig. 8e shows an X-TEM image of a film of another embodiment of the present disclosure;

Fig. 8f shows an X-TEM image of a film of another embodiment of the present disclosure;

Fig. 9 graphically represents stress characteristics of silicon films evaporated at lOOnm/min rate annealed at 600oC for 19 hours. The substrate temperature for each film is indicated on the plot. DETAILED DESCRIPTION OF EMBODIMENTS OF THE DISCLOSURE

In some forms the deposition occurs at a substrate deposition temperature of less than approximately 460°C while achieving full crystallization.

In some forms the silicon is doped polysilicon. In some forms the silicon is doped with boron or phosphorous.

In some forms the silicon is boron doped and the deposition temperature is between approximately 350°C and approximately 460°C. In some forms the silicon is phosphorous doped and the deposition temperature is between approximately 320°C and approximately 400°C.

In some forms the silicon has a low doping level. A low doping level is a low concentration of dopant within the silicon. In some forms the silicon is doped at a concentration of less than or approximately 5x1018/cm3.

In some forms deposition occurs at between 10 - 400 nm/minute.

In some aspects, disclosed is a thick polysilicon film formed by depositing silicon by ultra-high vacuum electron beam deposition. A thick film includes a film of greater than 20μπι, or in some cases a film of greater than 30μπι.

In some forms the process occurs at a substrate deposition temperature of less than approximately 460 degrees Celsius.

In some forms the silicon is doped polysilicon.

In some forms the silicon is doped with boron or phosphorous. In some forms the silicon is boron doped and the deposition temperature is between approximately 350°C and approximately 460°C.

In some forms the silicon is phosphorous doped and the deposition temperature is between approximately 320°C and approximately 400°C.

Complete crystallisation occurs at these low temperatures.

In another aspect, disclosed is a process for fabricating a Micro-Electro-Mechanical Systems device comprising depositing a thick silicon film directly onto an optimised integrated circuit or directly onto a CMOS die.

In yet another aspect, disclosed is a Micro-Electro-Mechanical Systems device fabricated by the methods described.

In another aspect, disclosed is a process for fabricating a piezoelectric actuator comprising depositing a thick silicon film by ultra-high vacuum electron beam deposition, and depositing a piezoelectric film on at least one surface of the silicon film. In some forms the piezoelectric film is deposited on the two opposing surfaces of the film.

A piezoelectric actuator fabricated by the method described.

The process avoids high temperature processing for silicon films with low stress. This allows for a thick, low thermal budget silicon film to be formed even in heat sensitive applications. It has potential benefits in building MEMS structures directly on top of optimised integrated circuits allowing for low thermal budget and fewer steps in production of MEMS.

The low temperature low stress application means that layers such as a piezoelectric layer are not destroyed by the deposition process. Processing can occur post CMOS/MEMS processing.

Figures 1 through 8 show the results in the as-deposited near intrinsic or low doping concentration silicon films. Referring to Fig. 1, disclosed is a silicon film formed by ultra-high vacuum electron beam deposition. Fig. la shows a film evaporated at substrate temperatures of 200°C. Fig. lb shows a film evaporated at substrate temperatures of 400°C. Fig. lc shows a film evaporated at substrate temperatures of 500°C. Fig. Id shows a film evaporated at substrate temperatures of 625°C. The film in Fig. la is amorphous. That in lb is partially crystallised, that in lc and Id is fully crystallised.

Fig. 1 shows the X-TEM images for the silicon films evaporated at a rate of lOOnm/min for various substrate temperatures. Fig la shows a film deposited at a substrate temperature of 200oC, Fig lb shows a film deposited at a substrate temperature of 400oC, Fig lc shows a film deposited at a substrate temperature of 500oC, and Fig Id shows a film deposited at a substrate temperature of 625oC. Films evaporated at less than 300oC were amorphous. Those evaporated between 300oC and 400oC were semi-amorphous. Films deposited at 500oC were fully crystallised. Those deposited at 575oC or above were fully crystallised with a coarse grain

In some forms, the semi-amorphous E-beam evaporated silicon films have columnar microstructure on top of amorphous silicon layer as seen from Fig 1(b). On the other hand, semi- amorphous silicon films deposited by LPCVD have quite a different microstructure consisting of an amorphous silicon layer on top of devitrified elliptical grains.

The average distance at which early crystallization occurs above the Si/Si02 interface is measured for various substrate deposition temperatures and plotted in Fig. 2 which utilises a near-intrinsic silicon film. The distance represents the average thickness of the amorphous layer above the Si/Si02 interface. As can be seen from Fig. 2, the amorphous layer thickness reduces quite sharply initially for temperatures below 400°C with the onset of crystallization and then completely disappears at 500oC. The amorphous layer is reduced from 2500nm at 370oC to 600nm with only 30oC increase in substrate temperature. The reduction in the amorphous layer is only lOOnm when the substrate temperature increases from 430oC to 460oC.

Fig. 3(a) shows the determined fraction of crystallization of the as-evaporated near- intrinsic silicon films with dopant concentration of approximately 5 x 1014 or 5 x 1015/cm3 at various substrate deposition temperatures. The films deposited at 300oC and below are completely amorphous with zero percentage of crystallization while those evaporated at 500oC and above are fully crystallized showing 100% of crystallization. Between 300oC and 500oC, the films are partially crystallized with percentage of crystallization exceeding 90% at 460oC. These characteristics of the evaporated silicon films are quite different from those silicon films formed using other deposition techniques such as LPCVD. LPCVD silicon films are known to be amorphous even at 570oC and only fully crystallized above 610oC. Hence, the E-beam evaporated silicon films uniquely display early crystallization, that is, crystal grain formation at comparatively lower substrate temperature. Such behaviour is particularly attractive for low thermal budget applications.

Fig. 3(b) shows the determined fraction of crystallization of the as-evaporated silicon films with higher dopant concentrations at various deposition rates as a function of substrate deposition temperatures demonstrating that full crystallisation is achieved at 400°C or lower with the appropriate dopant concentration and deposition rate. An example of this is phosphorus concentration of 5x1018/cm3 at a deposition rate of 50nm/min.

To observe the effect of the substrate temperature during deposition on the

composition of grain orientations, the ratio of intensity peak for (110) oriented grains to that of (111) is plotted as a function of substrate temperature and presented in Fig. 4. The figure indicates that the ratio of (110) oriented grains to that of (111) increases exponentially with an increase in substrate temperature up to a substrate temperature of 575oC before it drops down quite sharply. A similar trend has also been observed for both (110)/(331) and

(110)/(311) crystal orientations. This behaviour of E-beam evaporated silicon films is considerably different from those of LPCVD deposited silicon films even though similar crystal orientations exist in both cases. Semi-amorphous LPCVD silicon films formed typically at a substrate temperature ranging from 570°C to 610°C tend to have a higher proportion of (111) oriented grains while fully crystallized films above 610°C have predominantly (110) oriented grains.

Average grain size is obtained from TEM measurements for films deposited at various substrate temperatures and shown in Fig. 5. One can identify two regions from the figure. The first region covers the substrate temperatures ranging from 370oC to 575oC. It is characterized by fine-grain formation and gradual increase in the average grain sizes. As the substrate temperature goes above 575oC, a second region that is characterized by a coarse- grain film formation is obtained. The average grain size in the second region is increased by 250% with only 50oC change in temperature.

Fig. 6 shows measured average residual stress of the E- beam evaporated near- intrinsic silicon films for various substrate temperatures. The amorphous films deposited at 300°C and below are tensile with the stress reducing as the substrate temperature increases. A rapid change in stress can be observed as the substrate temperature changes from 300oC to 370oC. This can be attributed to the onset of crystal grain formation in the film. The semi- amorphous films evaporated at 370oC and below 500oC exhibit low stress that vary from tensile at lower temperatures to compressive at higher temperatures. From observations of the TEM images, the semi-amorphous E-beam evaporated silicon films have a crystallized layer over an amorphous layer. The grain formation in the crystallized layer involves competitive crystal grain growth that results in the development of compressive stress in the layer. As the thickness of the crystallized layer increases with the substrate temperature above 370°C, the film becomes proportionally less and less tensile and eventually becomes compressive.

The stress-substrate temperature characteristic of the E-beam evaporated silicon films is very different from that of LPCVD silicon films in three ways: (i) The stress in as- deposited amorphous LPCVD silicon films is highly compressive while that of E-beam evaporated amorphous silicon films is highly tensile, (ii) The semi-amorphous LPCVD silicon films are highly tensile while those of evaporated semi-amorphous silicon films are low tensile or compressive, (iii) For as-deposited fully crystallized silicon films, LPCVD films are highly compressive while those of fully crystallized E-beam evaporated films exhibit relatively low compressive stress. It is important to note that the compressive stress in the fully crystallized E-beam evaporated silicon films change gradually with substrate temperature. Hence, it is more controllable. As the substrate temperature increases from 300oC to 370oC, a jump in surface roughness can be observed. This is related to the occurrence of film crystallization between these temperature ranges. As the deposition temperature increases further, the surface roughness increases only slightly up to 575oC and increases dramatically as the substrate temperature rises to 625oC. The stress levels in the as-deposited silicon films can further be controlled by the deposition rate. Fig. 7 shows the film stress for various deposition rates at a substrate temperature of 500oC. Results indicate that the compressive stress in the film reduces as the deposition rate increases. The stress in the film is only -25MPa at the deposition rate of 400nm/min.

Smoother films can be formed by reducing deposition rate. The rms surface roughness is reduced by more than 50% when the deposition rate goes down from lOOnm/min to 50nm/min. Although the surface roughness increases as the deposition rate rises, the increment in roughness is only small and even marginal at higher deposition rates. Similar trends have also been observed at other substrate temperatures.

The X-TEM images of E-beam evaporated silicon films formed at substrate temperatures of 200oC, 370oC and 430oC before and after annealing at 600oC are shown in Fig. 8. Annealing causes the amorphous and semi- amorphous films to fully crystallize. In the case of the amorphous film in Fig 8(a), the microstructure of the film after annealing as seen in Fig 8(b) is random and uniform across the cross-section. It is characterized by large grains, as large as Ι μπι. However, two different textures can be identified with the annealed semi- amorphous films in Fig. 8(d) and (f): fine columnar grains on the top and random large grains on the bottom. The fine columnar grains are from the crystallized layer of the film before the annealing and do not show any apparent change after the annealing. High tensile stress is not desirable in some applications as it may lead to film cracking and peeling. However, for fully and almost fully crystallized silicon films, annealing can help to relieve the compressive stress by introducing tensile stress.

Fig. 9 shows the post-deposition annealing behavior at 600°C for Si films deposited at 430oC, 460oC, 500oC, 575oC and 625oC substrate temperatures for various durations. The compressive stress reduces and even becomes tensile as the annealing time progresses. Most of the change in stress occurs during the first one hour of annealing and from there on further stress change is minimal. The reduction in the compressive stress is associated with the occurrence of recrystallization, which is known to cause film shrinkage in fine-grained polysilicon films and hence introduce tensile stress. It should be noted that this phenomena is different from the as-deposited film where the competitive crystal grain formation during deposition produced compressive films. The introduced tensile stress is moderate and proportional to the difference between the deposition substrate temperature and the annealing temperature. The higher the difference is, the higher the introduced tensile stress is.

Annealing the films evaporated at 460oC, 500oC and 575oC has introduced an additional 70MPa, 50MPa, and 20MPa tensile stresses, respectively. Annealing the film evaporated at 430oC substrate temperature resulted in comparatively higher tensile stress.

The surface morphology study on the annealed films has shown that the annealing does not affect the surface morphology of the evaporated films appreciably.

While the disclosure speaks of using the process of deposition of thick film polysilicon for MEMS fabrication, the process can be utilised for alternative polysilicon film applications.

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

In the claims which follow and in the preceding description of the disclosure, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Claims

Claims:

1. A process for forming a thick silicon film, the process comprising:

depositing a thick film of silicon by ultra-high vacuum electron beam deposition.

2. A process as defined in claim 1, wherein the deposition occurs at a substrate deposition temperature of less than approximately 500oC.

3. A process as defined in claim 2, wherein the substrate deposition temperature is less than approximately 460oC.

4. A process as defined in any of the preceding claims, wherein the silicon is doped polysilicon.

5. A process as defined in claim 4, wherein the silicon is doped with boron or phosphorous.

6. A process as defined in claim 5, wherein the silicon is boron doped and the deposition temperature is between approximately 350oC and approximately 460oC.

7. A process as defined in claim 5, wherein the silicon is phosphorous doped and the deposition temperature is between approximately 320°C and approximately 400°C.

8. A process as defined in any one of claims 4 through 7, wherein the silicon has a low doping level.

9. A process as defined in claim 8, wherein the silicon is doped at a concentration of less than approximately 5x1018/cm3.

10. A process as defined in any of the preceding claims, wherein the deposition rate is between 10 - 400 nm/minute.

11. A thick polysilicon film formed by depositing silicon by ultra-high vacuum electron beam deposition to a thickness of greater than 20μπι.

12. A film as defined in claim 11, wherein the process occurs at a substrate deposition temperature of less than approximately 460°C.

13. A film as defined in claim 11 or 12, wherein the silicon is doped polysilicon.

14. A film as defined in claim 13, wherein the silicon is boron doped and the substrate deposition temperature is between approximately 350°C and approximately 460°C.

15. A film as defined in claim 13, wherein the silicon is phosphorous doped and the deposition temperature is between approximately 320°C and approximately 400°C.

16. A process for fabricating a Micro-Electro-Mechanical Systems or a Nano-Electro Mechanical Systems device comprising depositing a thick silicon film directly onto an optimised integrated circuit.

17. A Micro-Electro-Mechanical Systems device or a Nano-Electro Mechanical Systems device fabricated by the method defined in claim 16.

18. A process for fabricating a piezoelectric actuator comprising depositing a thick silicon film by ultra-high vacuum electron beam deposition, and depositing a piezoelectric film on at least one surface of the silicon film.

19. A process as defined in claim 18, comprising depositing a piezoelectric film on two opposing surfaces of the silicon film.

20. A piezoelectric actuator fabricated by the method of claim 18 or 19.