Patent Application
20120097105
The present invention relates to a molecular beam epitaxy apparatus for producing wafers of semiconductor material.
Epitaxial gallium nitride (GaN) semiconductor has shown very interesting semi conducting properties for high power and high frequency applications such as high luminescence LED or radiofrequence transistors.
The epitaxy of thin layers of GaN can be done by two main techniques namely, Molecular Beam Epitaxy (MBE) and Metal Organic Chemical Vapor Deposition (MOCVD).
In the case of molecular beam epitaxy technique, atoms or molecules of elements or compounds, for instance Gallium, is evaporated from a melted metal in an effusion cell and the nitrogen element can be supplied by molecular nitrogen cracked by a plasma source or from ammonia in gaseous state from a gas injector which decomposes chemically at the surface of the substrate by thermal activation.
A molecular beam epitaxy apparatus usually comprises a loading compartment wherein a substrate to be coated is introduced, a compartment wherein the substrate is dehydrated, a compartment wherein an operation of deoxidation of the substrate is performed, a growth chamber and a pumping means pumping the residual elements of the growth chamber.
The substrate, silicon for instance, is introduced in the growth chamber wherein a vacuum of about 10−8 Pascal is performed. The substrate is heated at a temperature between about 300° C. and 1100° C. Then, gaseous precursor like ammonia is injected in the growth chamber and the metal in the effusion cell is heated to be evaporated. Gaseous ammonia reacts with the evaporated metal at the surface of the substrate to form an epitaxial layer of GaN.
A part of the ammonia which is not cracked during the growth process is trapped on the cryogenic panel covering the inner surface of the lateral wall of the growth chamber and surrounding the process area.
Because of:
the ammonia molecules adsorb easily on the growth chamber walls, which are usually sensibly at room temperature, and desorb with a time constant of minutes to hours.
It induces a no negligible vapor pressure and flow of ammonia in the system when ammonia injection is stopped. This residual ammonia may disturb the other processes ran in the growth chamber when ammonia is not desired.
As an example, when silicon substrates are used, an important background of ammonia pressure during the heat-up of the substrate nitrides its surface converting silicon into silicon nitride and so affects its electrical properties.
The object of the present invention is to provide a molecular beam epitaxy apparatus enabling to reduce efficiently the pressure of gaseous precursor in the growth chamber under a pressure limit, when gaseous precursor is not desired. This pressure limit corresponds to a gaseous precursor pressure under which the perturbation of the growth process by the precursor is limited or avoided.
To this end, the invention concerns a molecular beam epitaxy apparatus for producing wafers of semiconductor material comprising a substrate covered by a layer of material, said device further comprising:
According to the invention, the molecular beam epitaxy apparatus comprises an insulation enclosure covering at least the inner surfaces of the growth chamber walls, said insulation enclosure comprising cold parts having a temperature Tmin inferior or equal to melting point of the gaseous precursor, and hot parts having a temperature Tmax superior or equal to a temperature wherein the desorption rate of the gaseous precursor on said hot parts is at least 1000 times greater than the adsorption rate of said gaseous precursor.
The temperature of cold parts Tmin is sufficiently low to trap the gaseous precursor and not release this last. The temperature of hot parts Tmax is sufficiently high for avoiding fixation of gaseous precursor thereof.
The invention permits to shield the process area from the gaseous precursor desorbing from the growth chamber walls which are usually at room temperature (usually a temperature not comprised in the range between Tmin and Tmax), reducing or eliminating the perturbation of the process when gaseous precursor is not needed. The invention allows limiting the residual vapor pressure of the gaseous precursor (ammonia for instance) into the growth chamber. Pollution of the substrate, the effusion cell, the gas injector and the pressure gauge, is avoided.
According to various embodiments, the present invention also concerns the characteristics below, considered individually or in all their technical possible combinations:
The description of the invention is illustrated by the following drawings in which:
The molecular beam epitaxy apparatus comprises a growth chamber 1 surrounding a process area 2. The growth chamber 1 comprises a lateral wall 3, a lower wall 4 and an upper wall 5. Each of these walls has an inner surface. The growth chamber walls 3, 4, 5 form a unitary assembly having the general shape of a closed cylinder.
The molecular beam epitaxy apparatus comprises a main cryogenic panel having at least a lateral part 10 covering the inner surface of the lateral wall 3. This main cryogenic panel 10 is cooled with a cryogenic fluid like liquid nitrogen, for example. Glycol can also be used as cryogenic fluid. Preferably, the lateral part of the main cryogenic panel 10 has a cylindrical shape.
The molecular beam epitaxy apparatus comprises a sample holder 6 which can be eventually surrounded by a secondary cryogenic panel 7 having a cylindrical shape. The sample holder 6 is positioned at the top of the growth chamber 1 and supports the substrate. It comprises heating means for heating the substrate at a temperature between 300° C. to 1100° C.
The molecular beam epitaxy apparatus comprises at least one effusion cell 8 able to evaporate atoms or molecules of elements or compounds, and a gas injector 9 able to inject a gaseous precursor into the growth chamber 1. The effusion cell 8 and the gas injector 9 are positioned at the bottom of the growth chamber 1.
A part of the gaseous precursor is able to react with the evaporated atoms or molecules of elements or compounds on the surface of the substrate to form an epitaxial layer of material like GAN for example, and an other part of the gaseous precursor is not consumed.
The substrate can be silicon, silicon carbide, sapphire, aluminium nitride, diamond, gallium nitride templates, for instance.
The atoms or molecules of elements or compounds to be evaporated can be a metal of the group III and the element to be injected can be an element of the group V, for instance.
Preferably, the molecular beam epitaxy apparatus is used to obtain an epitaxial layer of GAN at the surface of a silicon substrate, the element of group III being gallium and the gaseous precursor comprising an element of the group V being ammonia (NH3).
Each effusion cell 8 can comprise a movable shutter (not represented) and being made of various materials like alumina, for instance.
The molecular beam epitaxy apparatus comprises pumping means 11 connected to the growth chamber 1 and able to provide high vacuum capability.
The pumping means 11 can comprise a pumping duct 30 having a wall 15. The pumping duct 30 is connected to a pumping device 21 by a first end 12 and emerges into the growth chamber 1 by a second end 13.
The pumping device 21 can be a primary pump associated with a secondary pump. The lateral part of the main cryogenic panel 10 is provided with a hole 22 positioned in front of the second end 13 of the pumping means 11.
According to an embodiment of the invention, the molecular beam epitaxy apparatus comprises insulation enclosure 14 covering at least the inner surfaces of the growth chamber walls 3, 4, 5.
The insulation enclosure 14 has cold parts and hot parts having temperatures such that the adsorption/desorption exchange process of the gaseous precursor is limited or avoided around the process area 2, in order to limit the partial pressure of the gaseous precursor in the growth chamber 1, when gaseous precursor is not desired.
The insulation enclosure 14 covers completely or nearly the inner surfaces of the growth chamber walls 3, 4, 5, in order to insulate the process area 2 from the inner surfaces of the growth chamber walls 3, 4, 5.
The temperature of cold parts Tmin is inferior or equal to the gaseous precursor melting point in order to trap the gaseous precursor not consumed on the cold parts.
The temperature of cold parts Tmin is such that the desorption time constant is high leading to negligible flow of ammonia in the process area 2. This temperature must be at least below to the melting point of the gaseous precursor, ammonia for instance.
For example, with a temperature below the ammonia melting point at −78° C., desorption time constant is reduced by 5 orders of magnitude compared to room temperature. Thus, the contaminating flow into the process area 2 is reduced by this factor hundred thousand too. The temperature of cold parts must be sufficiently low to trap ammonia and not release this last.
Advantageously, at the temperature of liquid nitrogen (−196° C.), there is a gain of 12 orders of magnitude.
Most of gaseous ammonia is trapped by cold parts. For ammonia, the pumping capacity of cold parts of insulation enclosure 14 is about 99% of the total pumping capacity. The residual species, which are not trapped by the cold parts, like nitrogen, carbon, water and hydrogen are pumped by the pumping means 11. The temperature of the hot parts Tmax are such that the desorption rate of gaseous precursor on hot parts is at least 1000 times greater than the adsorption rate of gaseous precursor. Said differently, the temperature of hot parts Tmax is sufficiently high for avoiding fixation of gaseous precursor thereof.
The temperature of hot parts Tmax is superior or equal to a temperature wherein the desorption time constant of the gaseous precursor is less than durations of transition periods of the processes, in order to limit the adsorption of the gaseous precursor on hot parts.
Typically, with transition time in the range of the minute, desorption time constant must be below the second. Concretely, on usual materials like stainless steel or tantalum, it needs a temperature Tmax higher than +100° C.
To resume, when using ammonia as gaseous precursor, the temperature of cold parts Tmin is inferior or equal to −78° C., and the temperature of hot parts Tmax is superior or equal to +100° C. The surfaces of the insulation enclosure 14 having a temperature between −78° C. and +100° C. are limited and preferably eliminated. Said differently, the temperature of the insulation enclosure 14 is not comprised in the range between Tmin and Tmax, (−78° C. and +100° C. for ammonia).
Theses temperatures are different when using an other gaseous precursor like oxygen, for example.
The residual pressure of ammonia must be less than 10−7 Pascal, when ammonia is not needed, i.e before and after injection of gaseous ammonia in the growth chamber 1. This is the limit pressure under which the growth process is not perturbed, nor stopped. Preferably, the residual pressure of ammonia is about 10−8 Pascal. For comparison, during growth process, when gaseous precursor is injected, the residual pressure of ammonia is about 10−5 Pascal.
The main cryogenic panel comprises the lateral part 10 covering the inner surface of the lateral wall 3, a lower part 23 covering the inner surface of the lower wall 4 and an upper part 26 covering the inner surface of the upper wall 5.
The cold parts of the insulation enclosure 14 comprise the lateral part 10, the lower part 23, and the upper part 26 of the main cryogenic panel.
The lower part of the main cryogenic panel 23 is provided with a first hole 24 for the effusion cell 8, and a second hole 25 for the gas injector 9.
The upper part of the main cryogenic panel 26 is provided with a hole 27 crossed by the sample holder 6 and eventually by the secondary cryogenic panel 7 which surrounds this last.
In an other possible embodiment, the cold parts of the insulation enclosure 14 comprise a cryogenic panel 16 covering completely or nearly the wall 15 inner surface of the pumping duct 30 of the pumping means 11.
We mean by “covering” the fact that the cryogenic panel is disposed along the walls with a space d between these inner surfaces and the cryogenic panels, such that condensed precursor on cold parts does not touch the walls. The space d is between 0.5 cm and 5 cm.
Preferably, the lateral part of the main cryogenic panel 10, the lower part of the main cryogenic panel 23 and the upper part of the main cryogenic panel 26 of the growth chamber 1 form a unitary assembly wherein the cryogenic fluid circulates.
This unitary assembly is adapted to the form of the growth chamber walls 3, 4, 5.
The parts of the main cryogenic panel is designed to cover the most of the inner surfaces of the growth chamber walls 3, 4, 5. The holes in this cryogenic panel is filled with hot parts including, further, the sample holder 6, the effusion cell 8, the gas injector 9 comprising heating means and the shutter of the effusion cell.
Alternatively, the lateral part 10, the lower part 23 and the upper part of the main cryogenic panel 26 are three distinct cryogenic panels.
As seen on
In a preferred manner, more than 80% of the growth chamber walls are shielded by hot and a cold parts of the insulation enclosure 14 facing the process area 2.
Alternatively, the lateral part of the main cryogenic panel 10 comprising an upper end 28, the cold parts of the insulation enclosure 14 comprise a first thermal wing 17 linked to the upper end 28 of the lateral part of the main cryogenic panel 10, as represented in
The cold parts of insulation enclosure 14 comprise equally a second thermal wing 18 linked to the outer wall of the secondary cryogenic panel 7.
These two thermal wings 17, 18 extend transversally. The second thermal wing 18 extends transversally from the outer wall of the secondary cryogenic panel 7. These two thermal wings 17, 18 surround the secondary cryogenic panel 7 and are close one to each other such as to insulate the process area 2 from the upper wall 5 of the growth chamber 1.
A space 29 is provided between the two thermal wings 17, 18. Preferably, the thermal wings 17, 18 are ring-shaped.
The first thermal wing 17 extends from the lateral part of the main cryogenic panel 10 to the vicinity of the secondary cryogenic panel 7. A space is provided between the end of the first thermal wing 17 and the wall of the secondary cryogenic panel 7.
The second thermal wing 18 is positioned under the first thermal wing 17. The second thermal wing 18 is shorter than the first thermal wing 17 and is positioned in front of the space provided between the end of the first thermal wing 17 and the wall of the secondary cryogenic panel 7.
The thermal wings 17, 18 act as thermal conductor and are cooled by the cryogenic panels.
The hot parts of the insulation enclosure 14 comprise a third thermal wing 19 linked to the gas injector 9. The third thermal wing 19 is positioned between the second hole 25 of the lower part of the main cryogenic panel 23 and the inner surface of the lower wall 4 of the growth chamber 1. The third thermal wing 19 surrounds the gas injector 9 such as to insulate the process area 2 from the inner surface of the lower wall 4 of the growth chamber 1.
The third thermal wing 19 extends from the surface of the gas injector 9. It can be ring-shaped. Its width corresponds approximately to the diameter of the second hole 25 of the lower part of the main cryogenic panel 23.
The circulation of ammonia in vapor state is limited between the third thermal wing 19 and the lower part of the main cryogenic panel 23.
The cold parts of the insulation enclosure 14 comprise a fourth thermal wing 20 inserted into the second hole 24 of the lower part of the main cryogenic panel 23 and extending from the lower part of the main cryogenic panel 23. The fourth thermal wing 20 surrounds the gas injector 9 in order to insulate the process area 2 from the inner surface of the lower wall 4 of the growth chamber 1. The fourth thermal wing 20 can be ring-shaped.
In an other possible embodiment of the invention, the sample holder 6 is lower than the lower end 31 of the secondary cryogenic panel 7 for limiting or avoiding thermal exchanges between the sample holder 6 and the lower end 31 of the secondary cryogenic panel 7.
The invention allows limiting the residual vapor pressure of gaseous precursor into the growth chamber 1 when injection of gaseous precursor is stopped. Thus, the growth process is not perturbed.
| Number | Date | Country | Kind |
|---|---|---|---|
| 09305570.5 | Jun 2009 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP10/58569 | 6/17/2010 | WO | 00 | 12/19/2011 |
