Microtechnology: Material Deposition
This page talk about the different ways to deposit materials in microtechnology (refered to with barbarian words such as lpcvd, sputtering, evaporation - thermal and e-beam, pecvd etc.). It is not at all exhaustive, but corresponds to what I know! It is enough to understand how high is the number of possibility to create a fabrication process. This always start from an already made wafer. Wafer fabrication uses a different scheme than what is introduced here. Here you'll find a description of the technics that allow the deposition of a material layer above another material.
To talk about deposition, it is preferable to make the distinction between kind of materials. Most of time, a fabrication process only involve silicon based materials and metals. But there exists other ones, such as metal oxides, piezolectric materials, III-V semiconductors, etc. We won't talk much about materials here. This would be in another section!! All that needs to be known is that metals require a far lower temperature than what can silicon based materials stand. So the choice of the machine used to deposit a precise material is not only made by material characteristic considerations. I'll distinguish two families of machines:
- Furnace and x-CVD use chemical reactions to get a layer on the substrate.
- PVD, that makes whole molecule bind on the substrate (by a way or by another!).
So, furnaces are used to growth silicon dioxide on silicon. This needs quite high temperatures (about 1100°C) and take some times. Of course, since in microtechnology, everything must be as controlled as possible, dioxide growth furnaces have a controlled ambience. Temperature, pressure are regulated, and gases inside are too! To get an oxide growth, you can use either a dry (pure oxygen) or a wet (water) environment. Yes, i said water! Remember that at 1000°, there is very few chance of anything condensating somewhere... Wet oxidization is faster and allows thicker layers than dry oxidization. But the last one provides a better electrical isolation.
There is another use of furnace in microtechnology. It concerns doping of materials. Sometimes you need to make more conductor a semi-conductor. The physics explanation won't be given here, just accept the fact that you need to insert some atoms in the crystal structure. There are several ways to force the atoms getting inside the material you want to dope. But then, you have to help them getting a place in the crystal mesh. The energy they need to get in place can be provided by heat. So the furnaces can be used to activate dopants inside the material.
A classic furnace can perform this task, but a fast processing helps reducing uncontrolled diffusion of the dopants through the material. So special furnaces exist, with the ability to heat up and cool down fast enough to activate dopants while limiting their diffusion. This process is called Rapid Thermal Annealing (RTA). It uses temperature in the range of 900-1100°C.
All x-CVD devices imply the use of gases with a highly controlled flowing rate. The nature and ratios of the gases flow will determine the nature and quality of the deposited material.
Like its name suggests, APCVD use an atmospheric pressure furnace with gases flow to deposit materials. It allows temperature in the range of a few hundreds degrees.
Low Pressure Chemical Vapor Deposition devices do exactly what they seem to do: deposit materials at low pressure. Reducing the pressure limits gas phase parasite reactions and you get a more uniform layer. This process allows reaching high temperature up to the range of 1000°C and is very common to deposit silicon based material, such as polycrystal silicon (usually called polysilicon, that is silicon, but since it is very tough to get a perfect crystal, silicon is got by kind of an assembly of small crystals), silicon nitride, silicon oxide (not an xydation process, but a true deposition of oxide). This last one allows a lower temperature than oxidization, and so is called Low Temperature Oxide (LTO). LTO is a much poorer isolant than thermal oxyde, but the LPCVD process can deposit easily several microns of oxide, at temperature in the range of 400-500°C.
There are other materials that can be deposited with LPCVD technique, but this page has no intention to be exhaustive. LPCVD is typically a high temperature process and allows deposition of several micrometers thick layers.
In the PECVD devices, gases is ionized by a plasma. The ions are more reactive than a neutral gas, so the deposition can be done at lower temperatures than in LPCVD for example. The materials are often poorer in quality, being porous and having weak electrical isolation properties, but this process allows the use of silicon based material in low temperature fabrication process.
The ALCVD process is a recent technique that aim to deposit a layer as thin as possible, even only one molecule thick layer! The principle is to use two gases: the first one can bind to the surface, and the second one can only bind to the first kind of molecules. If you let the substrate in gas1 for a long time enough for the surface to be completely filled, and you entirely replace the gas1 with the gas2 inside the chamber, you should have, in theory, on a flat surface, a perfect one molecule thick layer.
Chemical vapor deposition use gas chemical reactions to get a material layer deposited on the substrate. Physical vapor deposition directly bind complete molecules to the surface of the substrate. It is used for - for example - depositing metal layers. Once again, there are several techniques to deposit that kind of materials. They all have pros and cons, depending of the quality, uniformity, covering properties of the process.
Sputtering consists in using a plasma to tear off molecules from a target and let them flow to the substrate. It is done under in a chamber in vacuum environment. The covering is very good and it is well adapted to deposit materials on non flat surface. The main con is that plasma can have a tendancy to affect the materials on the surface of the substrate.
Thermal evaporation is a very simple process consisting in using a heating resistance to vaporize the target material so that it condensates on the substrate. It has the lowest covering properties of the three processes introduced here, but this can be interesting for some special process such as lift-off. There is one point to check carefully with thermal evaporation: to avoid contamination of the deposited layer with the resistance material, thermodynamics properties of the target must show a higher vapor pressure phase than the resistance material one. Thermal evaporation is done under high vacuum.
To eliminate the pressure management problem with thermal evaporation, another technique uses an electrons beam to heat the target material and let vaporized molecule condensate on the substrate. This allows use of higher pressure. More materials can be deposited with electrons beam evaporation. The higher pressure also gives a better covering of the structures on the surface.
There are others techniques both for CVD and PVD processes to deposit materials. The ones introduces here are probably the most used ones at present. You can find some other resources throughout internet.
Example of considerations during deposition
As explained earlier, there are a lot of things to care about while planning a deposition step. I will not cover everything here, but just take two examples of the kind of things we must think about.
Most of the deposition technics imply a temperature increase compared to ambiant. This can be light or really high, that's why we talk about cold process and hot process. In every case, these temperature changing can have consequencies on the deposited material, and the underlying material layer.
First of all, the simple thermal treatment that is done to the wafer in the case of CVD deposition, for example. If any of the materials composing the layers on the substrate can't stand the furnace temperature, the wafer is lost, the deposited layer is useless, and, worst of all: depending on the weak material reaction, all of the wafers around and even the chamber itself could be polluted.
Besides of the materials becoming unstable at high temperature, there also evolution of mechanical and electrical properties in the materials. For example, doped materials, that are semiconductors in which atoms have been introduced to alter their electrical properties, are sensitive to temperature treatment: the dopants migrate. Migration of the dopants means a different thickness of doped region, and a lower density of dopants in this region. This can switch the state of a device from efficient to non-operative.
Another point to take care with while depositing materials at high temperature is stress. Materials have a natural residual stress (see the introduction to solid mechanics). But if you combine it with the different thermal expansion coefficient of the substrate and the material, you can increase or decrease the residual stress.
For example, depositing a material at high temperature on silicon needs to take care of polysilicon thermal expansion. If you deposit the same material on a silicon on which you have already deposited a silicon dioxide layer (SiO2), you will see that the dioxide has a lesser thermal expansion and the final stress on the wafer is not the same!
Since stress can lead to cracks, and so to the destruction of the device, this is another point to be very careful about.