Choosing Silicon Wafers for your project can be very confusing. In this section we will help make your decision a more informed one. At this time we will not be going into great detail about choosing wafers for device processes because this decision is normally addressed on a device by device and a process by process basis.

This Information is geared to be used in the selection of wafers that will eventually be used mainly for R & D, semiconductor tool development, and related applications. This includes the building of both complex and simple test structures. Please remember this information is not coming from any book, it comes from experience.

Common Types of Wafers

PRIME WAFERS

Description: There is a lot of confusion about what is considered "Prime" in the wafer business and there are definitely many grades of prime wafers being offered for sale today. A true prime wafer as far as we are concerned is a device quality wafer that any major fab could use to build the latest technology semiconductor devices on. These wafers can be hard to find in small quantities and if you do find them they are very expensive. For most projects of a very exacting nature this is the "Prime" that we recommend, i.e. wafers that are Non-EPI, No Backseal and Site inspected for flatness, meeting a spec of at least 0.3um on a 20mm x 20mm site (backside referenced). If you hear the words "Global Flatness" beware if you intend to use the wafers for high end (0.25um-0.5um) Photomasking. Also insist on the wafers being double bagged and that the outer bag is the original foil bag from the manufacturer. If you can, ask that the original factory labels be left on the cassette. One more spec that is important is resistivity, if you are paying for good prime wafers the resistivity grouping must be tight. An example would be 5-10 Ohm/cm or 10-20 Ohm/cm etc. This will insure you will get an excellent film process with a very tight uniformity spec. Our favorite manufacturers of Prime wafers are Wacker, SEH and Sumitomo.

Uses for Prime Wafers: The wafers described above may be used for many high end projects such as very uniform LPCVD Nitrides, 0.25um-0.5um Photolithography (with most areas on the wafer having the pattern resolved), accurate CMP Qualification and other high end uses such as particle monitors. Normally the polish is of a very high quality so if you are concerned about surface roughness these wafers will work very well for you. These are the wafers we choose to build thin film standards on.

EPI-PRIME WAFERS

Description: Again, a true Prime wafer with a perfect or near perfect Epitaxial layer is a wafer that is suited for manufacture of advanced semiconductor devices. These wafers if you can find them in small quantities can be very expensive. The EPI-Prime wafers that are normally available for a reasonable price can have some minor defects that could influence the results of the project you are working on. EPI-Prime wafers usually come in two flavors. The first kind is the EPI substrate wafer. This is a highly doped N or P-type wafer usually <1 Ohm/cm without the Epi layer. The second is the true EPI wafer. This wafer also has a highly doped N or P-type substrate with a layer of N or P-type Epitaxial silicon (various doping) on the front surface of the wafer. Both of these wafer types may have what is called an oxide backseal on them usually 3-5,000Å. This oxide is placed on the backside of the wafer before the Epitaxial Deposition to prevent Boron (P) or Phosphorous (N) from outgassing from the backside and contaminating the front side Epi layer during the high temperature Epitaxial Silicon deposition process. So if you see this Oxide on the backside of the wafer you know it is typically an Epi wafer. In our opinion in most cases the first wafer (the one without the Epi layer) will give you the best results for R&D processing. The wafer that has the EPI layer can have problems in the Epi layer such as spikes, crowning, slip, resistivity non-uniformity, haze and other issues which make these wafers unsuitable for some applications including thin and /or very uniform film depositions. On the other hand they can be very flat and very suitable for sub-micron photolithography. The Prime EPI substrate wafers (the ones without the Epi layer) can also be very flat and normally will produce a fairly uniform deposited layer 2-5% but we would not recommend them for highly accurate or very thin films.

Uses for EPI-Prime and Prime EPI substrates: Both of these types of wafers can be of great use to Semiconductor tool companies looking for a cheaper alternative to the high priced Prime wafers discussed earlier on the page. Uses we have found they work well for are, submicron 0.25um-0.50um Photolithography. Also EPI-Prime and Prime EPI Substrates seem to work very well in CMP characterization and in most cases the Prime EPI substrates work well as particle monitors. They work well with most deposited thin films.

TEST WAFERS

Description: The term "Test Wafer" has become a catch all for many different types of wafers. Test wafers can be wafers that fell out of one or more specifications at the factory in the attempt to make Prime wafers and are now test. They can get their start (as most test do nowadays) as Coin Roll. What is coin roll you ask? It is the material that falls out at all different steps of the Silicon wafer manufacturing process. Some of the wafers are unpolished. Most are scratched. They are different resistivities etc., etc. Silicon vendors buy these wafers in large Styrofoam lugs with hundreds of wafers stacked on top of each other inside. These stacks resemble a giant stack of coins hence the name "coin roll". Next these wafers are sorted for type and resistivity and sent to a polishing vendor to polish the scratches and other defects out and the Test wafer is born. Do not misunderstand. These wafers work fine as test, but remember test wafers have no flatness spec and the resistivity range they are normally sold under is 0-100 Ohm/cm (a very wide range). As long as your wafer vendor will guarantee they will meet SEMI Specifications for test wafers everything should be fine. Also most test wafers sold have no backside specification so don't panic if you see scratches or other defects on the backside of your test wafers. Of course if you can find the factory sealed test wafers they are normally the best choice for your money, the backsides are usually better and the polished front side is normally of a higher quality (even though they cost a little more).

Uses For Test Wafers: Test wafers are acceptable for a wide range of uses when low cost material is needed for testing equipment running marathons and other uses where large numbers of wafers are needed. Films normally deposit well on test wafers as long as 3-7% variations in uniformity are not an issue. A lot of our customers use test wafers for Photolithography 0.7um-5.0um and this is not a problem, as long as you realize the pattern will not be resolved in all areas of the wafer (because of the non-flat nature of test wafers). Of course most deposited and sputtered thin films usually go down well on test wafers, (as long as tight uniformity is not an issue). Even if you are doing very high end R&D work it is much more cost effective to develop a process using test wafers and then do the final checks using Prime or Epi-Prime wafers.

RECLAIMED WAFERS

Description: The name Reclaim sometimes strikes fear into the heart of semiconductor people and it really should not! Years ago some poor reclaim vendors used to turn out some pretty bad product that unfortunately gave reclaim wafers a bad name. Today there are many good reclaim vendors that turn out a very good product, in many cases as good as the coin roll test described in the last section. Reclaim wafers are used prime and test wafers that have been stripped of thin films and re-polished to remove all patterns and scratches. Todays reclaim vendors will sort your wafers for thickness and resistivity and in most cases reclaim wafers will perform as good as normal test wafers. Of course the reclaim wafers will have the same limitations test wafers do when it comes to film deposition and Photolithography. Remember reclaim wafers (like test) are not flat and normally the resistivity spec they are sold under is very wide, usually 0-100 Ohm/cm. Reclaim wafers can be a little thinner but most reclaim vendors will pull and dispose of the ones that are too thin and might cause you problems.

Uses for Reclaim Wafers: Reclaim wafers can be used for many of the same things as test wafers. Including film deposition (with uniformities of 3-7%), Photolithography 0.7um-5.0um as long as you realize the pattern will not resolve all across the wafer. For thin film deposition and many other uses, just do not expect the kind of results you will get from factory sealed Prime, Epi-Prime or test wafers.