|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. Process Specialties over the past 8 years has processed well over a million wafers, all shapes, sizes, types and grades and we know what works well and what doesn't. If you use this information you will in a lot of cases be able to save money on Silicon and still get good results in your application.
|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 are hard to find 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
Non-EPI, No Backseal and Site inspected for flatness, meeting a spec of at least .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 (.25um-.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 should be >1 Ohm/cm and the grouping must be tight
an example would be 5-10 Ohm/cm or 10-20 Ohm/cm etc. This will insure you will get a excellent
Thermal Oxidation 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 Thermal Oxides, .25um-.5um Photolithography (with most areas on the wafer having the pattern resolved), accurate CMP Qualification and other high end uses such as particle monitors and 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.
|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 will
be very expensive. The EPI-Prime wafers that are normally available for a reasonable price do have
some minor defects that can 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 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 P-type substrate with a layer of N-type Epitaxial silicon (lightly doped) on the front
surface of the wafer. Both of these wafer types will 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) from outgassing from the backside and contaminating the front side Epi layer (N)
during the high temperature Epitaxial Silicon deposition process. So if you see this Oxide on the
backside of the wafer you know it is an Epi wafer no matter what the vendor told you they were when
you bought them. In our opinion in most cases the first wafer (the one without the Epi layer) will give
you the best results. 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 Thermal Oxides. 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 oxide layer 2-5% but we would not recommend them for highly accurate or very thin
Thermal Oxides. |
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 found they work well for are, submicron .35um-.8um Photolithography (understand of course they are not usually as flat as Prime and the pattern may not resolve in all areas of the wafer). 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. We do not recommend them for very thin or accurate Thermal oxides but of course they work well with most deposited thin films.
|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. Thermal Oxides normally grow 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 .7um-5.0um 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, Again as long as high 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.
|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 preform as good as normal test wafers. Of course the
reclaim wafers will have the same limitations test wafers do when it comes to Thermal Oxidation 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 to 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 Thermal Oxides (with uniformities of 3-7%), Photolithography .7um-5.0um as long as you realize the pattern will not resolve all across the wafer. 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.
© 1996 Process Specialties Inc.