Logitech
doesn't only provide lapping and polishing equipment. We also supply
bonding, impregnation, cutting and test equipment. 
In the field of materials processing, it is sometimes difficult to distinguish when it is best to use a lapping process over a grinding process. While it all depends on what you will doing with the material once it has been processed, it is wise to assume that the better the surface finish the easier it will be to use the sample in later processing or fabrication processes.
Logitech's vast experience in materials processing ensures we have developed techniques that give the best results, time and time again. As a result, here is our definition of why you should always try and gain the best results possible.
WAFER GRINDING
Q: What happens when a sample is ground?
A: Grinding is by definition an aggressive machining process, which is suited to ductile materials such as metals. Brittle materials, including elements in crystal form, transmit the stresses at the cutting point through the rest of the sample, producing a layer of shattered material. This is typically three times as deep as the minimum particle size in the grinding wheel. Consequently, even a very fine 35 micron wheel can leave a damaged layer up to 100 microns deep on the sample surface.
Q: So what does this mean for wafer backthinning?
A: It means that the material used should be fairly robust, to endure the high speed, high load grinding process. It is well-suited to Silicon preparation, but Gallium Arsenide is a much more brittle material.
Q: If using a cassette-to-cassette grinder, what problems can this cause?
A: The problems caused fall into two categories: Firstly, there is the possibility that the GaAs wafer will cleave under the stresses of the grinding process. Clearly, any breakage represents a financial loss and a reduction in productivity. However, in an automated cassette-to-cassette system, the grinder automatically transfers the wafers through the system; one breakage may leave wafer pieces within the cassette which can then ruin the remainder of the wafers within the cassette – multiplying the effect. Here, the automation of the system may not be such an advantage.
Secondly, even if the wafer completes the grinding process, the damage to the crystal lattice will be considerable. This damage must be removed for any subsequent processing – usually by polishing. This means that the wafer must still be relatively thick after grinding, and that the amount of material to be removed during polishing will be fairly large. This in turn both increases the polishing duration, and also increases the danger of the wafer losing flatness and becoming convex after a long polishing stage.
Q: Can the sub-surface damage be removed?
A: Of course – by polishing. However, the aim in backthinning operations is to reduce the polishing time required to a minimum. This is for reasons of productivity - you can produce more wafers in a given time – but it is also to avoid over-long polishing times. Wafers which are polished for too long become convex, losing the flatness which has been generated in the earlier stages of the backthinning process.
WAFER LAPPING
Q: What happens when a sample is lapped?
A: Lapping is a relatively gentle operation requiring little applied load, the material being by fine abrasive particles in suspension 'rolling' between the wafer and the lapping plate. A range of abrasives and plates is available, and their use will be dependent on the material being lapped. Typically, a lapping abrasive with a 9 micron grit size is used, and the wafer sample is mounted on a Precision Lapping Jig, such as the Logitech PP5 or PP6.
Q: So what does this mean for wafer backthinning?
A: The jig allows the user to alter the loading on the wafer – meaning that even the more brittle materials can be lapped without fear of cleavage. The system is therefore just as suited to Silicon backthinning as to that of Gallium Arsenide.
Q: Does using a lapping machine create the same problems as a grinder?
A: No. With a properly set-up lapping system, there is no danger of wafer cleavage under the stresses of the process. The success rate is thus far greater, and the lack of an automated cassette-to-cassette system means that if, due to operator error (eg a breakage due to poor wafer-to-support bond quality), a wafer is lost, this effect is not multiplied through the entire cassette. Here, in fact, the lack of full automation for serial production can be seen as an advantage.
Secondly, the less aggressive lapping process and the fine abrasive used results in damage to the crystal lattice which is much less than if grinding. This means that the amount of material to be removed during polishing will only be in the order of 20-30 microns – reducing the polishing time and reducing the likelihood of a loss of wafer flatness.
Q: Can the sub-surface damage be removed?
A: Of course. In backthinning operations where the polishing time required should be as short as possible, the option of chemo-mechanical polishing, where the GaAs wafer is polished for a matter of less than two minutes, is ideal. More finished wafers can be produced, and these wafers demonstrate the high flatness and parallelism which is a pre-requisite of backthinning processes.