Introduction

If the material is in the form of an ingot, it can be cemented to the worktable of a MICROSLICE or ULTRASLICE diamond saw and sliced into wafers. The MICROSLICE gives fine, straight cuts with minimum damage and loss of material. The slices are cut serially and normally remain fixed to the saw worktable until the retaining cement is dissolved. ULTRASLICE saws allow for more versatile sample mounting, particularly for long or irregular shaped samples.

The resulting wafers can be polished directly, but it may be necessary to lap their surfaces to remove sawing marks and surface damage and to achieve the required thickness and flatness. The index movement of the saw is selected to give the required wafer thickness plus cutting losses; the "as cut" wafer thickness should oversize on the finished, polished thickness to allow for lapping. The amount of wafer surface which has to be lapped away depends upon the material but will be less than 0.1mm per surface. If single crystal material has to be oriented so that it is cut accurately with reference to a crystal plane, a goniometer table allows the ingot to be transferred from the X-ray machine (used for orientation) to the saw without losing alignment. "As received" wafers are lapped and polished directly.

Lapping

Lapping is carried out most economically on a MULTIPOL polishing machine, using a scrolled cast iron plate UMI191. The lapping material is medium grit (10 to 15 micron) silicon carbide powder suspended in a lapping oil. (Other lapping materials are medium grit alumina, diamond or self adhesive silicon carbide papers). One side of the wafer is lapped to produce a flat surface; it is then cleaned, reversed and the second surface lapped until it is flat and the required wafer thickness is obtained.

Polishing

After cleaning, the previously lapped surface is polished on a polyurethane faced plate UMI206, using suspensions of colloidal silica (0.125 micron) or fine alumina (0.3 micron). The slurry suspensions are pumped continuously over the plate.

The highest quality of surface finish is obtained by giving a second polish using a very fine (0.05 micron) alumina suspension, preferably on a self adhesive cloth pad fixed to a plain stainless steel plate UMI199. Damage to the crystalline structure is reduced by using very low loadings on the wafers, especially towards the end of the polishing process.

The flatness of the samples is, of course, controlled by the flatness of the plate used. The plates are kept flat by allowing annular conditioning rings to run on them. The rings run in the roller arms of the machine, whose positions are adjusted towards or away from the plate center to correct a convex (domed) or concave (hollow) plate.

With MULTIPOL 2, the glass-faced conditioning ring UMI153 is used on all plates except (final finishing) Cloth pads, which cannot be conditioned and the polishing time using them should be kept to a minimum, otherwise sample flatness will be impaired.

Sample Mounting

Wafers can be "free lapped" by allowing them to run inside the conditioning rings, using suitable plastic separator discs for the smaller diameters. An alternative method of mounting wafers is to cement them to the sample plate of precision jig, which gives precise control of load, parallelism of surfaces and flatness. The jig method is preferred for the most delicate materials. Jig UMI165 can be used for samples up to 50mm diameter as standard and up to 4 inches diameter with modification. Wafers should be cemented to discs and jig plates using the minimum capillary wax method described in the ULTRA TEC Application note "Back Lapping Semiconductor Wafers".

Further Reading

1. ULTRA TEC Application Note – Back Lapping Semiconductor Wafers
2. ‘Rapid Routes to Planar Polishing’, by Tim Hazeldine and Joseph Rubin, Materials World, Feb 1997, Published by The Institute of Materials
3. ‘Cutting and polishing optical and electronic materials’, by Fynn and Powell, 2^nd Edition, published by Adam Hilger
4. ‘Annular Sawing’, by Tim Hazeldine, European Semiconductor October 1997, published by Angel Publishing