PSL Wafer Standards - Calibration Wafers
PSL Wafers can come in two styles: Full Deposition (Full Dep) or Spot Deposition (Spot Dep).
Full Dep - Request a Quote
A Full Deposition PSL Wafer Standard is used to identify two aspects of an SSIS tool: size accuracy and uniformity of scan across the wafer. The surface of the wafer is deposited with a specific PSL size, leaving no portion of the wafer un-deposited. In other words, The peak of the PSL size distribution detected by the SSIS should be sized according to the size deposited on the wafer, and the uniformity of scan across the wafer should indicate the SSIS is not overlooking certain areas of the wafer during the scan. Count accuracy of a full Deposition wafer is not as accurate as a spot deposition wafer.
Spot Dep - Request a Quote
Spot Deposition PSL Wafer Standards are used primarily for size accuracy calibration of the SSIS.
But a Spot Deposition wafer also has a 2nd advantage in that the spot of PSL Spheres deposited on the wafer is clearly visible as a spot, and the remaining wafer surface is left free of any deposition. The advantage is that over time, one can tell when the PSL Calibration Wafer is too dirty to use as a size reference standard. Spot Deposition forces all the desired PSL Spheres onto the wafer surface at a controlled “spot” location, thus very few PSL spheres and much higher count accuracy is the result. Applied Physics uses a Model 2300XP1 with a Differential Mobility Analyzer (DMA) to ensure the NIST traceable PSL size output and count is accurate. A CPC is used to control count accuracy. The combination of the DMA size control minimizes unwanted Haze, Doublets and Triplets are deposited in the background. Several companies in the industry use Direct PSL Deposition to deposit Wafer Standards, as discussed below; which can not prevent these unwanted affects to the wafer surface. Lower prices do not mean you get a NIST Traceable size standard, which is a requirement of ISO 9000 companies.
Applied Physics can use both methods, but it is best to use DMA control for PSL Spheres under 1um in size, and PSL Direct for spheres above 1um in size.
PSL Direct Deposition
The PSL Direct Deposition method simply takes what was in the PSL bottle and deposits the aerosolized PSL Spheres onto the wafer surface. This method is OK for large PSL Spheres above 1um, but not below 1um.
If several companies producing the same size of PSL spheres are compared, for example at 0.2um, one will see a difference in the peak size of the two PSL spheres from the companies, often by as much as 10%. Manufacturing methods and measuring techniques cause this delta. However, t This means that any Wafer Deposition tool using only PSL Direct Deposition to deposit PSL Spheres directly from a PSL bottle, are relying on the bottle manufacturer to be precise. But, if size differences vary as much as 10%, what are you buying with a system that has no NIST DMA calibration control in the deposition of the wafer standards.
DMA Deposition Control
The second method, DMA (Differential Mobility Analysis) Deposition Control, applies more control over the PSL Spheres. The DMA system is calibrated to NIST Standards at 0.1007um, 0.269um and 0.895um. The bottle PSL spheres are then compared to this NIST Calibration, and only that correct portion of the PSL size distribution is deposited from the bottle. This ensures that even if various PSL manufactures have variations in PSL size, the DMA based deposition will deposit only that portion of the PSL size distribution that conforms to NIST calibration.
If for example, 0.2um PSL Spheres from several different companies were deposited using a DMA Controlled, PSL Wafer Deposition System, the DMA would ensure that only the correct size, as compared to a NIST size response, would be deposited onto the wafer surface.
A DMA based system also has far better count control, as well as computer recipe control over the entire deposition.