Applied Physics provides Particle Wafer Standards to calibrate the size peak accuracy of wafer inspection systems. KLA-Tencor SP1 and SP2 use these particle wafer standards to verify, calibrate as needed, the size response of the wafer inspection system across a broad size range of 40 nm to 10 microns. PSL Calibration Wafers are also used to visually verify the scanning response of surface contamination is uniform across across a wafer surface.
Particle Wafer Standard
Full Deposition across the Wafer Standard
Spot Deposition using several peak sizes deposited around the Wafer Standard
PSL Wafer Standard is provides as a full deposition or spot deposition with multiple sizes deposited around the wafer surface.
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 Deposition of Particle Wafer Standard is used primarily for size accuracy calibration of the SSIS.
100nm size peak
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.
The Technology in Produce Particle Wafer Standards
polystyrene latex particles and beads
Particle wafer standards are deposited using two methods of control: Direct Deposition and DMA Controlled Deposition.
DMA control is best suited for particle depositions from 40nm to 1 micron. Direct Deposition is useful to deposit PSL spheres and polystyrene latex particles above 1 micron.
Direct Deposition control
The Direct Deposition method simply takes what is in the polystyrene latex bead bottle and deposits the aerosolized PSL Spheres onto the wafer surface. This method is OK for large PSL Spheres above 1 micron.
If several companies producing the same size of PSL spheres are used to deposit PSL Spheres on a wafer standard, example at 200 nm, one might see the 200nm peaks of the two different PSL manufacturers differ in the peak size by as much as 5%. The reason for this is that manufacturing methods differ, and the methods by which a particle size peak is measured differs. Manufacturing methods and measuring techniques are the cause of this delta. Aerosol laser particle counters are designed with laser tubes or solid state lasers, both which differ in laser power, beam uniformity, beam diameters, etc. Assuming both sphere manufactures use NIST SRM, particle size standards, to verify size response of the 200 nm PSL spheres produced, the peak size delta of the two manufactured PSL spheres at 200nm should be under 3% variation. So, when depositing PSL Spheres onto a wafer standard, the polystyrene latex particle peak has been tested and verified to a NIST SRM, typically at 60nm, 100nm, 269nm or 895nm. To minimize the variation, a method called DMA Control is discussed below for deposition of particle wafer standards from 40nm to 1 micron.
DMA Deposition control
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 (200nm) from several different companies were deposited on a particle wafer standard using Direct Deposition, one may find one PSL Manufacture provides a 199nm size peak and a second supplier provides a 202nm size peak. The DMA Controlled Deposition has the ability to scan the two different peaks, and pick 200nm as a preferred size peak, depositing 202 nm on the particle wafer standard.
A DMA based system also has far better count control, as well as computer recipe control over the entire wafer deposition.