PSL Wafer Standard or Calibration Wafer Standard is deposited with polystyrene latex beads to calibrate the size accuracy of KLA-Tencor Surscan SP1, SP2, SP3, SP5, SPx tools
A Calibration Wafer Standard is a Certified, NIST Traceable, Size Standard deposited with polystyrene latex spheres on a prime silicon wafer at specific PSL peak sizes with narrow size distribution between 40nm and 10um. A PSL Wafer Standard can be provided with a spot deposition, multiple spot depositions or full surface deposition and controlled particle count between 100 and 10,000 count for each size deposited. 200mm and 300mm wafer standards deposited at 100nm (nanometers) sphere diameter or more are scanned by a KLA-Tencor Surfscan SP1. PSL wafer standards deposited with polystyrene latex spheres below 100nm are scanned by a KLA-Tencor Surfscan SP5. Calibration Wafer Standards are used to calibrate the size accuracy response of scanning surface inspection systems (SSIS) using Photo Multiplier Tubes or lower powered lasers to detect surface particle contamination.
PSL Wafer Standards come in two types of depositions: Full Deposition (Full Deposition above left) or Spot Deposition (Spot Deposition above right).
Wafer at right is a 3X SPOT Deposition on an AlCu Film wafer, deposited with 0.5um, 0.7um and 1.2um PSL Spheres
Full Deposition - Request a Quote
A Calibration Wafer Standard is used to identify two aspects of an SSIS tool: size accuracy of each size deposited and size accuracy across a wide dynamic range of the SSIS tool. Uniformity of scan across the wafer is generally analyzed in a FULL Wafer Deposition. The surface of the wafer is deposited with a specific PSL size, leaving no portion of the wafer not deposited with PSL Spheres. During the scan of the PSL Wafer Standard, The SSIS tools detects the peak of the PSL size distribution, which should be the size deposited on the wafer. The uniformity of scan across the wafer should indicate the SSIS is not overlooking certain areas of the wafer during the scan. Count accuracy on a Full Deposition wafer is subjective, since the Count Efficiency of two different SSIS tools (Deposition site and Customer site) are different, sometimes as much as 50 percent. Thus, the same Particle Wafer Standard deposited with a highly accurate size peak of 204nm at 2500 counts and counted by SSIS tool 1, may be scanned by SSIS 2 at customer site and the count of the same 204nm peak may be counted anywhere between 1500 count to 3500 count. This count difference between the two SSIS tools is due to the count efficiency of each PMT (photo Multiplier Tube) operating in the two separate SSIS tools.
Spot Deposition - Request a Quote
PSL Wafer Standards with a Spot Deposition are used for size accuracy calibration of the SSIS.
A Calibration Wafer Standard with a Spot Deposition has the advantage in that the spot of PSL Spheres deposited on the wafer is clearly visible as a spot, and the remaining wafer surface around the spot deposition is left free of any PSL Spheres. The advantage is that over time, one can tell when the Calibration Wafer Standard 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 using DMA (Differential Mobility Analyzer) technology to ensure the NIST traceable PSL size peak to be deposited is accurate. A CPC is used to control count accuracy. The DMA size control minimizes unwanted Haze, Doublets and Triplets from being deposited on the wafer surface. Several companies in the industry use Direct PSL Deposition to deposit Wafer Standards, as discussed below; which is not able to prevent Haze, doublets and triplets from being deposited on the wafer surface. However, the Direct Deposition, Particle Deposition System is less expensive to produce PSL wafer Standards from 40nm to 5 microns.
Applied Physics is able to use both DMA Deposition control and Direct Deposition control. DMA control provides the most size accuracy below 150nm by providing very narrow size distributions with minimal Haze, doublets and triplets deposited in the background. Excellent count accuracy is also provided. PSL Direct Deposition provides good depositions from 80nm and above, all the way up to 5 microns.
PSL Direct Deposition
The PSL Direct Deposition method uses a PSL Sphere source, diluted to the appropriate concentration, mixed with a highly filtered (20nm) airflow or dry nitrogen flow and uniformly deposited over a silicon wafer or blank photo mask as a full deposition or a spot deposition. The Direct Deposition, Particle Deposition System is less expensive, and best used for Peak PSL Size depositions from 80nm to 5 mircons.
If several companies producing the same size of PSL spheres are compared, for example at 204nm, one may measure a difference in the peak size of the two PSL depositions from the companies, possibly as much as 3 percent. Manufacturing methods, measuring instruments and measuring techniques cause this delta. However, this means that any Wafer Deposition tool using only PSL Direct Deposition to deposit PSL Spheres directly from a PSL bottle relies on the accuracy of the PSL size peak in the bottle source.
DMA Deposition Control
The second method, DMA (Differential Mobility Analyzer) Deposition Control, applies more control over the PSL Spheres. The DMA is calibrated to NIST Standards at 60nm, 102nm, 269nm and 895nm. The PSL spheres are diluted with DI Water to the desired concentration, aerosolized into an air stream, mixed with a Dry Air or Dry Nitrogen to evaporate the DI water surrounding each PSL sphere and nuetralized to remove double and triple charges. The air stream and PSL spheres are then processed through the DMA, which strips away the left and right side of the size peak to provide a narrow PSL Size peak, which is then directed down to the wafer surface as a FULL Deposition of SPOT Deposition, while simultaneously being counted for size accuracy. The DMA calibration to NIST SRM accuracy ensure the DMA peak is highly accurate and narrow, so as to provide superb calibration for a KLA-Tencor SP1 and KLA-Tencor SP2.
If for example, 204nm PSL Spheres from two different manufacturers were used in a DMA controlled, Particle Deposition System, the DMA would ensure that a precise 204nm would be deposited onto the wafer surface.
A DMA controlled, Particle Deposition System is also able to provide much better count accuracy, as well as computer recipe control over the entire deposition. IN addition, a DMA based system can deposit real process particles, such as Si3N4, SiO2, W, etc.