Metal contamination in a semiconductor manufacturing environment can greatly reduce wafer yield or create long term product reliability issues. For these reasons’ manufacturers are constantly monitoring their tools for low level metal contamination. Vapor Phase Decomposition Inductively Coupled Plasma Mass Spectroscopy (VPD-ICPMS) is the preferred analytical technique to measure trace metals on silicon wafers. VPD-ICPMS can measure levels of metals in the parts per billion and even parts per trillion range. However, to do this the process requires class 1 cleanroom areas and strict protocols so the samples are not contaminated in the lab prior to analysis.
Appropriate handling prior to submitting the wafers to the lab is also very critical. Below are 6 very important rules to follow to prevent cross-contamination from sources such as an unfiltered environment or human contact.
- Always handle wafers with vacuum wands, never with hands (gloved or not) and on the opposite side of the wafer from which you want analyzed. Touching the wafer with a gloved hand can deposit calcium and zinc on the surface.
- Open wafer boxes and perform all wafer movements in a cleanroom or at least in a laminar flow hood.
- Ship wafers in a cleaned wafer box (cassette, FOUP (Front Opening Unified Pod or Front Opening Universal Pod) or FOSB (Front Opening Shipping Box)). In our experience analyzing 1000s of wafers, single wafer carriers (aka pucks or clamshells) are dirty and result in an increased level of metals on the wafer.
- Place wafers in the cassette with the side to be analyzed facing up. Automated wafer preparation tools are designed to run the top surface so there is no extra handling required.
- After wafers are loaded in the wafer cassette, tape the cassette closed to prevent accidental opening and contamination from the outside air. Outside air can contain significant amounts of aluminum, calcium and other elements that can deposit on unprotected wafers.
- Double bag the cassette with plastic and seal before placing in a box for shipping.
Improper wafer handling can lead to confusing and inaccurate data. We take extreme measures to ensure our processes are clean. For the best results, however, the process starts at the customer site. Following these simple steps will ensure your samples are not contaminated by secondary sources.
There is a bit of a battle in the analytical arena when it comes to the best method for measuring trace contaminants on the surface of a silicon wafer. We described the analysis of trace metals on silicon wafers by Vapor Phase Decomposition – Inductively Coupled Plasma Mass Spectroscopy (VPD-ICPMS) in a previous blog post. TXRF or Total Reflection X-Ray Fluorescence is another technique that is often used in semiconductor manufacturing to monitor contamination. Where VPD-ICPMS collects the contaminants in a droplet that is then analyzed by mass spectroscopy, the TXRF instrument uses an x-ray beam to excite the wafer surface. Elements fluoresce after being excited, the fluorescence is then measured to determine what elements are present and in what amounts. There is considerable overlap between the two techniques, but each has its own unique features.
VPD-ICPMS on one hand is a very well-defined process and provides analysis of the top surface of the wafer, about 20 Angstroms. Whereas TXRF is more versatile and can be used on different wafer surfaces. Below is a comparison of the two techniques.
|VPD-ICPMS ||TXRF |
|Contaminants from the entire wafer surface are collected and analyzed. ||Only the area excited by the X-Ray beam (~2cm spot) is analyzed. To do the entire wafer multiple spots are required e.g., a 300 mm wafer will need 350 spot analyses to cover the full surface of the wafer. |
|Results are cumulative for the wafer surface, no spatial information is available. ||TXRF can produce maps showing the impurity distribution on the wafer surface. |
|10-100X greater sensitivity. || |
|Detects light elements like Li, Na and Mg where TXRF cannot. ||Detects non-metals like Cl, and Ar where VPD-ICPMS cannot. |
|Considered destructive analysis ||Non-destructive analysis. |
|Wafer can only be analyzed 1x. ||Wafer can be reanalyzed as needed. |
|Analyzes ~20 Angstroms or thickness of oxide film. ||X-Ray beam penetrates ~ 500 Angstroms. |
|Can only analyze bare Si or Si with SiO2 films. ||Will analyze oxide and any amorphous or crystalline film beneath it. |
VPD-ICPMS and TXRF use very different methods to analyze for contamination on wafer surfaces. These different mechanisms however provide certain benefits for each. In fact, chemists and engineers realized the advantages and now many semiconductor fabrication facilities use an integrated VPD-TXRF system to monitor contamination.