We need our results to be correct, no matter how small the dimensions may be. Implementing smaller device dimensions requires cleaner chemicals and, more importantly, cleaner production processes. Here at Cerium Labs, we regularly use a technique called Vapor Phase Decomposition Inductively Coupled Mass Spectroscopy. 

This is a method by which trace elements on the surface of a silicon wafer are collected into a liquid sample. Inductively coupled plasma mass spectrometry (ICP-MS) analysis of the resulting liquid sample is a direct approach for measuring the cleanliness of production processes run in the wafer fab. Required analytical measurements at <108 atoms cm−2 have forced the need for lower detection limits, lower instrument background, higher sensitivity and well-controlled analytical blanks in VPD-ICP-MS analysis. Don’t worry, it’s nothing our top team of scientists cannot handle! Regularly, we take measures to improve detection limits for VPD-ICPMS analysis. This is done through improved measurement methods using high resolution ICP-MS and the elimination of potential contaminants during the VPD sample preparation procedure.

Vapor phase decomposition is a proven method of wafer preparation for the subsequent measurement of trace metal contamination. Analyzing trace metals, which means a very small amount, is made much more simple with Vapor Phase Decomposition Inductively Coupled Mass Spectroscopy! This long phrase is often shortened to VPD-ICPMS. All in all, it is a 3 step process.

Step 1: Vapor phase decomposition

Step 2: Wafer surface impurity collection using a scanning droplet

Step 3: Scanning droplet metal analysis using ICP-MS

The procedure starts with the vapor phase decomposition sample preparation technique, by which trace elements on the surface of a silicon wafer are collected into a liquid sample to be analyzed by HR-ICP-MS. The silicon wafer (either 150, 200, or 300 mm) is exposed to hydrofluoric acid vapor in a sealed chilling chamber. The hydrofluoric acid vapor forms a condensate on the chilled wafer surface. This condensate etches the oxide layer off of the wafer surface along with any trace metals that are present. 

After this, the condensate is collected by scientists who roll a drop of scan solution across the surface of the wafer. This solution is typically a dilute mixture of hydrogen peroxide, nitric acid, and hydrofluoric acid. The drop is transferred from the wafer surface into a clean sample vial. The liquid sample is then analyzed for trace metals using HR-ICP-MS. 

The VPD technique is capable of measuring metallic contaminants at concentrations ranging from 1E6 to 1E14 atoms/cm2. It is particularly useful in measuring light elements on bare silicon or in hydrofluoric acid soluble thin films.

The advantages of using this procedure are numerous. It cannot be overstated just how important it is to find trace levels of toxic metals. Even a very small amount can be extremely dangerous, and even deadly. With VPD-ICPMS, we can trust the results to be completely and totally accurate.