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.
Analytical chemistry of water
What do you think of when you hear the term “Inductively Coupled Plasma Mass Spectroscopy”? A CSI episode? College Chemistry? Or maybe nothing at all? Most likely you are not familiar with Inductively Coupled Plasma Mass Spectroscopy or ICPMS for short. But one type of analysis performed with ICPMS is highly relevant to our health and safety and that is the analysis of metals in water.
While you may not have heard of ICPMS, you probably have heard of lead, the soft, malleable silvery gray metal. Lead poisoning has been documented for centuries but not until the early 1970’s did the US government start to regulate lead to reduce its effect on the environment and wildlife. In the last few decades regulations for cleaner gasoline, use of lead-free plumbing and paints and many other products have greatly reduced the problem, but not completely. This is because in many cases, old infrastructure is still in use, older buildings are being gentrified and antique items with the original lead-based paints are now chic. These are just some of the ways lead could be ingested or leak into the water supply.
Science tells us that even low-level lead exposure can cause neurological and cardiovascular disease, infertility, and decreased kidney function. Higher than healthy amounts of lead have been linked to learning and behavioral problems, lower IQ, and other health issues in young children. These adverse health effects can last a lifetime! This is why health experts agree that any level of lead in one’s blood, no matter how small, is cause for concern. This is where Inductively Coupled Plasma Mass Spectroscopy can help. ICPMS can measure lead at extremely low levels in water, soil, or other materials like paint.
The ICPMS technique can measure a long list of metallic contaminants in the part per trillion concentration range. To visualize how minute this is, think of 1 drop of water in 10,000,000 gallons of water! An Olympic size swimming pool holds 660,000 gallons of water so 1 part per trillion is 1 drop in about 15 full Olympic size pools. Furthermore, some elements, Pb included, can be measured in the parts per quadrillion range!
You cannot taste, see, or smell low levels lead in the water, which makes it pretty scary. But knowing that scientists have a method of “seeing” it should make us all feel a little safer. With the help of a professional lab you can be sure your house, soil, child’s toy or grandma’s antique armoire are not sources of lead in your home. Inductively Coupled Plasma Mass Spectroscopy is actually highly relevant to protecting ourselves and our children’s health!
Implementing smaller device dimensions requires cleaner chemicals and, more importantly, cleaner production processes. One example of a potential contamination source is polymeric seals often found in process chambers. Over time, in aggressive environments such as plasmas, these materials can break down and release metals that can become incorporated in the product processed in that chamber. Low levels of metals such as Na, K, Li, Cu, Zn, Fe, and Ti, can alter the electrical characteristics and affect long term reliability of semiconductor devices. Here at Cerium Labs, we regularly use a technique called Vapor Phase Decomposition Inductively Coupled Plasma Mass Spectroscopy (VPD-ICPMS) to measure the trace metals on silicon wafers and identify the source of contamination.
The 3 process steps are:
Step 1: Vapor phase decomposition
Step 2: Wafer surface impurity collection using a scanning solution
Step 3: Analysis by ICP-MS
The procedure starts with the vapor phase decomposition sample preparation technique, by which trace metals on the surface of a silicon wafer are released so that they can be collected. The silicon wafer (from 75 up to 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 surface silicon dioxide layer on the wafer surface along with any metals that are present.
After this, the condensate is collected by carefully scanning the entire wafer surface with a droplet of ultra-pure scanning solution. This solution is typically a dilute mixture of hydrogen peroxide, nitric acid, and hydrofluoric acid. The scanning process is done by either our automated scanning system or manually by a certified chemist. The droplet is then transferred from the wafer surface into a clean sample vial. Once the droplet is collected, it is diluted and analyzed on the ICPMS.
Our VPD-ICPMS 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 where TXRF analysis falls short.
The advantages of using this procedure are numerous in semiconductor manufacturing. Metal contamination comes from numerous sources and can lead to catastrophic failures and loss. VPD-ICPMS and Cerium Laboratories can help you identify the source and improve your wafer yield.
Analyzing small and volatile molecules is made not only possible but simple with Gas chromatography-Mass Spectrometry (GC-MS). GC-MS is the separation technique of choice for smaller volatile and semi-volatile organic molecules such as hydrocarbons, alcohols, and aromatics, as well as pesticides, steroids, fatty acids, and hormones. When GC is combined with the detection power of mass spectrometry (MS), the process can be used to separate complex mixtures, quantify analytes, identify unknown peaks, and determine trace levels of contamination.
This technique can detect picogram quantities of material. Automated Thermal Desorption (ATD) GC-MS is a powerful tool for identifying organic contaminants. These may be present as an adsorbed film on silicon wafers, as airborne vapors in the manufacturing environment, as dissolved components in ultrapure water or process chemicals, or as vapors that outgas from plastics, coatings, garments, o-rings, and similar materials.
GC-MS can be used to study liquid, gas, or solid samples. How does it all work? Well, tell you! It all begins with the gas chromatograph. It is here that the sample is effectively vaporized into the gas phase and separated into its various components using a capillary column coated with a stationary, solid, or liquid, phase. The compounds are then propelled by an inert carrier gas. This is typically nitrogen, helium, or hydrogen. As components of the mixture are separated, each compound elutes from the column at a different time based on its boiling point and polarity.
The time of this elution is referred to as a compound retention time. Once the components leave the GC column, they are ionized and fragmented by the mass spectrometer. This is done using electron or chemical ionization sources. These molecules and fragments, now ionized, are accelerated through the instrument’s mass analyzer. This is most often a quadrupole or ion trap, but not always. Here, the ions are separated based on their different mass-to-charge (m/z) ratios. This type of data acquisition can be performed in either full scan mode, to cover either a wide range of m/z ratios, or selected ion monitoring (SIM) mode. A second option is to gather data for specific masses of interest.
The process is nearly complete! The final steps of the GC-MS process involve ion detection and analysis, with fragmented ions appearing as a function of their m/z ratios. These peak areas are proportional to the quantity of the corresponding compound. When a complex sample is separated by GC-MS, it will produce many different peaks in the gas chromatogram and each peak generates a unique mass spectrum used for compound identification. Thankfully, there are extensive commercially available libraries of mass spectra available to scientists across the world. Using this great resource, unknown compounds and target analytes can be identified and quantified.
This method has the capacity to resolve complex mixtures or sample extracts containing hundreds of compounds. Wow! If you need AFM, Cerium Labs is here to help! We have experts in Gas Chromatography-Mass Spectrometry (GC-MS) here in-house, ready to get to work for you and your project!