Secondary Ion Mass Spectroscopy (SIMS)

university wafer substrates

What Substrates are Used for SIMS Measurements?

When selecting substrates for SIMS measurements, several specifications are typically considered to ensure reliable, consistent data and accurate depth profiling. Although exact needs vary by application, common substrate specs include:

  1. Material & Crystallographic Orientation

    • Material: Silicon is most common, but other semiconductors (e.g., GaAs, InP, SiC) or insulators can be used.
    • Orientation: Often (100) or (111), chosen based on the sputtering characteristics and specific application requirements.

  2. Doping Type & Concentration

    • Type: Intrinsic (undoped), n-type (e.g., phosphorus, arsenic), or p-type (e.g., boron, gallium).
    • Concentration/Resistivity: May range from lightly doped (high resistivity) to heavily doped (low resistivity). Precise doping levels help create calibration standards and reference wafers.

  3. Wafer Size & Thickness

    • Diameter: Commonly 2", 4", 6", 8", or even 12" wafers.
    • Thickness: Typically between 275–725 µm for standard silicon wafers, with thickness tolerance depending on the manufacturer.

  4. Surface Finish

  5. Uniformity & Defect Density

    • Doping Uniformity: Ensures reproducible SIMS depth profiles and better calibration.
    • Low Defect Density: Minimizes localized variations in sputtering rates and secondary ion yields.

  6. Special Considerations

    • Oxide Layers: May be intentionally present for certain measurements.
    • Metallization or Other Layers: Some applications require known metal films or additional layers on top of the substrate.
    • Chemical Cleanliness: Minimizing contamination is crucial for ultra-trace SIMS analyses.

In summary, the most important substrate specs for SIMS include well-defined doping type and concentration, precise wafer dimensions, high-quality surface polish, and low defect density. These parameters ensure repeatable sputtering rates, accurate mass-spectral data, and reliable depth profiles in semiconductor research and other high-precision material analysis.

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Substrates Used for Measuring Electron Mobility

An undergrade requested help with their research project.

Question:

I am conducting a research project that is about measuring the electron mobility of n-doped silicon as a function of concentration.

What I want to do is have around six different samples with known conductivities and apply a voltage across the sample and measure its current density in order to find the electron mobility. I do not need large samples for this project, and the concentration is the most important factor in this project. Do you have any suggestions?

Answer:

Reference #132111 for specs and pricing.

We can supply you a wide range of n-type silicon wafers, with resistivities from 0.001 to 20,000 Ohmcm, hence dopant concentrations from 8E19/cc (1,200ppma) to 5E11/cc (0.00001ppma). However, we have neither mobility nor dopant concentration measurements for individual wafers or even batches of wafers. We do have Resistivity measurements but even these are relatively broad ranges for groups of wafers.

To appreciate why this is so you have to understand how the silicon ingots and wafers are made. To grow a crystal, one charges a certain amount of dopant into a batch of very pure silicon which is then molten and from it a crystal is drawn. However, the dopant concentration in the crystal is not the amount of dopant charged divided by the amount of silicon in the charge. During crystallization, the dopant is preferentially excluded form the crystal (in some cases it is preferentially included in the crystal) so that during crystallization, dopant concentration in the melt rises and in the "pot scrub", in the bottom of the crucible, it can be very high indeed. In the monocrystalline part of the ingot, the resistivity changes from top to bottom, by a factor of 2 for p-type doped with Boron and by a factor of 3 for n-type doped with Phosphorus.

Thus to measure mobility, you have to measure, in the same place on the wafer, both Resistivity (Ro) and Dopant concentration (Nc). Dopant concentration has to be measured with a tool such SIMS (Secondary Ion Mass Spectroscopy). From Ro and Nc you calculate mobility.

We can supply you the sample wafers, but you have to do your own Resistivity measurements and SIMS analysis.

Another approach is to use Silicon wafers with Silicon Epitaxial layer. In growing the Epitaxial layer one can control the dopant concentration in the gas from which the Epi layer is deposited, hence know the dopant concentration in the Epi layer that is deposited to an accuracy of at least +/-10%. Of course in practice, one measures Resistivity (Ro) to verify the Dopant Concentration (Nc) under the assumption that mobility is constant, whereas you want to see the relatively small variations of dopant mobility dependant on dopant concentration. So, you are back to doing SIMS analysis. n-type Nc in Epi layers can be controlled from about 1E18/cc to 5E12/cc.

In summary, we can supply you Silicon wafers or Silicon Epi wafers, with wide range of n-type dopant concentrations, but you have to do Resistivity measurements and SIMS analysis, yourself.

Note: On polished wafers one has to measure Resistivity with a non-contact gauge (which measures the eddy currents induced within the silicon). One can use a 4 point probe Resistivity measurement only on "as-cut" and or ground or lapped wafer surfaces (not on alkaline etched silicon surfaces). You would have to grind or sand-paper the polished wafer surface for the 4-point probe to make good contact to give you a valid resistivity measurement.

Note: We do have some broken silicon wafers, with a wide range of dopant concentrations, which are available cheaply but which can be used for the measurements that you need to make.

There are very many n-type wafers that you can use for your purposes.
I have chosen a small sample for your review, concentrating on 2" wafers and [100] orientation wafers, and otherwise covering the resistivity range of 0.001 to 20,000 Ohmcm.
Item Material Orient. Diam (mm) Thick (μm) Surf. Resistivity Ωcm Comment
1511 n-type Si:As [111] 2" 300 P/E 0.002-0.005 SEMI Prime, 2Flats, hard cst
C327 n-type Si:As [100] 2" 300 P/E 0.001-0.005 SEMI, 1Flat, hard cst
1611 n-type Si:Sb [100] 2" 300 P/E 0.010-0.020 SEMI Prime, 2Flats, hard cst
B121 n-type Si:P [100] 4" 450 P/E 0.3-0.5 SEMI, 2Flats, Empak cst
B531 n-type Si:P [100] 4" 450 P/E 0.3-0.5 SEMI Prime, 2Flats, hard cst, cassettes of 5 & 7 wafers.

What Is Secondary Ion Mass Spectroscopy (SIMS)?

Secondary Ion Mass Spectrometry (SIMS) is an analytical technique used to characterize the composition of the surfaces (and near‐surface regions) of materials. It works by bombarding a sample’s surface with a focused beam of primary ions, causing atoms and molecular fragments (secondary ions) to be ejected from the sample. These ejected ions are then collected and analyzed by a mass spectrometer to determine their mass-to-charge ratios. From this data, one can identify which elements or molecular species are present and, in many cases, measure their concentration and spatial distribution.

Here is a more detailed breakdown of how SIMS works and why it is widely used:

  1. Ion Bombardment

    • A beam of primary ions (commonly oxygen, cesium, or noble gas ions) is directed at the sample surface.
    • This bombardment causes sputtering—i.e., ejection of atoms, clusters, and molecules from the sample.

  2. Secondary Ion Collection

    • Ejected species can be neutral or charged. The charged particles (secondary ions) are extracted and fed into a mass analyzer.
    • Only a small fraction of sputtered atoms/molecules become ions, but this fraction is sufficient for highly sensitive measurements.

  3. Mass Analysis

    • The mass spectrometer separates the secondary ions by their mass-to-charge ratio (m/z).
    • Typical mass analyzers used in SIMS include time-of-flight (TOF), magnetic sector, and quadrupole mass spectrometers.

  4. Imaging and Depth Profiling

    • Imaging mode: By scanning the primary ion beam across the sample and measuring the mass of secondary ions at each position, SIMS can generate a two-dimensional “map” of composition.
    • Depth profiling: By sputtering for a controlled duration and monitoring the evolving secondary ion signals, SIMS can provide elemental concentration profiles as a function of depth into the material.

  5. Applications

    • Semiconductor industry: Measuring dopant distributions in silicon wafers.
    • Thin films and coatings: Verifying composition and detecting contaminants.
    • Geological and environmental studies: Analyzing mineral compositions or detecting trace elements in geological samples.
    • Biological and pharmaceutical research: Investigating localized chemical distributions in cells or tissues (using specialized variants such as cluster SIMS).

  6. Advantages

    • High sensitivity: Capable of detecting very low concentrations (parts per million to parts per billion).
    • Spatial resolution: Can achieve lateral resolution on the order of tens of nanometers (depending on instrument design).
    • Depth profiling: Allows layer-by-layer analysis.

  7. Limitations

    • Destructive: Because SIMS relies on sputtering the sample surface, it damages and eventually consumes the analyzed area.
    • Quantification: Quantitative analysis can require calibration standards and careful consideration of matrix effects (the sputtering process can vary with different materials).

In summary, SIMS is a powerful technique for probing the elemental and molecular composition of surfaces with high sensitivity and spatial resolution. It is particularly valued in fields that require precise information on surface chemistry and depth profiling, including semiconductor manufacturing, materials science, and various research applications.