Metal Impurity Content Identified in Mono and Polycrystalline Silicon Crystals

Metal Impurity Content Identified in Mono and Polycrystalline Silicon Crystals

The impurity elements in crystalline silicon materials mainly include non-metallic impurities such as carbon, oxygen, boron, and phosphorus, and metal impurity such as iron, aluminum, copper, nickel, and titanium. Metal impurities generally exist in interstitial states, substitution states, complexes or precipitations in crystalline silicon, which often introduce additional electrons or holes, resulting in changes in the carrier concentration in silicon, and direct introduction into deep energy level centers. Become the recombination center of electrons and holes, greatly reduce the life of minority carriers, increase the leakage current of p-n junction; reduce the emitter efficiency of bipolar devices; cause the oxide layer of MOS devices to be broken down, etc., resulting in the reduced performance of silicon devices.

Therefore, the detection of the main metal impurity content in crystalline silicon materials is particularly important in the development and utilization of devices. The most commonly used methods for the detection of impurities in silicon materials are: Inductively Coupled Plasma Mass Spectrometry (ICP-MS), Glow Discharge Mass Spectrometry (GDMS), and Secondary Ion Mass Spectrometry (SIMS). Take SIMS method as an example.

The FZ polycrystalline ingot from PAM-XIAMEN is used as the sample for SIMS testing, specification as follows:

1. FZ Silicon Ingot for Determining Metal Impurity Content

PAM201109 – FZSI

FZ Polycrystalline Ingot Spec.

Length >1700mm
Bow <2mm
Ovality <2mm
Diameter 150mm
Resistivity >3,000Ωcm
MCC Lifetime >1,000μs
Donor Elements
(P, As, Sb)
Acceptor Elements
(B, Al)
Oxygen Content <0.2ppma
Carbon Content <0.1ppma

2. What is Secondary Ion Mass Spectrometry?

SIMS is a method for the analysis of surface compositions and impurities of solid materials. The experimental principle is to bombard the sample with a primary ion beam under high vacuum conditions, generate secondary ions by sputtering, and then analyze the secondary ions by mass spectrometry. Provides detection limits as low as 10-12~10-6 levels, and is largely unaffected by crystal doping.

Mass spectrometry can be used to obtain information on molecules, elements and isotopes on the surface of the sample, to detect the distribution of chemical elements or compounds on the surface and interior of the sample. The surface of the sample is scanned and peeled off (sputtering peeling speed can reach 10 microns/hour), and a three-dimensional image of the surface layer or internal chemical composition of the sample can also be obtained. Secondary ion mass spectrometry has high sensitivity, which can reach the order of ppm or even ppb, and can also perform micro-area composition imaging and depth profiling. So it’s a good means for the analysis of metal impurities in silicon.

3. Operating Steps of Metal Impurity Content in Silicon Measured by SISM

1) Cut each sample (unknown sample, reference sample, blank sample) into small pieces to fit into the sample holder. The reference sample must contain 23Na, 27Al, 39K, 53Fe (or 54Fe) elements and so on or there are multiple reference samples, and each reference sample contains one or more of these elements. These preparations should be done to minimize metal contamination of the sample surface.

2) Load the sample into the SIMS sample holder.

3) Bring the sample holder into the sample compartment of the SIMS instrument.

4) Turn on the instrument according to the instruction manual of the instrument.

5) Set appropriate analysis conditions, which should contain methods to eliminate the mass interference of molecular ions.

5.1) Select the primary ion beam current, the scanning area of the primary beam and the transmission mode of the secondary ion mass spectrometer to obtain a suitable sputtering rate (less than 0.015 nm/s).

5.2) Select the appropriate mass spectrometer conditions to ensure the maximum secondary ion count rate, so that the dead time loss is less than 10%.

6) Ensure that the analysis conditions for metal impurities characterization are suitable and can meet the test requirements of the reference sample and blank sample of known concentration at the same time.

6.1) Ensure that the analysis sputtering rate satisfies: During the dissection process, each monitored element is counted more than or equal to one time per sputtering depth of 0.2 nm.

6.2) When using oxygen injection, in order to determine whether the oxygen leakage pressure is appropriate, a depth analysis of a sample is required to monitor the stability of the secondary ion yield of the main element in the first 10 nm (the change should be within 20%). This confirmation experiment was performed on a typical sample using the same sputtering rate used for the surface metal impurity test. If there is a significant change in ion yield, increase the oxygen leak pressure by a factor of 2 each time until it is stable.

6.3) Determine the efficiency ratio between the various detectors used during the test (for example, electron multipliers and Faraday cup detectors), based on the instruments used and the requirements of the actual test. This is achieved by comparing the secondary ion signal (minimizing dead time loss). The secondary ion count rate used here can be different from the count rate used for the analysis, and the sputtering rate at this time may be different from the analysis.

The content of metal impurities in silicon crystal is calculated as the table listed below. We haven’t listed all the data; if necessary please contact for the full data.

Bulk Metal Content <2ppbw
Na 0.4ppba
Ni 0.1ppba
Surface Metal Content  
Zn <0.4ppbw


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