Frequently Asked Questions (FAQ) on 2-dimension Ellipsometers

1.Basic Idea and Features of Measurement
2.On the sample data
3.Accuracy and Procedure of Measurement

1.Basic Idea and Features of Measurement

Q1-1. How is BEPE series compared with products of other companies?

A1-1. Our BEPE series are series of machines measuring the film thickness and refractive index in which two new revolutionary features, 2-dimension Ellipsometer (PEL) and multiple thin film ellipsometer (MFE), is unified in one machine. Conventional ellipsometer, a single ellipsometer (SEL) only measure one point at one time. On the other hand, PEL is a machine which has put together the features of 100,000 SELs in parallel. MFE can measure and calculate with multiple parameters for each point of multilayer film. Conventionally, it was believed that with single wavelength, only 2 parameters measurement was possible. However, with MFE algorithm, measurement of each layer of triple-layered film or simultaneous measurement of optical constants of film and substrate of monolayer film became possible.

Q1-2. Can you explain the method of calculation to measure the thickness of film?

A1-2. Calculation of film thickness is done by measuring ƒ (changes in the azimuth angle and the ellipcity, after linearly polarized incident light is reflected on the sample) of each point, then using Fresnel Formula with the refractive index as a parameter. The refractive index is optimized so that the error function which is shown as imaginary part of film thickness is minimized.

Q1-3. How is your product compared with products of other companies?

A1-3. When using an interferometer for film measurement, the refractive index is put as a parameter. Therefore, an interferometer is limited to the measurement for the sort of sample that has a fixed refractive index to obtain an accurate data. 1000Angstrom or more is the reliable number for film thickness of Silicon-dioxide (SiO2) on silicon. With SEL, a reliable simultaneous measurement of film thickness and refractive index can not be made with less than 200Angstrom. The refractive index has to be a constant, with film thickness less than this; otherwise, there will be huge repetitive error in the measured film thickness. However, the density and refractive index of oxide film in this range could vary according to the process parameters. Therefore, with less than 200Angstrom, SEL not only is unable to measure the refractive index, but film thickness can not be measured correctly. Probably, the reason for this limit in SEL is because of the beam size and sample*s roughness. In Ellipsometry, calculation is made with assumption that the incident light is a plane wave. Therefore, beam size of 50m which is about 100 times of the wavelength is the limit of accurate measurement. Normally, SEL measures with 1mm resolution. On the other hand, finer resolution is necessary for thin film because of irregularities on the surface and influence of barriers. The resolution of BEPE series depends on the relation of measurement area and the amount of pixels of the CCD Camera. We guarantee that our BEPE series, which are 2-dimension Ellipsometers (PEL), can measure refractive index and film thickness of up to 50Angstrom simultaneously.

2.On the sample data

Q2-1. The refractive index of a oxide film with 50Angstrom is 1.38 at maximum and 1.35 at minimum according to the result of your measurement. The difference seems quite large.

A2-1. In fact, the difference of this sample is rather small compared with others. A thin film thinner than 70Angstrom, in which the structure is not definite, the dispersion of density in thin film is larger than that of a thick film. Therefore the difference of the refractive index is usually larger.

Q2-2. Is it possible to measure etched oxide film which are normally less than 25Angstrom ?

A2-2. Actually, the film thickness and the refractive index of samples with 10Angstrom have been measured. But the result was somewhat different from the measurement by the machines of other companies, since their machines regard the refractive index fixed and measure only the film thickness.

Q2-3. What is the limit of thickness for BEPE series?

A2-3. In a case of a film that is too thick for the light to permeate through, our machine would either be unable to measure it or the accuracy would drop drastically. The depth of penetration of light is about/k. Around 20is the measuring limit for a silicon.

Q2-4. When a film of poly-silicon is formed on the oxide film, is it possible to measure each of them?

A2-4. Yes. It is possible to measure multilayer films such as Poly-Silicon/Silicon-dioxide/Poly-Silicon/Quartz substrate, SOI, AlGaAs/GaAs, ONO, OPO, etc. Please contact us, if you would like to see the data on the above.

Q2-5. Is it possible to measure a photo-resist film?

A2-5. Yes. Any film thickness which are in practical use can be measured by our products.

3.Accuracy and Procedure of Measurement

Q3-1. Can your PEL measure just the designated 9 points (1m~1m)? How long would be the measurement time?

A3-1. BEPE series have a great freedom in selecting the method of measurement and measuring area and point. They can be designated through the software. The measurement time is same, but the calculation time would change proportionally depending on the number of measuring points. The resolution of QR120K is about 100m. HR and FR have a resolution of ʂ order. HR can measure 400,000 points of each 9 areas with the resolution of 1m. You can freely designate the area and points to be measured from 1 to maximum points.

Q3-2. How would the resulted image be after measuring the same point repetitively for ten times?

A3-2. We have measured the refractive index and the film thickness of an SiO2 film of 30Angstrom , using a CCD camera with 40,000 pixel. The variance of film thickness was }1.5Angstrom . There is a Cold CCD for option, which improves the accuracy of repetitive measurement dramatically.

Q3-3. Would the diffused reflection from the edge of the wafer influence the result?

A3-3. No. Since our PEL is focused on the entire surface of the wafer, the diffused reflection would not influence the result. But you can not obtain an accurate measurement around edges, because there are slants, etc.

Q3-4. What is done against bends on the sample?

A3-4. Our machine smooth it by absorbing the sample stage with a vacuum. In case of a sample that has surface to be measured on both sides like a hard disk, it has to rise from the stage; therefore, the sample itself must be flat.

Q3-5. Would your PEL be accurate as the spectroscopic ellipsometer even with the use of a lens?

A3-5 . Our company succeeded in the miniaturization of the device by solving the problems such as the spherical aberrations of the lens, without losing its accuracy (QR400K). As a result of these developments, we came to introduce two dimensional spectroscopic ellipsometer. Our two dimensional spectroscopic ellipsometer is the device which has solved various problems (such as the purity of the source of light, the wavelength dependency of the refractive index that current spectroscopic ellipsometer had ) through an original optical design and the algorithm, and gives physically reasonable solutions to the structure of the sample and the wavelength dependency of the characteristic.

Q3-6.Why is the wavelength of the semiconductor laser a variant as a Peltier device?

A3-6. The wavelength is a variant to measure multilayer films. By drifting the wavelength the structure of film is surmised. Although the variable range is very narrow, sufficient number of measurement is possible for the structure of multilayer film because of the purity of wavelength. Later, multilayer calculation is done using the MFE algorithm. Stripe observation by measurement of Interferometry became possible, since BEPE measures as an image. Through controlling the temperature with Peltier device, the wavelength is more stabilized.

Q3-7. How long are measurement and calculation times?

A3-7. The measurement time is 15-30 seconds per one measurement.The calculation times for 1 point on a standard thin film are: 15 msec with 5-parameters (n1, k1, d1, ns, ks) calculation, 4 msec with 2-parameters (n1, d1) calculation, and 0.2msec with 1-parameter (d1) calculation. (See our specification sheet for further details)

Q3-8. What kind of PEL are there in BEPE series?

A3-8. There are:

(1) QR120K
 - With a wafer up to 300mm, it can measure the entire surface in one time, and in case of a wafer up to 300mm in two times. - Standard number of points it measures: 120,000 points - Resolution: 100m to 300m

(2) HR400K
- IT CAN MEASURE 300mm WAFER. - It can measure a scribeline and parts in a chip. - Area of measurement: 3~10 mm (at 4m resolution) - Standard number of points it measures: 400,000 points - Resolution: more than 1m

(3) FR1000K (currently under construction)
- It can measure delicate patterns inside a chip. - Standard number of points it measures: 1,000,000 points - Resolution: less than 0.2m - XY scan stage is equipped.

Q3-9. Is it possible to widen the measurement area for HR-type?

A3-9. Yes. It is possible when a camera with a high pixels is used, but the price would rise.


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