# US 7,411,665 B2 · Method for wavelength calibration of an optical measurement system

> Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt

- Number: US 7,411,665 B2
- Document ID: US7411665B2
- Status: Granted
- Kind: B2
- Application number: 11/329,434
- Filed: 2006-01-11
- Published: 2006-07-20
- Granted: 2008-08-12
- Earliest claimed: 2005-01-18
- Assignees: Leica Microsystems CMS GmbH
- Inventors: Thomas Iffland
- Web page: https://patentgrep.com/patent/US7411665B2

## CPC

- G01J 3/28: Investigating the spectrum (using colour filters G01J3/51)
- G01N 21/211: {Ellipsometry (optical thickness measurement G01B11/06)}
- G01N 21/274: {Calibration, base line adjustment, drift correction}
- G01J: MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY (light sources F21, H01J, H01K, H05B; investigating properties of materials by optical means G01N)
- G01N: INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES (measuring or testing apparatus or processes other than immunoassay, involving enzymes or microorganisms C12M, C12Q)

## Abstract

An optical measurement system having a spectrophotometer and a ellipsometer is calibrated, the spectrophotometer and the ellipsometer first being calibrated independently of one another. A spectrophotometer layer thickness (dphoto) of a specimen is then determined at an initial angle of incidence (θinit) using the spectrophotometer. An ellipsometer layer thickness (delli) of the specimen is then determined using the layer thickness determined with the ellipsometer. The spectrophotometer and the ellipsometer are matched to one another by varying the initial angle of incidence (θinit) until the absolute value of the difference between the spectrophotometer layer thickness (dphoto) and the ellipsometer layer thickness (delli) is less than a predefined absolute value.

## Claims

### Claim 1

A method for wavelength calibration of an optical measurement system having a spectrophotometer and a ellipsometer, comprising the steps of:
calibrating the spectrophotometer and the ellipsometer being independently of one another in an initial calibration,
ascertaining upon calibration of the spectrophotometer, an allocation of the wavelength (λ) incident onto a CCD chip to the position of the pixels of the CCD chip,
determining a layer thickness (delli) of a specimen using the ellipsometer, as a reference measured value;
determining a spectrophotometer layer thickness (dvisphoto) in the visual spectral region and a spectrophotometer layer thickness (dUVphoto) in the UV region, at an initial angle of incidence (θinit);
modifying coefficients of a wavelength-dependent function until the absolute value of the difference between a re-measured visual spectrophotometer layer thickness (dvisphoto) and a re-measured UV spectrophotometer layer thickness (dUVphoto) is less than a predefined absolute value; and
varying the initial angle of incidence (θinit) until the absolute value of the difference between the visual spectrophotometer layer thickness (dvisphoto) and the ellipsometer layer thickness (delli), and the absolute value of the difference between the UV spectrophotometer layer thickness (dUVphoto) and the ellipsometer layer thickness (delli), are less than a predefined absolute value.

### Claim 2 (depends on 1)

The method according to claim 1, wherein upon calibration of the spectrophotometer, the allocation of the position of the pixels of the CCD chip as a function of the wavelength (λ) is accomplished by way of a function λ(pixel)=a0·(pixel)2+b0·(pixel)+c0, and the coefficients a0, b0, c0 are accomplished by fitting to the measured values, in particular using a method of minimum mean error squares.

### Claim 3 (depends on 2)

The method according to claim 2, wherein the coefficients a0, b0, c0 are varied until the layer thicknesses (dUVphoto) and (dvisphoto) within a layer thickness interval are less than a predefined tolerance value.

### Claim 4 (depends on 3)

The method according to claim 3, wherein the predefined tolerance value for layer thicknesses less than 10 nm is equal to approximately 0.5 nm.

### Claim 5 (depends on 3)

The method according to claim 3, wherein the predefined tolerance value for layer thicknesses greater than 10 nm and less than 100 nm is equal to approximately 0.6 nm.

### Claim 6 (depends on 3)

The method according to claim 3, wherein the predefined tolerance value for layer thicknesses greater than 100 nm and less than 500 nm is equal to approximately 2 nm.

### Claim 7 (depends on 3)

The method according to claim 3, wherein the predefined tolerance value for layer thicknesses greater than 500 nm is equal to approximately 5 nm.

### Claim 8 (depends on 3)

The method according to claim 3, wherein variation of the coefficients a0, b0, c0 is accomplished using a function of the form a1=a0+Δ, b1=b0+Δ, and c1=c0+Δ, Δ being ascertained empirically.