Ellipsometer optical set-up, from Jobin Yvon

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Optical Set-up of UVISEL

Optical Setup of a UVISEL Ellipsometer

The source is a high pressure 75 or 150 watts arc Xenon lamp.

Optical fibres are employed to deliver light and provide a lot of flexibility in the system installation.

A mechanical shutter is used in order to evaluate the continuous background and thus to eliminate any influence of environmental radiation, e.g. from substrate and chamber wall heaters.

The polariser and the photoelastic modulator are mounted in the incident light path. A stepping motor allows rotation of the polariser-modulator combination, the angle between the polariser and the modulator being fixed. The reflection arm contains the analyser which can be also automatically rotated by means of a stepping motor. The analyser is mounted on a Microspot which allows one to focus the probing beam on a small area on the sample, providing a better spatial resolution. The diameter of the spot is 50, 100 or 1000 microns.

The modulator, the polariser and the analyser are mounted on 0.01 degree precision rotators.

Finally the energy of the light is analysed by a grating monochromator or a spectrograph.

A photomultiplier tube is used as detector. In this case the available wavelength range is 230 nm to 850 nm. However, this range can be easily extended towards the IR range, up to 1700 nm, by using low gap semiconductor detectors.

In-situ and Ex-situ configurations

The UVISEL provides two configurations : in-situ and ex-situ. The basic configuration is the in-situ one where the optical assemblies for incident and reflected beam are to be mounted on the process chamber. The ex-situ configuration includes the same hardware as the in-situ one plus variable angle of incidence goniometer and a sample stage. The ellipsometer hardware is modular in design so that input and output optical assemblies can easily be removed from the goniometer and mounted onto a deposition chamber and vice-versa.

A complete in-situ system includes :

mechanical and optical parts

electronics

calibration, data acquisition and modelling software

and a complete ex-situ system includes also :

variable angle of incidence goniometer and motorized sample stage

mapping software

UVISEL Multi-wavelength

The UVISEL Multi-Wavelength option is a spectrograph with multichannel detector. The spectrograph employs a holographic aberration corrected concave grating and is coupled with 16 detectors and is extendible up to 48 detectors.

The Multi-Wavelength option allows simultaneous kinetic data acquisition at the wavelengths chosen from the ellipsometer spectral range for real-time process control. The wavelength real-time data acquisition is considered as an optimal because of substantial reduction of acquisition time in comparison to spectroscopic measurements.

A particular example of an optimal choice of a multiple wavelength configuration is related to process control of compound semiconductors :

Optimum compositional ( for III-V and II-VI compound semiconductors ) sensitivity requires a wavelength at which dielectric function varies most rapidly with alloy composition, which is generally in the vicinity of a strong absorption edge.

Optimal thickness sensitivity requires a second wavelength at which the films are sufficiently transparent to pass the light that is reflected from interfaces.

Optimal surface sensitivity requires a third wavelength at which penetration depth of light is comparable with a characteristic width of surface roughness.

The wavelength can be chosen from a range which includes a specific spectral feature of the material under investigation.

Therefore, the Multi-Wavelength option set enables real-time monitoring of structural and compositional modifications and is computationally efficient due to a minimum of data processing.

Data Acquisition and Processing System

The read-out system consists of 128 multiplexed inputs. An automatic gain control amplifier combined with a 12 bit analog-digital converter is used in order to match the detected signal to the full range of the ADC. The high precision fast ADC ( 12 bit, 1 MHz ) is used to synchronously convert the signal. The Fourier analysis of the signal is then carried out by a Digital Signal Processor ( DSP ). This fast microprocessor system is dedicated to the fast Fourier transform computation. Finally the DC component together with the harmonics of the signal are continuously transmitted to the computer by a FIFO register. Moreover, the DSP allows the on-line control of the modulation amplitude and the read-out system. Finally the computer is devoted to the overall monitoring of the ellipsometer.

Real-time Output and Triggers

The real-time output of the ellipsometric signal and the triggers allows one to arrange endpoint or closed-loop feedback control of kinetic processes ( deposition or etching ). A control algorithm which depends on a model of the sample is incorporated in the real-time software to provide a signal related to the sample parameters ( thickness, phase composition, refractive index, etc. ).

the outputs provide real-time signal proportional to Psi and Delta

a TTL signal is generated each time an acquisition is started or stopped

a switching threshold can be programmed on a analog input to perform a program acquisition

General Ellipsometer Set-up

Optical Set-up of UVISEL

In-situ and Ex-situ configurations

UVISEL Multi-wavelength

Data Acquisition and Processing System

Real-time Output and Triggers