VUV Spectrophotometer
The UV/VUV - spectrophotometer system is a product of more than twenty years of research in the field of production and testing of advanced optical components for UV/VUV spectral range. The current system is based on a prototype spectrophotometer of the Laser Zentrum Hannover e.V., which was initially developed for typical analytical tasks in optical thin film technology. During the production phase of more than ten units for industrial companies and research institutes, the measurement performance of the system could be improved to an outstanding level, which qualifies the spectrophotometer in routine quality management and research applications.
Technical Data
In consideration of the presently available experience with several set ups constructed for research institutes and industrial companies, the following specifications can be expected (deuterium lamp with low operation time):
| Wavelength Range | 115 nm to 230 nm | single lamp |
| 115 nm to 230 nm and 160 nm to 320 nm | double lamp | |
| Spectral Resolution | < 1 nm | FWHM of spectral lines at 160nm |
| Reproducibilit | < 0.1 nm | verified at 160 nm and 121.5 nm |
| Operation pressure | < 1×10-5mbar | proven value for optimum measurement |
| Accuracy of measurements | ||
| Transmittance | <0.3% | |
| Reflectance | <0.5% | |
| Transmittance out of band | <0.1% | |
| Scattered Reflectance | >0.001% | 157nm |
|
Reproducability |
157nm | |
| Diffuse Reflectance | <5% | 157nm |
| Scattered Reflectance | <5% | 157nm |
| (@ aperture of 1mm in front of PMT , wavelength dependent |
Description of the VUV/UV-Spectrophotometer
The scheme in the figure 1 outlines the modular set-up of the DUV/VUV-spectrophotometer. The three segments lamp compartment, monochromator and sample compartment are the main parts of the device. The system includes an oil-free pumping unit, which consists of a turbo pump and a diaphragm pump, and the power supply for the pumps and stages. In more detail on the right side of the figure 1, two deuterium (D2) lamps are installed, which can be selected by switching a mirror to the corresponding position. The different exit windows of the lamps, MgF2 and quartz, offer different spectral ranges of radiation. A spectrum from 115 nm up to 230 nm is available with the MgF2-window and from 155 nm up to 310 nm with the quartz window, respectively. The upper limit of the spectral ranges is restricted by the second order of the grating. Before entering the monochromator unit, the radiation is focused on the entrance slit which has a typical width of 100µm. A holographic concave grating coated with an aluminium and a protective layer serves as dispersive element. The diffraction efficiency of the grating is sufficient in a range from 100 nm up to 300 nm. The concave grating images the entrance slit onto the exit slit. These three components are arranged in a Seya-Namioka configuration. The selection of the wavelengths is given by a simple rotation of the grating.
Figure 1: Principle of the spectrophotometer
After the second mirror, the collimated beam propagates into the sample compartment. A polarizing module is moved into the beam path for investigations using s- and p-polarized light (This is an option). The polariser, as sketched in figure 2, is rotatable in an angle range between 0° and 270°. Following the path into the sample chamber, a scraper is installed to reflect a part of the beam to a reference detector. The other part of the beam passes the sample and impinges onto the signal detector. The monitoring by an additional reference detector allows for the correction of the measurement signal in respect to fluctuations in lamp intensity or a possible drift behaviour.As typical signal detectors, CCD-arrays, photomultiplier-tubes (PMT), micro-channel-plates (MCP) and diodes come into consideration. In this case both, reference and signal detector, are VUV-optimised photomultiplier tubes (PMT) and provide high sensitivity in the spectral range below 400 nm.
Figure 2 Polariser in ROCHON-configuration;
Additionally, the signal-PMT is adjustable in height and angle position for a correction of the beam path through thick or tilted samples. The basis of the sample mount is formed by a precision 4-axis positioning system, which enables the movement of a specimen in all Cartesian directions and additionally, a rotation to vary the angle of incidence. This combination of movable signal-PMT and sample mount allows the investigation of optical components in reflection, directly.
The complete chamber system can be evacuated to a pressure range of approximately p <5 ×10-5mbar or can be flushed with nitrogen (N2) avoiding absorption by oxygen and reducing the contamination of surfaces by other chemical compounds.
Specifications of the VUV/UV-Spectrophotometer
Principle: Monochromator with grating and deuterium-lamp as light source, optional arrangements with two separate deuterium lamps, detection of reference and sample signal by side on photo- multipliers, out coupling of the reference signal by a scraper mirror.
Sample holder: Vacuum chamber with separate access and valve revolving mount with four positions, one position for 100%-calibration, sample mount diameter up to 50 mm, different other options available: scanning up to four axes.
Monochromator: Computer controlled monochromator with single holographic grating, separate monochromator chamber, isolated from the sample compartment by a MgF2-window or linear gate valve.
Source, detector: Deuterium lamp 30 W or two lamps with SiO2 and MgF2-window optional, side on photo multiplier for reference and signal channel, head on photo multipliers optional.
Controlling system: Stepping motors for grating, sample mount and x-,y-adjustment of the signal photomultiplier, IBM compatible PC, Windows-programme,
Vacuum system: Turbo-molecular pump with roughing pump according application (rotary, scroll or membrane type), vacuum monitoring system for the sample chamber and the monochromator system.
Extensions for the basic system
Different options concerning the state of polarisation of the measurement radiation, sample mounting, geometric scanning of the sample position and angle of incidence, as well as a variety of sample holders and vacuum system configurations are available upon request:
