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Sensing & Measurement

Designing for the UV Band

A single-beam, lock-in spectrophotometer minimizes loss to enable accurate reflection measurements at UV wavelengths.

From oemagazine January 2004
31 January 2004, SPIE Newsroom. DOI: 10.1117/2.5200401.0007

Aaccurate measurement of polarization-dependent reflectivity for UV wavelengths can be challenging. Spectrophotometers are optimally designed for transmission measurements. Sample compartment upgrades can be adapted for polarization-dependent reflectivity measurements, but this is only effective in the visible and near-IR regions. Faced with the growing need to make accurate reflectivity measurements for UV optics at 193 nm to support our opto-mechanical subassemblies for semiconductor equipment manufacturers, we turned to designing and building our own system (see figure).

A single-beam, lock-in-method design provides a high performance, economical solution for UV spectrophotometry.

In standard commercial spectrophotometers, losses from optical components significantly diminish sample beam power in the UV range. Such systems would require N2 purging to mitigate any further beam power loss due to absorption in O2. To address these issues, we designed a single-beam spectrophotometer with fewer optical components in the beam path and a simplified configuration. It yields higher sample beam power and N2 purging is not necessary at 193 nm.

Another issue for standard designs is that the beam-steering complexity of standard accessories can lead to beam clipping or cause variations in beam location on the detector, which, coupled with the detector's spatial non-uniformity, can result in measurement variations. These variables can be controlled with the single-beam, lock-in method spectrophotometer.

Making polarization-dependent measurements requires the isolation of orthogonal polarization states. Only a limited number of broadband polarizers transmit well in the UV region: we selected the Rochon prism. A birefringent MgF2 Rochon prism can be used to create two linearly and orthogonally polarized beams. The relatively small divergence of the polarized beams exiting from the prism requires a longer beam path to the device under test to ensure sufficient transverse beam separation, however. In a commercial spectrophotometer, this leaves little to no room for further sample compartment reflection accessories, but our design allows sufficient space to be able to isolate one of the two polarized beams exiting the Rochon prism; the second beam is extraneous and can be dumped.

We started with a deuterium lamp coupled with a monochromator for controlled wavelength selection. The exit beam passes through a collimator and then through the Rochon prism, which is mounted such that the linearly polarized sample beam can be rotated to the proper orientation for either S or P reflection measurements. The beam passes through a chopper and initially straight to the photomultiplier tube (PMT) where we can establish a baseline using lock-in detection techniques.

With a precision-machined, angularly adjustable fixture, we can then rotate the PMT to the proper location to take sample reflectivity measurements at angles of incidence from 7° to 50°. The beam path reflects off of the test sample to reach the PMT. The received PMT signal passes to the lock-in detector, which enables frequency-domain detection, allowing us to minimize noise level. Using a general-purpose interface bus controller along with test, measurement, and control software, the user can program data-acquisition routines to create reflectivity graphs and manage the data as needed.

One critical design factor for the single beam spectrophotometer is the requirement for a stable deuterium light source. Dual-beam spectrophotometers typically have reference beams to compensate for drift in light-source power; a single-beam system does not provide this capability. We selected the L2D2 lamp (Hamamatsu Corp.; Bridgewater, NJ) due to its stability of better than 0.5% per hr at λ = 193.3 nm.

For polarization-dependent reflectivity measurements in the UV spectral range, the single-beam, lock-in-method spectrophotometer is an economical solution that can result in more accurate and repeatable metrology. oe


Kelly Child, Emmanuel Rabinovich
Kelly Child is marketing engineer and Emmanuel Rabinovich is metrology manager at CVI Laser, Albuquerque, NM.