The Difficulties of Beam Profiling 193nm DUV

Author: 
Dick Rieley, Mid-Altantic Regional Sales Manager, Ophir Photonics

By Dick Rieley, Mid-Altantic Regional Sales Manager, Ophir Photonics Group

A research organization was developing a critical procedure that required a pulsed DUV laser beam at 193nm. The current equipment only produced a gray-scale image that told little about the distribution of the energy intensity across the beam profile. The solution was a CCD camera-based profiler.

Background
The organization was developing a new procedure that required the use of a pulsed DUV laser beam, specifically 193nm, produced by a Lambda Physik laser. Even after incorporating specialized DUV cameras, the current equipment was limited to producing gray scale images of the laser beam. The image was low on information related to the distribution of energy intensity across the beam profile. A typical image showed the following 2D beam image:

Figure 1. Original 2D beam image.
Figure 1. Original 2D beam image.

Although a live image of the beam, it did not provide the engineer with an understanding of how uniformly energy was being distributed across the beam, whether hot spots existed, or how uniform the beam was, side to side. Due to the critical nature of the process, it was imperative that beam quality be highly controlled and uniform in both X and Y directions.
 
Solution: BeamGage Camera and Software
Ophir-Spiricon offers a USB controlled CCD camera that has been calibrated to respond from 193nm to 1100nm. When used with BeamGage® beam diagnostic software, imaging the beam provided a true profile of the beam at 193nm. And it did so easily and in a cost-effective manner.
 
For the best resolution, the Ophir-Spiricon BGP-USB-SP620 camera, with an array 7.1mm x 5.4mm prepared with 4.4um x 4.4um pixels and a total matrix of 1600 x 1200 pixels, was selected. To allow this silicon-based camera to work properly and safely at 193nm, the camera was fitted with a 1x1 UV florescence lens. Prolonged exposure to DUV wavelengths can damage the silicon array, eventually degrading the performance of the array and the overall sensitivity of the camera.
 
CAUTION—Prepared by Ophir-Spiricon engineering:
Our CCD cameras respond to 193nm and will work until they fail. The failure mode is quite simple. The UV energy ablates the silicon structures until essential structures are removed and the device fails. The problem is actually more complex as there is a passivation layer that covers the silicon. The layer fluoresces but is eventually removed by the UV light. Once it goes, the response drops off because there is nothing left to fluoresce. This leaves low response areas across the camera, depending on where the laser beam has been. So the CCD will work but once it starts to lose the passivation layer, the response gets spotty, and then it stops working. How long does this take? It is all a matter of dose. If the beam is big and the fluence is low, the camera may last for tens of thousands of shots. If the beam is small and they exceed saturation, there will quickly be low response areas and then it will fail.

Figure 2. BGP-USB-SP620 Camera with UV Florescence Lens attached.
Figure 2. BGP-USB-SP620 Camera with UV Florescence Lens attached.

Results
The energy level of the 193nm source was extremely low, in the μW's of average power. And with the use of the florescence lens, multiple efforts were made to align and image the beam. After a number of failed attempts, it was determined the average power of the laser source was too low to effectively fluoresce the UV glass inside the lens assembly, preventing the camera from properly imaging the beam.
 
The next effort, in spite of possible damage to the silicon array, was to remove the UV lens and target the 7.1mm x 5.4mm silicon array directly with the UV beam. Concern for possible damage to the array, as mentioned above, was minimized due to the very low average power of the laser at this measurement location in the optical train. With this in mind, the BGP-USBSP620 camera was prepared with the bare array in the optical path of the 193nm laser.

Figure 3. Bare camera array.
Figure 3. Bare camera array.

BeamGage Professional, powerful beam diagnostic software, is the only profiling measurement package to include profiling algorithms that meet ISO standards. The software offers the option of summing frames of images when working with very low power laser sources. With the 193nm laser source targeting the array direction, a faint image of the beam was detected

Figure 4. BeamGage shows faint image of 193nm beam.
Figure 4. BeamGage shows faint image of 193nm beam.

The only method that allowed this very faint image (dark purple image) inside the circle to be seen was Ophir-Spiricon patented Ultracal algorithm. This patented, baseline correction algorithm helped establish the ISO 11146- 3 standard for beam measurement accuracy. The problems with cameras used in beam profile measurements are: (a) baseline, or zero, of the cameras drift with the temperature, and (b) include random noise. Ultracal is the only beam profiler algorithm that sets the baseline to "zero," and in the center of the noise. Competitive products use other less sophisticated algorithms that perform a baseline subtraction, but truncate the noise below the "zero" of the baseline. This leaves only a "positive" component, which adds a net value to all beam measurements.
 
Frame Summing
To enable a more enhanced image of this faint image, BeamGage offers a feature called Frame Summing. This allows a select number of frames to be compiled, or summed, to build up the image to one that is visible and representative of the laser source beam. In this case, several efforts were made: summing 25 frames, then 50 frames, then 100 frames. Eventually, it was determined that with 200 summed frames taking about 28 seconds to process, an image that was representative was fully achieved.

Figures 5 and 6. To get a representative image required 200 summed frames.
 

Figures 5 and 6. To get a representative image required 200 summed frames.

Figures 5 and 6. To get a representative image required 200 summed frames.

 

48% Difference in Power Density
What was important with this analysis is that the beam did not exhibit a degree of uniformity from side to side, as expected. BeamGage can be calibrated to power. In this case, it was 100μW's of power. By locating the cursor over a light yellow region in the SE quadrant area in the beam, the actual power density was determined. The power density was 1.228+3μW/cm2. (or 1228μW/cm2).

Figure 7. Power density measured at 1.228+3μW/cm2
Figure 7. Power density measured at 1.228+3μW/cm²

When the cursor was then positioned over the lighter blue-green area in the NW quadrant, the power density was measured at 8.283+2μW/cm² (or 829.3μW/cm²). That's a difference of 48%!

Figure 8. Power density measured at 8.283+2μW/cm2
Figure 8. Power density measured at 8.283+2μW/cm²

Summary and Conclusion
An initial analysis determined that the gray scale image did not provide useful information and it was limited in the quantitative data it could provide. However, use of the Ophir-Spiricion CCD camera and BeamGage Professional software provided a more meaningful analysis. Initial results demonstrated that the beam is not as uniform and consistent, as was expected. As the laser and optical variables are addressed, this beam profile process can document the improvement 'by the numbers' and help achieve a level of uniformity likely unachievable through any other means.

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