Adding Grayscale Layer to Chrome Photomasks - Professor Glenn ...

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Table 2: Laser power and OD measurements for ten squares at top of chrome photomask Square # Laser Power Used Optical Density 1 6.43X10 -2 W 0.0823 2 5.98X10 -2 W 0.0780 3 4.59X10 -2 W 0.121 4 2.40X10 -2 W 0.311 5 2.07X10 -2 W 0.542 6 1.74X10 -2 W 0.655 7 1.40X10 -2 W 1.101 8 1.07X10 -2 W 1.233 9 7.372X10 -3 W 1.240 10 2.60X10 -3 W 1.291 The ten grayscale squares were written with the input argon laser power ranging from 2.6 mW to 64.3 mW resulting in optical densities from 0.0823 OD to 1.291 OD, respectively. The final Bi/In/Cr grayscale photomask has been sent to Benchmark Technologies for exposure and alignment testing. The grayscale mask will be tested in a commercial I-line 4X stepper system. The photomask will also be used to produce 3D structures on several photoresists that are typically used in the industry today. Analysis of the resulting structures using SEM will determine the feasibility of the bimetallic-chrome photomasks for commercial use. 6. OD CHANGE DURING LASER INDUCED OXIDATION OF BIMETALLIC GRAYSCALE CHROME PHOTOMASK In this section, we describe our investigation on how the differences in threshold power between chrome and Bi/In layers affect the film’s OD behaviour. The investigation consisted of measuring the change in OD with time for both chrome and bimetallic-chrome films as the laser light power is increased beyond the chrome’s threshold level. Another goal of the investigation was to see if there was a difference in rate of oxidation between chrome and Bi/In. If the rate of oxidation in chrome was found to be slower, OD changes in Bi/In could maybe be produced without affecting the chrome layer by scanning the Bi/In/Cr film very fast. We had previously measured changes in OD with respect to time in our investigations into the oxidation behaviour of Bi/In/O, Sn/In and Zn films [9]. Knowing how the OD of the Bi/In/Cr mask changes with time should shed light on the underlying oxidation processes taking place in the different layers of the mask. The OD transient response measurement system is shown in Figure 19. There are two identical photodetectors used in this setup: the Texas Instrument Monolithic Photodiode and Single-Supply Transimpedance Amplifier, OPT101P-ND. Due to the wide range of optical densities measured, the transmitted laser power, after the thin film, may vary from a few microwatts to about a hundred milliwatts. Previously, we had used a photovoltaic diode to measure low power laser light and a photoconductive diode to perform measurements in the high power range. In this paper, only one OPT101P-ND photodetector was used to measure light power for the entire range. To enable the measurement of this wide range of laser powers, the photodetector’s response was made to behave logarithmically with external diode circuits. To divert a portion of the laser beam for measuring purposes, a 60/40 non-polarizing beam splitter cube (Type BS010 from Melles Griot) splits the main laser beam into a sample beam and a reference beam. The sample beam goes to the mask and produces OD changes in the film. One of the photodetectors is placed beneath the mask to measure the remaining laser power after the beam passes through. The reference beam goes directly to one of OPT1010P-ND detectors allowing us to track changes to the main laser beam’s power. Taking the negative logarithm of the transmitted sample beam’s laser power over the reference beam’s laser power allows us to calculate the OD of the mask. Both the sample and reference photodiodes are reverse-biased externally at 30V. A Tektronix TDS2018 digital oscilloscope and a computer system connected to the oscilloscope’s communication module captures the outputs from the two photodetectors as the thin bimetallic film is exposed. The electro-optical shutter, which modulates the argon laser beam’s power when writing grayscale masks, is placed before the beam splitter; in this case, this acts to turn the writing beam on and off. A laser power versus photodiode voltage calibration is also performed on the system so that laser power information can be obtained from the photodetector voltages.
Figure 19. Transient time response measurement experimental system set up. The results for the transient response between chrome and Bi/In coated chrome films are shown in Figure 20. In all cases the laser beam was focused by the 50x objective to the 2 µm spot used in the mask writing. For the chrome film, there is a rapid decrease in the OD from 3 to about 1.5 in the first 10 ms of direct exposure to a 0.65 W beam and a gradual linear decrease to about 1 OD at 100 ms after exposure. We had previously suggested that this rapid change may be due to a combination of reduction in reflectivity and increased oxidation due to conversion to a liquid state. At 0.6W laser power, the Bi/In/Cr film exhibits a similar rapid decrease from about 4.4 OD to about 1.58 OD in the first 10 ms and decreases further to about 1.55 at around 20 ms after exposure. The OD then remained at this value unchanged up to and beyond 100 ms. At 0.65W laser power, the Bi/In/Cr film shows a different behavior, the OD initially shows a rapid decrease from 4.1 to 1.2 within the first 10 ms of exposure and then decreases linearly at a low rate to about 0.85 at 100 ms after exposure. These results follow very closely the behavior of chrome indicating that at 0.65W laser power the chrome layer’s OD is changing together with that of the Bi/In film. In contrast, at 0.6W, as soon as the film reaches its saturated OD of about 1.55 in about 20 ms no further change in OD is observed. The low rate drop at 0.65W is probably due to a significant amount of laser light penetrating the Bi/In layer and oxidizing the chrome. Whereas at 0.6W, the amount of laser power penetrating the Bi/In layer is low enough so that oxidation in the chrome does not continue after 20 ms. The lack of OD change after 20 ms means a much lower amount of chrome is converted at this power level. The result also implies that at low enough laser power changes in the OD of the Bi/In layer can be made without damaging the underlying chrome layer. 5 Optical Density (OD) 4 3 2 1 0 0 0.02 0.04 0.06 0.08 0.1 Time (second) Bi/In/Cr 0.6W Cr 0.6W Bi/In/Cr 0.65W Cr 0.6W Bi/In/Cr 0.6W Bi/In/Cr 0.65W Figure 20. Optical density transient time response for conventional chrome and Bi/In coated chrome photomasks.
Page 1 and 2: Adding Grayscale Layer to Chrome Ph
Page 3 and 4: 3. FILM DEPOSITION The first step i
Page 5 and 6: underlying chrome patterns, and wri
Page 7: Figure 11. Test square structures w
Page 11 and 12: Figure 21. Illustration of UV photo

Adding Grayscale Layer to Chrome Photomasks - Professor Glenn ...

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