Optimization of Post Sulfuric Acid/Hydrogen Peroxide Dump Rinsing Processes
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programming and temperature. Our optimized dump rinsing process, resulted in a factor of five saving in water consumption and factor of two reduction in cycle lifetime. EXPERIMENT The experiments were performed at the Hewlett Packard ULSI fabrication facility in Palo Alto, CA. Conductivity measurements were made as a function of time during the rinsing cycles. These measurements were used to determine the amount of chemical residue in the rinse tank and served as an in situ probe to help determine the quality of the rinsing process. Conductivity and pH measurements were made in an automated wetbench during rinsing using an Accumet model 50 pH/conductivity meter. The probe was calibrated using NIST standard buffers for pH measurements and NIST standard conductivity solutions for conductivity measurements. Our experimental procedure was; (1) load 25 to 50 wafers, (2) robotically transfer wafers to a sulfuric acid/hydrogen peroxide mix (SPM) chemical bath, (3) process wafers for one minute, (4) robotically transfer wafers to the rinse tank and (5) rinse wafers as a function of time, flow rate and quick rinsing dump combinations. The SPM ratio was 4 to 1 sulfuric acid to 30% hydrogen peroxide. We used 150 mm wafers that were placed in two cassettes holding 25 wafers each. The spacing between wafers was 0.476 cm. The deionized water (DIW) flow rate was varied from 0.5 to 10 gpm and the pullout velocity of the wafer cassettes from the SPM and rinse tanks was 10 cm/sec. The total transfer time from the SPM to the rinse tank required approximately 18 seconds. The temperature of the SPM bath was varied from 25 to 120 'C and the temperature of the DIW was either 25, 40 or 80 'C. We determined the quality of the optimized rinsing process relative to the standard rinsing process from the in situ conductivity measurements, sulfate analysis and also from ex situ post-rinsing measurements of light point defects (LPDs) and sulfur residue on the wafer surfaces. We measured LPDs in the range 0.2 to 0.3 gim in effective diameter using a Tencor particle counter. LPDs were measured for wafers as received and after the standard and optimized rinsing processes. Sulfur residues on the wafer surfaces were determined by total reflectance x-ray fluorescence (TXRF) [11]. The TXRF measurements were made on wafers as received and after the standard and optimized rinsing processes. RESULTS We performed several preliminary experiments to calibrate the conductivity measurements. First, we wanted to accurately determine the volume of the carryover layer from SPM chemical bath to the rinse tank. To do this we studied the conductivity in the rinse tank as a function of a predetermined amount of SPM titrated into the rinse tank. We found that conductivity had a linear dependence on the amount of SPM for 0 to 70 ml of SPM added. We then performed the same experiment with an empty cassette and with a cassette loaded with 1 to 50 wafers. The conductivity had a predictable dependence on the number of wafers. We found the carryover of the empty cassette w
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