Production of semiconductors is at an all-time high as global demand shoots through the digital (and literal) roof. With an ever-growing digital world, production will likely continue to expand as more and more goods and services entrench themselves into the world of 1s and 0s.
Monitoring pH and conductivity plays a vast role in semiconductor production. The delicate, highly detailed surfaces of microchips are constructed by ‘wafers’ and must be thoroughly polished via a specialized brush and pH-sensitive slurry before they are ready for use. The conductivity of this slurry is also monitored, ensuring that the correct slurry composition is utilized. When this process is complete, the wafers are washed with ultra-pure water (UPW), requiring additional conductivity monitoring. This process is known as chemical mechanical planarization (CMP).
Challenges in CMP Slurries
Wafer surfaces in semiconductors don’t exit the production line pristine and ready for use. The wafer surface must be polished to remove even the smallest abrasive particles. Typically, the wafers are constructed from a silicon base. However, wafers can also be formed from copper and other fine metals.
So, a simple spit shine and the wafer is ready to go? Well, not quite. The polishing process involves a pH & conductivity-dependent chemical slurry that passes over the surface as it is being polished. Maintaining stable pH and conductivity values ensures the chemical stability of the slurry over time, which is important for achieving consistent polishing results throughout the entire wafer batch. pH fluctuations can alter the reactivity of chemical agents and the abrasive efficiency of the slurry, leading to inconsistent or uneven polishing. Conductivity fluctuations can indicate a change in the composition of the slurry, hindering its effectiveness. This can cause defects or variations between wafers, reducing the overall yield in semiconductor manufacturing.
The high ionic strength of the slurry can cause inconsistencies in pH readings. Typical pH sensors might struggle with this complexity, making calibration and measurement more challenging. CMP slurry can be either highly acidic or highly basic, depending on the materials being polished (acidic for copper, basic for silica). This can create a corrosive environment for pH sensors. Additionally, monitoring the conductivity of the slurry requires a sensor with a wide conductivity range. The sensor’s material of construction must also be considered to achieve chemical compatibility with the slurry. The additional conductivity sensor for the ultra pure water bath must maintain accuracy within low conductivity ranges.
Maintaining Stable pH and Conductivity Values in CMP Slurry
The demands of wafer polishing require reliable, durable pH sensors that can withstand varying ranges of pH levels. The SE 554 Memosens pH sensor is the ideal solution tailored for a wide range of applications. Designed to withstand the most aggressive process across a wide range of pH values, the SE 554 is equipped to handle the varying pH values and chemical composition of CMP Slurry.
The SE 656 conductivity sensor is a corrosion-resistant sensor constructed of PFA in a smooth, easy-to-clean design. PFA is the ideal material when extended service is required in hostile environments involving chemical, thermal, and mechanical stress. This toroidal conductivity sensor maintains accuracy across a wide range and maintains material compatibility with the demands of CMP slurry measurement.
For UPW requirements, the SE 625 conductivity sensor is the ideal solution. The sensor has robust, coaxially-arranged titanium Grade 2 electrodes. Operators can easily clean the sensor thanks to the detachable outer electrode. Additionally, an integrated temperature element assures that all conductivity measurements are accurately compensated for the process media temperature.
Introducing real-time diagnostics can also further improve the efficiency of the polishing process. When working within necessarily precise values, technicians can utilize the diagnostic features of the Stratos Multi transmitter to make real-time, necessary adjustments to the process. Dosing agents and other techniques used to regulate pH are thus optimized using the diagnostic features of the Stratos Multi transmitter.
Return on Investment
Improved Yield and Quality: Consistent pH and conductivity levels in the CMP slurry ensure uniform material removal and smooth wafer surfaces, leading to better yield.
Reduced Equipment Downtime and Maintenance: Maintaining accuracy with pH and conductivity monitoring can detect the buildup of contaminants or ions that might precipitate and clog equipment, reducing maintenance-related downtime. Chemical imbalances can be closely monitored with sensor diagnostics via the Stratos Multi, providing further insight into the process.
Optimized Resource Consumption: Maintaining consistency with pH and conductivity measurements leads to fewer adjustments and wash cycles, reducing water and energy consumption.
