How Do I Select a Pressure Transducer for Semiconductor UHP Liquid Applications?

Selecting a pressure transducer for semiconductor ultrahigh-purity (UHP) liquid applications requires careful consideration of material compatibility, contamination control, accuracy, repeatability and durability. These applications often involve aggressive chemicals and ultrapure water, making the construction and performance of the pressure instrument especially important.

Pressure sensors, including pressure transducers and pressure transmitters, play a critical role in semiconductor manufacturing. They measure pressure to help control the flow and distribution of UHP gases and liquids safely and effectively throughout the production process.

In this article, we'll focus on liquid processes, including how UHP liquids are used and the pressure transducer features that can help provide reliable measurement. We also have a separate article that focuses on pressure transducers for semiconductor UHP gas applications and one that addresses how air systems are monitored in these applications. 

How are liquids used in semiconductor manufacturing?

Liquids are used throughout semiconductor manufacturing for processes such as polishing, etching and cleaning silicon wafers. Because contamination can affect semiconductor quality, these processes require precise control and extremely pure materials.

Semiconductor manufacturing converts a blank silicon wafer into hundreds or thousands of individual microelectronic chips. Silicon starts as 99.99% pure laboratory-grown silicon ingots measuring approximately 200 to 300 mm (7.9 to 11.8 in.) in diameter. These ingots are sliced into wafers well under 1 mm (0.04 in.) thick.

Materials are then layered onto the wafer to form the pathways and junctions that enable electronic data to flow and be stored. Exceptional precision is required throughout this process to minimize contamination at the atomic level and maintain the purity of each device.

What are the primary liquid processes in semiconductor manufacturing?

Three important processes that rely on liquids during semiconductor manufacturing are chemical mechanical planarization (CMP), wet etching and wafer cleaning. Each introduces different chemical, purity and material compatibility challenges for pressure measurement equipment.

1. How are liquids used to polish semiconductor wafers?

Chemical Mechanical Planarization (CMP) uses a chemical slurry to help achieve the extremely flat wafer surface required for semiconductor manufacturing.

The slurry can be alkaline or acidic and contains nanoscale particles that chemically and abrasively remove material from the wafer. This process also helps eliminate impurities, particularly metals, that could create surface defects.

Any surface that contacts the slurry must resist:

• Abrasion
• Chemical breakdown
• Leaching

Pressure transducers used in these applications can incorporate high-purity fluoropolymers such as Teflon™, PTFE or PFA to withstand these challenging process conditions.

However, fluoropolymers have relatively poor mechanical properties for precise pressure measurement. One approach is to combine a PTFE/PFA diaphragm that contacts the process fluid with a ceramic sensor element. This design takes advantage of the chemical-resistant properties of the fluoropolymer while supporting precise and reliable pressure measurement.

2. How are liquids used during wet etching?

Wet etching uses powerful acids or alkalis to selectively remove semiconductor material and create the required structures or patterns on the wafer.

Chemicals commonly used during wet etching include:

• Potassium hydroxide (KOH)
• Tetramethylammonium hydroxide (TMAH)
• Hydrochloric acid (HCl)
• Hydrofluoric acid (HF)
• Nitric acid (HNO₃)

These liquids can extract impurities from material surfaces and permeate certain materials. As a result, pressure transducers used in wet etching systems must use wetted materials that are compatible with these aggressive chemicals while helping maintain the required process purity.

3. How are semiconductor wafers cleaned?

Semiconductor wafers are typically cleaned with ultrapure water to remove etching chemicals and stray particles without introducing additional contamination.

The purity requirements for this water are extremely stringent. Semiconductor fabrication facilities also require large quantities of ultrapure water, with a typical fab using more than 2 million gallons (8 million liters) per day.

Standards including SEMI F63 and ASTM D5127 establish requirements related to ultrapure water. Once water reaches these purity levels, it becomes an aggressive solvent. Therefore, materials used in process piping, controls and measurement devices must be exceptionally pure and compatible with the application.

What should I consider when selecting a pressure transducer for semiconductor UHP liquids?

A pressure transducer for semiconductor UHP liquid applications should combine high-purity, chemically compatible wetted materials with the accuracy, repeatability and durability required by the process.

Due to the harsh nature of semiconductor liquid applications, pressure instruments such as the Ashcroft® ZL91 Fluoropolymer Pressure Transducer must meet demanding requirements for both material compatibility and measurement performance. Here's a little more detail about each consideration:

1. Choose high-quality, compatible materials

Pressure transducer materials should be compatible with the process media while minimizing the potential for contamination.

High-purity grades of fluoropolymers such as polytetrafluoroethylene (PTFE) and perfluoroalkoxy (PFA) are commonly used in semiconductor liquid applications because of their chemical resistance and ability to help prevent contamination.

Material compatibility is particularly important when the transducer will be exposed to aggressive acids, alkalis, chemical slurries or ultrapure water.

2. Look for repeatable performance

Repeatability helps ensure that the pressure transducer provides consistent measurements when the same pressure conditions are applied repeatedly.

In semiconductor manufacturing, you need to trust the measurement provided by the instrument. Ideally, a pressure transducer should provide a consistent reading each time without requiring frequent zero or span adjustments.

3. Consider durability

A durable pressure transducer should withstand the mechanical conditions it may encounter without sacrificing measurement performance.

Potential challenges can include:

• Shock
• Vibration
• Pressure spikes
• Pulsation

If the transducer is not appropriately designed for these conditions, exposure can damage the instrument or affect its performance. Selecting a transducer designed for rugged operating environments can help support reliable, long-term operation.

4. Confirm the required accuracy

The pressure transducer's accuracy specification should meet the requirements of the specific semiconductor process.

Consider the sensing technology and how the manufacturer defines the instrument's accuracy. Some instruments offer ±1% of span accuracy out of the box, with zero and span setting errors already included in the specification.

In these cases, the pressure transducer can be installed without additional calibration adjustments to compensate for those initial setting errors.

How do I choose the right pressure transducer for my UHP liquid application?

The right pressure transducer depends on the process fluid, required purity, operating conditions and measurement performance your semiconductor application requires.

When evaluating a pressure transducer, consider:

• Process media: Identify the acids, alkalis, slurries, ultrapure water or other liquids that will contact the transducer.
• Wetted materials: Confirm that the materials are chemically compatible with the media and meet the application's purity requirements.
• Accuracy: Determine the level of measurement accuracy required for the process.
• Repeatability: Look for consistent measurement performance under repeated operating conditions.
• Durability: Account for shock, vibration, pressure spikes and pulsation that may occur in the system.
• Calibration requirements: Understand whether zero or span adjustments are required before installation.

Evaluating these factors together can help identify a pressure transducer that provides reliable measurement while maintaining the chemical compatibility and purity required for semiconductor UHP liquid processes.

Ready to learn more?

Semiconductor manufacturing continues to evolve as technologies such as artificial intelligence (AI), autonomous driving and the Internet of Things (IoT) increase demand for advanced semiconductor devices. As manufacturing processes evolve, pressure measurement equipment will continue to play an important role in maintaining process control and purity.

Check out the related resources below or contact us if you need help selecting a pressure measurement solution for your semiconductor application. In the meantime, download our guide to learn more.