The temperature impact on the performance of UHP pressure transducers is discussed.
BY YANLI CHEN, Ph.D. and MATTHEW MILBURN, P.E., UCT, Hayward, CA
As the semiconductor industry develops new films that require heated delivery systems, all related components need to be characterized at elevated temperatures. Vacuum pressure measurement components, typically called manometers, have been used at elevated temperatures for many years. In fact, many of the vacuum measurement transducers are internally heated to a known temperature to stabilize the mechanical relationships between moving parts and the sensors used to measure the movement. This stabilization enables the precision and inaccuracy of the measurement to be greatly improved. For positive pressure UHP transducers, this elevated temperature characterization has not been done. Based on the testing performed at UCT, temperature related performance variations are very real and must be carefully considered before choosing a positive pressure transducer for elevated temperature use. Since the industry is driving toward higher delivery system operating temperatures, temperature effects will become more important.
The UHP pressure transducer is a widely-used component in the semiconductor industry and the performance is very important for process control and process monitoring. Selecting a proper UHP pressure transducer with good performance for the specific application is challenging, because different UHP pressure transducers manufacturers have different parameters listed in their data and specification sheets. Behind the data presented, it was found that different test procedures and data processing methods were used to determine and report performance characteristics. This reality creates a situation where, without standardized test method or reporting format, neither the specifier nor the end user can compare the performance of different brands of pressure transducers. To date, the industry has not recognized the full scope of the specification problem nor developed a standardized testing and reporting program. A new push toward standardization has become available with the publishing of SEMIF113 “Test Method For Pressure Transducers Used In Gas Delivery Systems” in November of 2016.
In order to have a better understanding about the performance of different UHP pressure transducer manufacturers’ products, UCT initialized a comprehensive performance evaluation project with a participation of three major UHP pressure transducer manufacturers (MFG A, MFG B and MFG C). The totality of the project covered a total of nine test categories, including warm up time test, input voltage sensitivity test, repeatability, linearity, hysteresis and inaccuracy test, reproducibility test, thermal coefficient test, drift test, accelerated lift cycle test, proof and burst test. The topic of this paper is the thermal coefficient test. Interested readers can find the other article “Comprehensive performance evaluation of UHP pressure transducers” published on the VOL. 59 NO. 4 of Solid State Technology (June 2016), which demonstrated the test method of repeatability, linearity, hysteresis and inaccuracy.
Ideally, a pressure transducer would sense pressure and remain unaffected by other environmental changes. In reality, however, the signal output of every pressure transducer is somewhat affected by variations in environment and fluid temperature. Temperature changes can cause the expansion and contraction of the sensor materials, fill fluids, housings, and electronics. Temperature changes also can affect the sensor’s resistors and electrical connections through the thermoelectric effects. Typically, a sensor’s behavior regarding changes in temperature is characterized by two temperature coefficients: temperature effect on zero (TC zero) and temperature effect on span (TC Span). TC zero is expressed as a percentage of full scale and indicates the greatest deviation of a pressure transducer at zero setpoint per equal temperature change (such as 10K or 50°C) during the operating temperature range. TC span is also expressed as a percentage of full scale and indicates the greatest deviation of a pressure transducer at 100%FS setpoint per equal temperature change (such as 10K or 50°C) during the operating temperature range. FIGURES 1, 2 and 3 list the TC zero and TC span of pressure transducer products of MFG A, MFG B and MFG C, respectively.
Comparing the three thermal coefficient specifications above for MFG A, MFG B and MFC C, it is not possible to conclude which manufacturer’s product is the best for thermal behavior. Therefore, a standard test method and data process for thermal effects evaluation is needed.
Test setup and procedure
Three major UHP pressure transducer manufacturer (MFG A, MFG B, and MFG C) participated in this comprehensive performance evaluation project by providing test samples. Table 1 shows the detailed information of all the devices under tests (DUTs). Twelve DUTs were installed in a test fixture designed by UCT for running simultaneous tests. The schematic of the test fixture is shown in FIGURE 4. The benefit of this design is to save significant time that would be otherwise used for assembly, disassembly, and testing, and eliminates the potential for setup errors if each transducer was tested separately in the battery of tests.
The test was conducted in a temperature controlled environmental chamber (see Figure 5). The following sequence of steps were taken:
• A leak integrity test
• Make the initial zero adjustment per the manufacturer’s instructions
• Adjust the temperature of the environmental chamber to 0°C and allow the temperature to stabilize for a minimum period of two hours.
• Adjust the pressure to 0% FS (-14.7 psig), and record the signal output of all the DUTs and the pressure reference device after the pressure stabilization.
• Adjust the pressure to 100% FS(235.3 psig),andrecord the signal output of all the DUTs and the pressure reference device after the pressure stabilization.
• Repeat the same procedure for the temperature setpoints of 20°C, 40°C and 60°C at the pressure setpoints of 0%FS and 100%FS.
Results and discussion
The TC zero (0%FS) and TC span (100%FS) values of all DUTs are listed in Table 2. For each manufacturer’s sample group, the highest value for the thermal coefficients at zero and span are highlighted in red; the lowest value for the thermal coefficients at zero and span are highlighted in green. To reiterate, the smaller the TC value, the better.
• For the DUTs from MFG A, the smallest TC zero is 0.0022%FS/°C and the smallest TC span is 0.0324%FS/°C.
• For the DUTs from MFG B, the smallest TC zero is 0.0012%FS/°C and the smallest TC span is 0.0099%FS/°C.
• For the DUTs from MFG C, the smallest TC zero is 0.0102%FS/°C and the smallest TC span is 0.0215%FS/°C.
• For the DUTs from MFG A, the largest TC zero is 0.0127%FS/°C and the largest TC span is 0.0564%FS/°C.
• For the DUTs from MFG B, the largest TC zero is 0.0042%FS/°C and the largest TC span is 0.0155%FS/°C.
• For the DUTs from MFG C, the largest TC zero is 0.0283%FS/°C and the largest TC span is 0.0354%FS/°C.
The extreme TC values for each manufacturer are summarized in Table 3. As shown in this table, the MFG B product has the lowest value (0.0042%FS/°C) and MFG C product has the highest value (0.0283%FS/°C) for the TC zero. For the TC span, the MFG B product still has the lowest value (0.0155%FS/°C), and the MFG A product has the highest value (0.0564%FS/°C).
To compare the results to the published specification from MFG A, the results needed to be converted and are listed in Table 4.
Comparing test results with the published specifications (FIGURE 1), the MFG A devices are meeting their thermal coefficient specification.
To compare the results to the published specification from MFG B, the results needed to be converted and are listed in Table 5.
Compared with the published specifications (FIGURE 2), the MFG B devices are meeting their thermal coefficient specification at zero. All the MFG B devices except DUT 6 meet the of the thermal coefficient specification at span. However, the TC span for DUT 6 is 0.15550%FS/10K, which is very close to the specification value (0.15%FS/10K).
To compare the results to the published specification from MFG C, the results needed to be converted and are listed in Table 6.
Compared to the published MFG C specifications (FIGURE 3), the MFG C devices are meeting their thermal coefficient specification.
The error change with the temperature increase of all the DUTs at 0%FS is shown graphically in FIGURE 6. Comparing the three plots, it can be seen that the DUTs from manufacturer C have the largest thermal variation across the temperature range of the test as well as device to device variation. The DUTs from manufacturer B have the smallest thermal variation across the temperature range of the test as well as device to device variation.
The error change with the temperature increase of all the DUTs at 100%FS is shown graphically in FIGURE 7. Comparing the three plots, it can be seen that the DUTs from manufacturer C have the largest thermal variation across the temperature range of the test as well as device to device variation. The DUTs from manufacturer B have the smallest fluctuation across the temperature range.
Based on this study, transducers marketed as comparable to each other display dramatically different performance levels within a relatively small temperature range which could lead to process reproducibility challenges. As the demand for higher temperature applications increases, these temperature performance variances will become more pronounced. These variations may prove to be very problematic with tool-to-tool process replication or when a transducer is replaced as a repair activity and the new transducer does not have the same performance characteristic as the old unit. The test results also demonstrate that the published specifications need to be standardized to improve direct comparison by end users. In addition, a uniform test procedure and data processing method needs to be adopted by the industry. The pressure measurement task force of SEMI North America Gases and Facilities Committee has developed and published a new pressure transducer measurement standard in November of 2016 based on this study.
Temperature-related shift not only contributes to the overall inaccuracy of a pressure transducer in a particular application, but they also factor into the economics of designing and manufacturing pressure transducers. This is due to the fact that temperature compensation is a complex, time-consuming, and expensive process that requires a significantly larger investment in production equipment and a deeper understanding of the influencing parameters.
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6. SEMI F113-1116 Test method for pressure transducers used in gas delivery systems