Load Lock Gauge

Abstract

The loadlock pressure gauge comprises a housing configured to be coupled to a loadlock vacuum chamber. The housing supports an absolute vacuum pressure sensor providing an instantaneous high vacuum pressure signal over a range of high vacuum pressures, and a diaphragm differential pressure sensor providing an instantaneous differential pressure signal between the loadlock pressure and ambient pressure. The housing further supports an absolute ambient pressure sensor. A low vacuum absolute pressure is calculated from the instantaneous differential pressure signal and the instantaneous ambient pressure signal. A controller of the housing can recalibrate the diaphragm differential pressure sensor based on a measured voltage of the sensor and a measured ambient pressure during normal operation of the gauge through routine cycling of pressure in the loadlock.

Description

Related Applications

[0001] This application is a continuation of U.S. Patent Application No. 17 / 154,698, filed January 21, 2021. The entire teachings of the above application are incorporated herein by reference. [Background technology]

[0002] A loadlock is a standard feature in industrial and laboratory vacuum process chambers. As shown in FIG. 1, the samples 102 are fed into the loadlock 106 through the loadlock port 104 while the loadlock 106 is vented to ambient (atmospheric) pressure. Most loadlocks are expected to remain particle-free when exposed to the environment. Once the samples are placed in and secured, the loadlock port is closed and pumping down to high vacuum through the vacuum pump 108 begins. When the pressure in the loadlock is low enough, the transfer port 110 is opened to connect the loadlock chamber 106 to the process chamber 112, which has its own vacuum pump 112 and pressure gauge 107. The samples are then transferred through the transfer port to the process chamber where they will be processed. Once the process is complete, the samples are transferred back to the load lock, which is still at high vacuum, the transfer port is closed, and the load lock port is opened to the ambient or a front opening unified pod (FOUP), and the load lock is vented so that the processed samples can be removed.

[0003] During proper loadlock operation there are several important requirements.

[0004] The loadlock must remain particle free at all times. This requires ensuring that the loadlock port, when opened, must be at a slightly positive pressure relative to the ambient pressure. This gives an assurance that any airflow will exit the chamber when the loadlock port is opened, preventing room dust particles from entering the loadlock chamber. In the absence of ambient air inflow, dust particle intrusion is avoided. An accurate differential pressure measurement between the loadlock chamber and ambient is required to ensure that the loadlock port is only opened when the loadlock chamber pressure is greater than the ambient pressure. The loadlock pressure gauge is expected to provide a very accurate differential measurement.

[0005] The loadlock pressure must also be carefully monitored during pumpdown. The transfer port is only opened at a low enough pressure to preclude the transfer of dust particles from the loadlock chamber to the process chamber. Typically, a pressure of less than 1 Torr is required in the loadlock chamber to preclude the entrainment of dust particles into the high vacuum process chamber during transfer. Accurate absolute pressure measurement at the high vacuum of the loadlock chamber is necessary and required by the loadlock operator.

[0006] Therefore, the load lock chamber must be equipped with a high-precision pressure measurement gauge capable of performing two very important measurements with high accuracy: a. Measure the pressure differential between the load lock chamber and ambient prior to opening the load lock port to ambient conditions. b. Measure the absolute pressure at high vacuum inside the load lock chamber, especially before opening the transfer port to the process chamber.

[0007] Gauges used for pressure monitoring and control in load lock chambers are often referred to as load lock gauges.

[0008] A load lock gauge that met the above requirements was disclosed in U.S. Pat. No. 6,672,171. The gauge included both a high vacuum absolute pressure sensor in the form of a Micro-Electro-Mechanical System (MEMS) Pirani sensor, specifically a MicroPirani™ gauge, and a differential pressure sensor in the form of a differential diaphragm sensor with one side of the diaphragm exposed to the pressure of the load lock and the other side of the diaphragm exposed to ambient pressure. Although the device met the above requirements, the MEMS Pirani gauge, which provides high accuracy at high vacuum pressures required to control the transfer ports, only has an accuracy of about + / - 25% at ambient. However, some process engineers had expectations of accurate absolute pressure measurements in the load lock throughout the pressure range from ambient to high vacuum. U.S. Pat. No. 6,909,975 addressed that additional requirement.

[0009] In one embodiment of US Pat. No. 6,909,975, the differential pressure sensor was replaced with two absolute pressure sensors: a piezoresistive diaphragm absolute pressure sensor that was accurate at the low vacuum pressure of the loadlock, and an absolute pressure sensor that was accurate at the ambient pressure outside the loadlock. The differential pressure requirement to open the loadlock port was then met by calculating the difference between the low vacuum pressure sensor and the ambient pressure sensor. The full absolute pressure range was obtained by basing the output of the Pirani sensor at high vacuum, the absolute piezoresistive diaphragm sensor at low vacuum, and the mixture of the two outputs at the intermediate vacuum level. An early implementation of that embodiment was the MKS390 pressure gauge. The MKS390 gauge included a MEMS Pirani sensor, an absolute piezoresistive diaphragm sensor, and a barometer. It further included an ionization pressure gauge, as also proposed in US Pat. No. 6,909,975, to provide very high vacuum absolute pressure readings at even higher vacuums than could be supported by the Pirani sensor.

[0010] The difficulty with using a separate absolute gauge to calculate the target differential pressure is that the target differential pressure is not directly sensed, and calculations based on the sensed pressures of separate sensors are prone to error. Process engineers prefer the highly reliable direct differential readings from differential diaphragm sensors.

[0011] US Patent No. 6,909,975 presented a method in which the diaphragm differential pressure sensor output could be used to provide an accurate absolute pressure reading at low vacuum pressure. Thus, a correlation coefficient between the absolute pressure measurement and the differential pressure measurement was taken where the absolute pressure of the chamber could be accurately and reliably measured, and the correlation coefficient was stored. The differential pressure measurement was then adjusted with the correlation coefficient to provide a virtual absolute pressure measurement. Although the correlation coefficient could be taken and stored at atmospheric pressure when the differential pressure measured zero, the preferred method was to measure the output of the differential pressure sensor at high vacuum, which is equal to negative ambient pressure, and store the absolute value as the correlation coefficient. That correlation coefficient could then be added to the differential output of the differential pressure sensor to provide absolute pressure readings throughout the entire range of pressures of the loadlock. The output of the Pirani sensor then provided an accurate high vacuum absolute pressure reading, the virtual absolute pressure calculated from the differential pressure sensor output provided the absolute pressure at low vacuum, and a mixture of the two provided the absolute pressure at the intermediate vacuum pressure. That approach was used in the highly successful MKS901P gauge, which was the most widely used load lock gauge in the industry for over a decade.

[0012] The MKS901P gauge is based on a MEMS Pirani sensor for the absolute high vacuum (low pressure) output and a Differential Piezoresistive Diaphragm (DPRD) sensor for the differential and low vacuum virtual pressure outputs. Each time the loadlock is pumped below approximately 1 Torr, the absolute value of the differential pressure is measured and stored in memory as a constant magnitude positive number. This is essentially the most recent measurement of ambient pressure performed with high accuracy on the DPRD itself. From then on, all differential readings from the DPRD can be subtracted from that constant value to provide the absolute pressure in the loadlock chamber. Because the DPRD is used to generate a stored value of ambient pressure (during pumpdown) which is then used to calculate the loadlock chamber pressure during venting, the readings are more accurate at low vacuum than those provided by a Pirani sensor. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] U.S. Patent No. 6,672,171 [Patent Document 2] U.S. Patent No. 6,909,975 Summary of the Invention [Problem to be solved by the invention]

[0014] The MKS901P gauge uses the virtual pressure approach of U.S. Pat. No. 6,909,975 to provide the required very accurate high vacuum pressure from a Pirani sensor for controlling the transfer ports, high accuracy differential pressure for controlling the load lock ports, and even high accuracy absolute pressure at low vacuum. To maintain that high accuracy low vacuum absolute pressure, it is important that the correlation coefficient corresponds to the actual ambient pressure. Therefore, the correlation coefficient is updated every time the load lock pressure is reduced to high vacuum. However, ambient pressure fluctuations of + / - 10 Torr can occur throughout the day. If the correlation coefficient is not updated often enough, the ambient pressure fluctuations can lead to inaccuracies in the low vacuum absolute pressure readings. Although those readings are not critical to the differential of the transfer ports or load lock ports, they are less than optimal. To provide even better accuracy even at low vacuum pressures, another absolute pressure sensor, such as the absolute piezoresistive diaphragm sensor used in the MKS390 product, can be added to the gauge. However, such a sensor would greatly increase the overall cost of the gauge and would suffer from undesirable hysteresis in itself. [Means for solving the problem]

[0015] To improve the accuracy of the absolute low vacuum pressure readings derived from the diaphragm differential pressure sensor at a reasonable cost, an absolute ambient pressure sensor is added to the gauge. Such gauges include a diaphragm that is exposed to a reference vacuum on one side and to ambient pressure on the other side. They are widely used as barometers, for example in altimeters, and therefore provide very accurate ambient pressure readings at low additional cost. With the ambient pressure sensor included in the gauge, it is no longer necessary to store correlation coefficients at specific process conditions. Rather, accurate instantaneous ambient pressure can be added to a very accurate differential pressure over the entire range of load lock pressures. The ambient pressure sensor included in the gauge also allows synchronization of the Pirani gauge at ambient pressure and automatic recalibration of the differential pressure sensor during normal operating cycles based on the sensed ambient pressure.

[0016] The pressure gauge comprises a housing configured to be coupled to a vacuum chamber. An absolute vacuum pressure sensor within the housing is configured to be exposed to the vacuum chamber and is operable at high vacuum to provide an accurate instantaneous high vacuum pressure signal over a range of high vacuum pressures. A diaphragm differential pressure sensor within the housing comprises a diaphragm configured to be exposed on one side to the vacuum chamber and on an opposite side to ambient pressure to provide the instantaneous differential pressure signal. An absolute ambient pressure sensor within the housing is exposed to ambient pressure to provide the instantaneous ambient pressure signal. A controller within the housing receives the high vacuum pressure signal from the absolute vacuum pressure sensor, the differential pressure signal from the diaphragm differential pressure sensor, and the ambient pressure signal from the ambient pressure sensor. The controller calculates a calculated absolute pressure of the vacuum chamber from the instantaneous differential pressure signal and the instantaneous ambient pressure signal, both sensed throughout a range of low vacuum pressures. The controller provides as outputs a differential pressure output from the diaphragm differential pressure sensor signal, an absolute vacuum pressure output taken from the absolute vacuum pressure sensor signal at high vacuum, and an absolute vacuum pressure output from the calculated absolute pressure at low vacuum.

[0017] The controller may be further configured to calibrate the diaphragm differential pressure sensor based on an ambient pressure bridge voltage of the diaphragm differential pressure sensor when the vacuum chamber is at ambient pressure, a full-scale bridge voltage of the diaphragm differential pressure sensor when the vacuum chamber is at full-scale high vacuum, and a full-scale ambient pressure measured by the absolute ambient pressure sensor when the vacuum chamber is at full-scale high vacuum.

[0018] The controller may be configured to calibrate the diaphragm differential pressure sensor by measuring an ambient pressure bridge voltage of the diaphragm differential pressure sensor when the vacuum chamber is at ambient pressure, measuring a full-scale bridge voltage of the diaphragm differential pressure sensor when the vacuum chamber is at full-scale high vacuum, and measuring full-scale ambient pressure with the absolute air pressure sensor when the vacuum chamber is at full-scale high vacuum. The controller may determine a gain for the diaphragm differential pressure sensor from the ambient pressure bridge voltage, the full-scale bridge voltage, and the full-scale ambient pressure. An instantaneous differential pressure signal may then be determined from the instantaneous bridge voltage, the ambient pressure bridge voltage, and the gain. The gain may be

[0019] a=(V b,0 -V b,FS ) / P amb,FS

[0020] where a is the gain and V b,0 is the ambient pressure bridge voltage, and V b,FS is the full-scale bridge voltage, and P amb,FS is the full-scale ambient pressure. The pressure difference is

[0021] P D =(V b -V b,0 ) / a

[0022] where P D is the instantaneous pressure difference, and V b is the instantaneous voltage from the diaphragm differential pressure signal.

[0023] The pressure gauge may further provide an output of instantaneous ambient pressure from the absolute ambient pressure signal.

[0024] The absolute vacuum pressure sensor may have an accuracy of 7% or better at a high vacuum pressure of 5 Torr. The calculated absolute pressure from the instantaneous differential pressure readings may have an accuracy of 1% or better at 60 Torr.

[0025] The controller may be configured to calculate a mixed pressure from the instantaneous high vacuum pressure signal and the calculated absolute pressure at the intermediate vacuum pressure. The controller may output as the chamber pressure signal the instantaneous high vacuum pressure signal over a range of high vacuum pressures, the calculated absolute pressure over a range of low vacuum pressures, and the mixed pressure at the intermediate vacuum pressure.

[0026] The controller may be configured to zero the absolute vacuum pressure signal at ambient pressure.

[0027] The absolute vacuum pressure sensor may be a thermal conductivity sensor, for example a MEMS Pirani sensor. The diaphragm differential pressure sensor may be a differential piezoresistive diaphragm pressure sensor. Other differential diaphragm gauges may also be used, but piezoresistive gauges have the advantage of being small and inexpensive. The absolute ambient pressure sensor may comprise a diaphragm exposed to a reference vacuum on one side and ambient pressure on the opposite side.

[0028] In a method of controlling a loadlock port and a transfer port of a loadlock, an instantaneous high vacuum pressure signal may be provided from an absolute vacuum pressure sensor. An instantaneous differential pressure signal may be provided from a diaphragm differential pressure sensor having a diaphragm exposed to the loadlock on one side and exposed to ambient pressure on an opposite side. An instantaneous ambient pressure signal may be provided from an absolute ambient pressure sensor. In an electronic controller, a calculated absolute ambient pressure may be calculated from the instantaneous differential pressure signal and the instantaneous ambient pressure signal. Only after the loadlock reaches a sufficiently low pressure as indicated by the instantaneous high vacuum pressure signal from the absolute vacuum pressure sensor. Only after the instantaneous differential pressure signal from the diaphragm differential pressure sensor indicates a loadlock pressure at least as high as ambient pressure is the transfer port opened to ambient. An absolute vacuum pressure output from the controller is an absolute vacuum pressure output taken from the absolute vacuum pressure sensor at high vacuum and from the calculated absolute pressure at low vacuum. [Brief description of the drawings]

[0029] The foregoing will be apparent from the following more particular description of exemplary embodiments, as illustrated in the accompanying drawings, in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the embodiments. [Figure 1] 1 is a schematic diagram of a loadlock having a loadlock gauge, the loadlock being coupled to a process chamber. [Diagram 2] 1 is a cross-sectional view of a load lock pressure gauge embodying the present invention; [Figure 3A] 3 is a cross-sectional schematic diagram of the diaphragm differential pressure sensor of FIG. 2 in one of three different pressure states of the load lock. [Figure 3B] 3 is a cross-sectional schematic diagram of the diaphragm differential pressure sensor of FIG. 2 in yet another of three different pressure conditions in the load lock. [Figure 3C] 3 is a cross-sectional schematic diagram of the diaphragm differential pressure sensor of FIG. 2 in yet another one of three different pressure conditions in the load lock. [Figure 4] FIG. 3 is a schematic cross-sectional view of the ambient pressure sensor of FIG. 2. [Diagram 5] FIG. 3 is a schematic cross-sectional view of the MEMS Pirani gauge of FIG. 2. [Figure 6] 3 is a flow chart of the pressure measurement loop of the gauge of FIG. 2. [Figure 7] 4 is a graph of the bridge voltage output from the differential pressure sensor of FIGS. 3A-C plotted against differential pressure. [Figure 8A] 13 is a pressure gauge controller microprocessor flow chart for determining a differential pressure sensor offset. [Figure 8B] 13 is a controller microprocessor flow chart for slope determination and full-scale calibration of a differential pressure sensor. [Figure 9] FIG. 2 is a schematic diagram of a system similar to that of FIG. 1, but with an additional very high vacuum pressure sensor coupled to the load lock. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] A description of an exemplary embodiment follows.

[0031] The multiple sensor gauge provides the ability to accurately measure both absolute and differential pressure in the load lock chamber throughout the entire range of pressures without the limitations of current equipment. The MEMS Pirani sensor provides high vacuum absolute pressure for transfer port control. The Differential Piezoresistive Diaphragm (DPRD) pressure sensor provides the single sensor differential pressure readings that the industry has come to expect from the MKS901P and its load lock switches.

[0032] The addition of the barometer allows for real-time ambient pressure measurement updates, providing the most up-to-date absolute pressure inside the loadlock chamber from the differential pressure sensor, and provides an improvement over the fixed ambient pressure correlation coefficient that is stored by the 901P and updated only during loadlock pumpdowns. No need to perform repetitive pumpdowns throughout the day to know the ambient pressure and calculate an accurate loadlock chamber absolute pressure from the DPRD measurements.

[0033] There is also an opportunity to synchronize sensors.

[0034] The differential sensor can be synchronized with the ambient sensor whenever the high vacuum is less than about 0.5 Torr inside the load lock chamber. When the absolute pressure is less than 0.5 Torr, which is approximately the typical accuracy of a barometer, it can be ensured that the ambient sensor and the DPRD read within the same 0.5 Torr or so. If not, the differential pressure sensor can be automatically recalibrated as discussed below. The ambient sensor continues to track the shifting ambient pressure with high precision, ensuring accurate calculation of the load lock chamber absolute pressure. The ambient sensor is precisely calibrated prior to use and can be relied upon during operation for reliable measurement of ambient pressure.

[0035] The MEMS Pirani atmospheric reading can be synchronized whenever the load lock chamber is vented to ambient. At this point the differential sensor reads zero differential pressure and the Pirani is synchronized to the barometer. This improves the accuracy of the Pirani sensor at near ambient pressure.

[0036] FIG. 2 shows a pressure gauge with a particular application as a load lock gauge, which is based on but improves upon the MKS901P gauge. A block 202 has a flange 204 for mounting the gauge to the side of the load lock chamber. A volume 206 within the block is exposed to the load lock chamber. A conduit 208 from the volume 206 through the block exposes a tube 210 to the load lock pressure. The tube 210 extends to a differential pressure sensor 212, which may be a differential piezoresistive diaphragm sensor (DPRD) (the gauge is shown only as a solid block in FIG. 2; further details are shown in FIGS. 3A-C). The differential sensor 212 is mounted to a sensor board 213. It includes a protective can 215 that allows exposure of the differential sensor to ambient pressure from outside the gauge.

[0037] A second conduit 214 of the block 202 extends to a volume 216 to which a MEMS Pirani absolute pressure sensor 218 is exposed (the sensor 218 is shown only as a solid block in this view and is described further with respect to FIG. 5). The Pirani gauge 218 is suspended below the sensor board 213 by electrical pins 219.

[0038] An additional sensor not available in the 901P gauge is an ambient pressure sensor 220, which may be a conventional barometer. The sensor 220 is exposed only to ambient pressure. Barometers are widely available with high accuracy of less than plus or minus 2 Torr. With recalibration, the accuracy can be improved to plus or minus 0.5 Torr, preferably plus or minus 0.25 Torr, or better. Barometers usually include temperature compensation.

[0039] Additional electronics, including a controller microprocessor 226, are mounted on board 213 and on a microprocessor board 222. The sensors and electronics are enclosed within a protective housing 223. Communication with the pressure gauge is through electrical connection 224. As will be discussed in more detail below, the microprocessor 226 is responsible for pressure measurement and output. In particular, the microprocessor is responsible for pressure synchronization of the Pirani sensors and synchronization and calibration of the differential diaphragm sensors.

[0040] 3A-C are schematic diagrams of differential pressure sensor 212 in three different states. The sensor includes a diaphragm 302 exposed to ambient pressure in an upper volume 304. Its opposite face is exposed to loadlock pressure in a chamber 306 that is in fluid communication with tube 210. Diaphragm 302 may be, for example, silicon, with a piezoresistive element mounted thereon. When the pressures in volumes 304 and 306 are the same, the diaphragm has zero deflection, as shown in FIG. 3A, and a calibrated device provides a zero differential pressure output. When a positive pressure is applied to the loadlock, the diaphragm is pushed upward, as shown in FIG. 3B, providing a voltage output that is converted to a positive differential pressure output. When a vacuum is drawn in the loadlock, the diaphragm is pulled downward, as in FIG. 3C, and the sensor outputs a negative voltage output that is converted to a negative differential pressure reading. This is a direct, gas species independent differential pressure reading.

[0041] 4 is a schematic diagram of a barometer for use in an ambient pressure sensor. It also includes a diaphragm, such as a silicon diaphragm 402, exposed at its top surface to a volume 404 that is exposed to ambient pressure through a hole 408 in a housing 410. The bottom surface of the diaphragm 402 is exposed to a closed chamber at a reference pressure. The sensor may be a piezoresistive diaphragm sensor that provides a voltage output by bending of the diaphragm driven by ambient pressure relative to the reference pressure in the chamber 406. When properly calibrated, the voltage output is converted into a very accurate reading of the ambient pressure.

[0042] The high vacuum absolute pressure sensor 218 is a thermal conductivity sensor, specifically a MEMS Pirani sensor. Other sensors, for example, wire-based Pirani sensors, may also be used. Thermal conductivity sensors measure pressure based on the relationship between the temperature of a heated sensor resistor and the amount of heating power applied to the sensor resistor. For example, a thin wire (sensor resistor) is heated to a certain temperature (T s ) may be monitored. As the gas pressure increases, the thermal conductivity of the gas increases and the gas will conduct more heat away from the heated wire, increasing the heating power required to maintain the wire at a constant temperature. A gas species dependent calibration curve relating heating power to pressure allows for pressure measurement. Calibration is usually performed in the factory against pure nitrogen gas. This is an indirect pressure measurement in which the heating power is proportional to the gas pressure.

[0043] This principle is used in the well-known Pirani gauge, where the heat loss is measured with a Wheatstone bridge network that serves both to heat the sensing element and to measure its resistance. In the Pirani gauge, a temperature sensitive resistor is connected as one arm of the Wheatstone bridge. The temperature sensitive resistor is mounted in a chamber exposed to the vacuum environment in which the pressure is to be measured.

[0044] Conventional Pirani gauges are calibrated for some known pressure to determine the relationship between the pressure of the gas and the power lost to the gas or bridge voltage. Then, assuming end and radiation losses remain constant, an unknown pressure of the gas may be determined directly by the power lost to the gas or related to the bridge voltage at the bridge balance.

[0045] The specific thermal conductivity sensor used here is a MEMS Pirani sensor, as shown in FIG. 5. This sensor includes a silicon nitride membrane 502 that extends across a chamber having volumes 504 and 506. Volumes 504 and 506 communicate with the pressure gauge volume 216, and thus the load lock pressure. A heated resistive element is formed on the membrane 502 at 508. Due to the geometry of the sensor, convection cannot occur within the cavity, and as a result, the sensor is insensitive to mounting position. Gas molecules are passed by diffusion only to the heated element, where heat loss to the gas is measured. As with conventional Pirani gauges, heat is conducted from the heated resistor to the ambient gas at 510 and 512 from the resistive element on the membrane 502. As with conventional Pirani sensors, as the pressure of the gas increases, the thermal conductivity of the gas increases, the gas takes more heat from the heated resistor, and the heating power required to maintain the wire at a constant temperature increases. A calibration curve relates heating power to pressure. The MEMS Pirani sensor provides very accurate pressure readings at high vacuum and is therefore particularly suitable for controlling transfer ports to high vacuum process chambers. However, as mentioned above, it is not as accurate at higher pressures approaching ambient pressure.

[0046] To obtain accurate absolute pressure readings at low vacuum without the need for expensive and perhaps less accurate absolute pressure sensors, the system determines the absolute pressure of the loadlock pressure from the differential pressure sensed by the DPRD sensor. Since the differential pressure is related to the ambient pressure, the absolute vacuum pressure can be determined by adding the ambient pressure. This calculation is similar to the approach of U.S. Pat. No. 6,909,975, except that the actual instantaneous ambient pressure is used in the calculation rather than a stored correlation coefficient. If both sides of the differential pressure sensor are exposed to the ambient, the differential pressure reading is zero. Thus, calculating the absolute pressure from the ambient differential pressure requires that the ambient pressure sensed by the barometer be added to the differential pressure. Similarly, with loadlocks evacuated to lower pressures, their absolute pressures are: P LL =P D +Pamb (1) can be calculated from, where P LL is the load lock absolute pressure, P D is the pressure difference, P amb is the instantaneous ambient pressure sensed by the barometer, i.e., the ambient pressure at the moment of calculation.

[0047] The loadlock pressure gauge outputs the absolute loadlock pressure P, derived from the Pirani gauge at high vacuum (very low pressure) and the differential pressure of the DPRD sensor plus the ambient pressure from the barometer at low vacuum (pressure closer to ambient). LL Provides the load lock pressure P LL Since the single output for transitions from a Pirani measurement to a differential plus ambient measurement, differences in the two measurements can be expected if the gas in the loadlock is other than the pure nitrogen gas for which the sensor was calibrated. To avoid a step in the output due to that transition, a technique well known in the industry and used in the MKS901P product is to blend the two measurements over a range of pressures. In one example of blending, two factory set thresholds define when measurements are taken solely from the Pirani sensor, solely from the differential and barometer sensors, or a blend of the two. Threshold P X,L Below the threshold P, only the Pirani measurements are used. X,H If it exceeds, P D and P amb Between those thresholds, some form of mixture may be used to differentiate between pure Pirani measurements and pure P D Plus P amb The blend allows for a continuous transition from the measured value to the Pirani value. The blending may be linear or non-linear, but near the lower threshold the blended value is weighted more towards the Pirani measurement, and towards the upper threshold the measured value is weighted more towards the calculated measurement. A typical lower threshold is 20 Torr and may be user configurable, and the upper threshold is 60 Torr and may be user configurable.

[0048] The upper and lower thresholds may be the same, or may be simply a value P X, for example, about 40 Torr. In either case, there is no mixing, but there is a sharp transition between the readings as the pressure rises and falls, and the pressure output P LL There are possible steps to this.

[0049] 6 is a flow chart of pressure measurements of a manometer via a measurement loop. In any order, at 602 the loadlock pressure is measured with a Pirani sensor, at 604 the difference between the loadlock pressure and ambient pressure is measured with a DPRD sensor, and at 606 the ambient pressure is measured with a barometer. Each of these measurements is available to the loadlock operator via a measurement loop. At 608 the Pirani pressure reading P MP is the lower mixing threshold P X,L If so, then in 610, the load lock pressure P LL P MP If the Pirani pressure in 608 is not less than the lower threshold, then in 612 the Pirani pressure is set equal to the upper blend threshold P X,H If the Pirani pressure is above that threshold, then in 614 the load lock pressure P LL , P D and P amb If the Pirani pressure is not below the lower threshold and is not above the upper threshold, then the pressure is in the mixed pressure range and at 616 the load lock pressure P LL is the Pirani pressure P MP and the calculated pressure P D Plus P amb The pressure defined in step 610, 614, or 616 is made available to the user. The system then loops back to take another set of measurements. During operation, the pressure P provided by the Pirani sensor is MP , the differential pressure P provided by the DPRD D , P provided by the barometer amb , and the determined load lock pressure P LL is available to the user for both control and monitoring purposes.

[0050] The DPRD sensor is factory calibrated and has an accuracy of plus or minus 1% Torr or better. The sensor output is very stable and linear (typically better than 0.1% linear over the pressure range and plus and minus 1% repeatable) and the sensor will hold calibration properly. However, over time there may be variations in voltage offset and gain that are necessary to maintain accuracy. Having a real-time ambient pressure measurement during operation allows for periodic calibration of the DPRD during normal operation.

[0051] The output signal from the DPRD is the bridge voltage V, which is approximately linearly related to the differential pressure across the diaphragm. b The relationship between the bridge voltage and the differential pressure is shown in Figure 7. LL P amb Therefore, P D When the load lock chamber is exposed to the ambient, such that V is equal to 0, the bridge voltage is offset by some b,0 When the load lock is evacuated to vacuum pressure, the bridge voltage becomes more negative. LL is approximately 0, usually less than 1.2 Torr, probably about 0.5 Torr, so the maximum negative deflection of the DPRD diaphragm towards the load lock pressure is the full scale bridge voltage V b,FS To, P D =-P amb,FS As mentioned above, V b and P D The relationship is approximately linear, such that the following relationship holds:

[0052] V b =V b,0 +((V b,0 -V b,FS ) / P amb,FS )P D (2)

[0053] Here, V b,0 is the voltage offset at zero differential pressure. P D The slope a in Figure 7 is the gain added to

[0054] a=(V b,0 -V b,FS ) / P amb,FS (3)

[0055] This can be seen from the formula: b = offset = V b,0 Simplifying equation 2, we get the pressure difference P D teeth,

[0056] P D =(V b -b) / a (4)

[0057] It is.

[0058] The MKS901P gauge is factory calibrated. The calibration is normally for the life of the gauge, but the gauge is not calibrated for differential pressure P D It can also be recalibrated in the field using a precision external ambient pressure calibration gauge and by exercising the load lock gauge through ambient and high pressure conditions, using a special code through communication connection 22 to determine the offset b and slope a for subsequent measurements of . The gauges disclosed herein are capable of self-calibrating during normal operation.

[0059] Whenever the load lock is exposed to the environment, as indicated by the system controller through communication input 224, an offset V b,0 can be checked and the stored value updated if necessary. Then, at approximately zero load lock pressure, the full-scale V b,FS and P amb,FScan both be measured to allow for the calculation of the slope (gain), a. This allows the DPRD calibration parameters of offset and slope to be redetermined at each cycle of operation, if necessary. During operation, the calibration procedure performed by the manometer microprocessor 226 may be substantially the same as that performed at the factory. A significant advantage of including the ambient pressure sensor 220 in the manometer is that the procedure can be performed automatically in the field whenever it is determined to be necessary, for example when the absolute pressure indicated by the differential pressure calculation at high vacuum does not equal the negative of the measured ambient pressure.

[0060] 8A illustrates the determination of the offset under microprocessor control. At 802, when the loadlock is vented to ambient, for example when signaled by the system controller, the loadlock pressure P LL is the ambient pressure V amb and the differential P D is known to be zero. When the load lock is vented in 804, P D Bridge voltage at V = 0 b V b,0 is converted to a voltage V through a high-resolution analog-to-digital converter. b The measured voltage is stored in the calibration memory at 806 as a voltage offset b.

[0061] The slope is determined for a full-scale calibration in the microprocessor procedure of FIG. 8B. The loadlock is pumped down in normal operation to approximately zero loadlock pressure, within approximately barometric accuracy, as measured by the Pirani sensor. At that pressure, at 808, the pressure difference is -P amb,FS At 810, the full-scale bridge voltage V b,FS is measured and the exact full-scale ambient pressure P amb,FS is measured using a barometer. b,FS and P amb,FS7, which is the gain applied to determine the differential pressure, is calculated at 814. The slope a of FIG. 7 is stored at 816 in the calibration memory.

[0062] Then the stored value V b,0 and a is the pressure differential P using equation (4). D To calculate the instantaneous voltage V b Combined with the load lock pressure P LL , P D and P amb Using the real-time measurements of, it can be calculated using equation (1).

[0063] Like 901P, temperature compensation of the DPRD measurements may also be provided.

[0064] In addition to the multi-sensor embodiments described herein, the scope of the present disclosure can be expanded to very high and ultra-high vacuum levels adding a fourth sensor, such as an ionization sensor, to the array as shown at 902 in FIG. 9. If an ionization gauge (low or high temperature embodiment) is added, then the user can handle load lock chambers that need to be pumped down to a much lower pressure than the Pirani sensor can monitor before the transfer valve is activated. Such a multi-sensor is very versatile in enabling accurate venting of the load lock chamber to the ambient and in verifying high vacuum levels before transferring a sample to a process chamber at high vacuum (very low pressure).

[0065] This disclosure has focused throughout on the addition of a standard barometric pressure sensor to provide real-time continuous ambient pressure measurements. Barometers are very unique in that they allow measurement of ambient pressure between sea level and the highest mountain peaks with very high resolution. They are commonly used as altimeters. However, the scope of this disclosure extends beyond those standard sensors to include any sensing technology capable of accurately measuring ambient pressure. This includes high-end diaphragm deflection sensors, such as piezoresistive capacitive and optically deflected diaphragm sensors.

[0066] The system is not limited to the use of differential piezoresistive diaphragm sensors. The instrument could be similarly constructed using differential diaphragm sensors based, for example, on optical and capacitive measurements of differential diaphragm deflection. Capacitive and optical diaphragm deflection pressure sensors could be made, for example, from metal, ceramic, or sapphire membranes.

[0067] The teachings of all patents, published applications, and references cited herein are incorporated by reference in their entirety.

[0068] Although exemplary embodiments have been shown and described in detail, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the scope of the embodiments encompassed by the appended claims.

Claims

1. a housing configured to be coupled to a vacuum chamber; an absolute vacuum pressure sensor within said housing configured to be exposed to said vacuum chamber and operable at high vacuum to provide an accurate instantaneous high vacuum pressure signal; a diaphragm differential pressure sensor within the housing, the diaphragm configured to have one side exposed to the vacuum chamber and an opposite side exposed to ambient pressure to provide an instantaneous differential pressure signal; an absolute ambient pressure sensor within said housing exposed to ambient pressure for providing an instantaneous ambient pressure signal; a controller in the housing that receives the high vacuum pressure signal from the absolute vacuum pressure sensor, the differential pressure signal from the diaphragm differential pressure sensor, and the ambient pressure signal from the ambient pressure sensor; Equipped with the controller calculating a calculated absolute pressure in the vacuum chamber from the instantaneous differential pressure signal and the instantaneous ambient pressure signal, both sensed through a range of vacuum pressures; the controller providing as outputs a differential pressure output from the diaphragm differential pressure sensor signal, an absolute vacuum pressure output taken from the absolute vacuum pressure sensor signal at high vacuum, and an absolute vacuum pressure output from the calculated absolute pressure at low vacuum. Pressure gauge.

2. The controller: an ambient pressure bridge voltage of the diaphragm differential pressure sensor when the vacuum chamber is at ambient pressure; a full-scale bridge voltage of the diaphragm differential pressure sensor when the vacuum chamber is at full-scale high vacuum; and The full-scale ambient pressure measured by the absolute ambient pressure sensor. and further configured to calibrate the diaphragm differential pressure sensor based on 2. The pressure gauge of claim 1.

3. The controller: measuring an ambient pressure bridge voltage of the diaphragm differential pressure sensor when the vacuum chamber is at ambient pressure; measuring a full-scale bridge voltage of the diaphragm differential pressure sensor when the vacuum chamber is at a full-scale high vacuum; measuring a full scale ambient pressure with said absolute ambient pressure sensor; and determining a gain of the diaphragm differential pressure sensor from the ambient pressure bridge voltage, the full-scale bridge voltage, and the full-scale ambient pressure; and further configured to calibrate the diaphragm differential pressure sensor by the instantaneous differential pressure signal is then determined from the instantaneous bridge voltage, the ambient pressure bridge voltage, and the gain.

2. The pressure gauge of claim 1.

4. The gain is a=(V ｂ，０ -V ｂ，ＦＳ ) / P ａｍｂ，ＦＳ where: a is the gain, V ｂ，０ is the ambient pressure bridge voltage, V ｂ，ＦＳ is the full-scale bridge voltage, and P ａｍｂ，ＦＳ is the full-scale ambient pressure, and, The pressure difference is P Ｄ =(N ｂ -V ｂ，０ ) / a where: P Ｄ is the instantaneous pressure difference, V ｂ is the instantaneous voltage from the diaphragm differential pressure sensor; 4. The pressure gauge of claim 3.

5. The pressure gauge of claim 1 , wherein the controller further provides an output of instantaneous ambient pressure from the absolute ambient pressure signal.

6. 2. The pressure gauge of claim 1, wherein the absolute vacuum pressure sensor has an accuracy of 7% or better at a high vacuum pressure of 5 Torr.

7. 2. The pressure gauge of claim 1, wherein the calculated absolute pressure from the instantaneous differential pressure readings has an accuracy of 1% or better at 60 Torr.

8. The controller: calculating a mixed pressure from the instantaneous high vacuum pressure signal and the calculated absolute pressure at an intermediate vacuum pressure; outputting as a chamber pressure signal the instantaneous high vacuum pressure signal over a range of high vacuum pressures, the calculated absolute pressure over a range of low vacuum pressures, and the mixed pressure at the intermediate vacuum pressures; It is configured as follows:

2. The pressure gauge of claim 1.

9. The pressure gauge of claim 1 , wherein the controller is configured to zero the absolute vacuum pressure signal at ambient pressure.

10. The pressure gauge of claim 1 , wherein the absolute vacuum pressure sensor is a thermal conductivity sensor.

11. The pressure gauge of claim 10 , wherein the thermal conductivity sensor is a MEMS Pirani sensor.

12. The pressure gauge of claim 1 , wherein the diaphragm differential pressure sensor is a differential piezoresistive diaphragm pressure sensor.

13. 2. The pressure gauge of claim 1, wherein the absolute ambient pressure sensor comprises a diaphragm exposed to a reference vacuum on one side and to ambient pressure on an opposite side.

14. 1. A method for controlling a load lock port and a transfer port of a load lock, comprising: providing an instantaneous high vacuum pressure signal from an absolute vacuum pressure sensor; providing an instantaneous differential pressure signal from a diaphragm differential pressure sensor having a diaphragm exposed to said load lock on one side and exposed to ambient pressure on an opposite side; providing an instantaneous ambient pressure signal from an absolute ambient pressure sensor; calculating in an electronic controller a calculated absolute ambient pressure from the instantaneous differential pressure signal and the instantaneous ambient pressure signal; opening the transfer port to a high vacuum chamber only after the load lock has reached a sufficiently low pressure as indicated by the instantaneous high vacuum pressure signal from the absolute vacuum pressure sensor; opening the loadlock port to ambient only after the instantaneous differential pressure signal from the diaphragm differential pressure sensor indicates a loadlock pressure at least as high as ambient pressure; providing an absolute vacuum pressure output from said controller; Including, the absolute vacuum pressure output is taken from the absolute vacuum pressure sensor at high vacuum and from the calculated absolute pressure at low vacuum; method.

15. The controller: an ambient pressure bridge voltage of the diaphragm differential pressure sensor when the load lock is at ambient pressure; a full-scale bridge voltage of the diaphragm differential pressure sensor when the load lock is at full-scale high vacuum; and The full-scale ambient pressure measured by the absolute ambient pressure sensor. and further calibrating the diaphragm differential pressure sensor based on the The method of claim 14.

16. The controller: measuring an ambient pressure bridge voltage of the diaphragm differential pressure sensor when the load lock is exposed to ambient pressure; measuring a full scale bridge voltage of said diaphragm differential pressure sensor when said load lock is at full scale high vacuum; measuring full scale ambient pressure with the absolute ambient pressure sensor when the load lock is at full scale high vacuum; and calculating in the controller a gain of the diaphragm differential pressure sensor from the ambient pressure bridge voltage, the full-scale bridge voltage, and the full-scale ambient pressure; further calibrating the diaphragm differential pressure sensor by An instantaneous differential reading is then calculated from the instantaneous bridge voltage, the ambient pressure bridge voltage, and the gain. The method of claim 14.

17. The gain is a=(V ｂ，０ -V ｂ，ＦＳ ) / P ａｍｂ，ＦＳ where: a is the gain, V ｂ，０ is the ambient pressure bridge voltage, V ｂ，ＦＳ is the full-scale bridge voltage, and P ａｍｂ，ＦＳ is the full-scale ambient pressure, and, The pressure difference is P Ｄ =(N ｂ -V ｂ，０ ) / a where: P Ｄ is the instantaneous pressure difference, V ｂ is the instantaneous voltage from the diaphragm differential pressure sensor; The method of claim 14.

18. The method of claim 14 , wherein the controller further provides an output of an instantaneous ambient pressure from the instantaneous absolute ambient pressure signal.

19. 15. The method of claim 14, wherein the controller calculates a combined pressure from the instantaneous high vacuum pressure signal and the calculated absolute pressure signal at an instantaneous vacuum pressure.

20. The method of claim 14 further comprising zeroing the absolute vacuum pressure signal at ambient pressure.

Images