Abstract:
A device to implant impurities into a semiconductor wafer has a process chamber having a wall, a pressure compensation unit, a disk to support a plurality of semiconductor wafers within the process chamber. The disk has a radially extending slot arranged among the wafers. A beam gun is positioned within the process chamber to shoot an ion beam at the semiconductor wafers. A cryo pump minimizes the pressure within the process chamber. A first ion gauge is positioned between the process chamber and the cryo pump. A second ion gauge extends through the wall of the process chamber. A switching device selectively connects the first or second ion gauge to the pressure compensation unit. A faraday receives ions from the ion gun filter after the ions travel through the slot in the disk. A current meter counts the number of electrons flowing to the disk faraday to neutralize the ions.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     Impurities are implanted into semiconductor devices for a variety of reasons, including introducing electrons and holes into the semiconductor substrate in order to locally change the conductive properties of the substrate. For example, silicon has four electrons in the outer ring. Phosphorus has five electrons in its outer ring, one more than silicon. Boron has three electrons in its outer ring, one fewer electron than silicon. Boron can be used to introduce holes into the substrate. Phosphorous can be used to introduce electrons into the substrate.  
         [0002]     To enable implantation, the impurities are implanted as ions having one fewer electron than the neutral species. During the implantation process, the electron deficit can be used to determine how much impurity has been implanted. Specifically, it is not possible to accurately count the number of ions (or atoms) leaving the ion gun. Therefore, a predetermined portion of the ions is directed to an ion counter instead of the semiconductor wafer(s). The ion counter may be embodied has a disk faraday. When an ion strikes the disk faraday, an electron is pulled to the disk faraday in order to neutralize the ion. The number of electrons pulled to the disk faraday is counted using a current meter. It is presumed that the number of ions striking in the disk faraday is proportional to the number of ions striking and entering the semiconductor wafer.  
         [0003]     The current (electrons per second) represents the rate at which impurities are introduced into the wafer. If the implanter detects that one area of the wafer is receiving impurities at a slower rate than other areas of the wafer, then the implanter spends more time implanting on the deficient area. In this manner, the implanter can work to achieve uniform total dosing across the surface of the wafer.  
         [0004]     When the ions hit the semiconductor wafer, they may destroy a portion of a resist layer formed on the wafer. This process releases an outgas into the implant chamber, which would otherwise kept at a very low pressure. Electrons from the outgas can neutralize a portion of the ions, before the ions reach the disk faraday or the semiconductor wafer. Although the ions are neutralized by the resist outgas (rather than being neutralized at the disk faraday or within the semiconductor wafer), the neutral species is still implanted and still causes the desired change to the substrate. However, because the neutral species contains the correct number of electrons, there is not disk faraday current flow for neutralization. Therefore, the neutral species are not counted.  
         [0005]     In order to count the impurities implanted as atoms, rather than ions, a pressure sensor is used. As the pressure increases from resist outgassing, it is presumed that a larger percentage of the impurities are introduced into the wafer as atoms rather than ions.  
         [0006]     The following equation represents how pressure is taken into consideration to determine the number of ions implanted. 
 
 I   DISK   =I   DOSE   ·e   −KP  
 
         [0007]     In the above equation, I DISK  is the current flowing to the disk faraday. This current is proportional to the number of ions implanted. I DISK  is the rate at which impurities (ions+atoms) are implanted. P is the pressure as sensed by the ion gauge/pressure sensor within the device. K is a factor determined by the engineer and input into the implanter. K represents how a pressure change is presumed to effect ion neutralization.  
         [0008]     Instead of, or in addition to, the K-factor shown above, a pressure compensation factor P-COMP can be used. The mathematical relationship between K and P-COMP is as follows:  
         P   -   COMP     =     100   ⁢     (       ⅇ     K   /   10000       -   1     )           
     or     
       K   =     ln   ⁢           ⁢     (     1   +       P   -   COMP     100       )     ⁢           ⁢     (   10000   )           
 
         [0009]     Because K and P-COMP are interchangeable through simple math, the term “pressure compensation factor” is used hereinafter to represent both K and P-COMP with the understanding that the two parameters are interchangeable through the above mathematical relationships.  
         [0010]     The process chamber is kept at a very low pressure. By detecting pressure increases, the ion gauge is able to calculate the number of ions within the chamber. The chamber is held at a near-vacuum through cryogenic pumps. The conventional ion gauge is located outside of the process chamber, near a cryogenic pump. However, this location reduces the accuracy of the pressure reading for two reasons. First, resist outgassing causes dramatic increase of pressure near the wafer. This high pressure is localized and drops with distance. At the location of the ion gauge, the pressure has dropped significantly, causing an artificially low pressure reading. Second, the ion gauge is in close proximity to the cryogenic pump, which reduces chamber pressure. The cryogenic pump also reduces the pressure reading of the ion gauge.  
         [0011]     With high energy implants, the pressure increase is sufficiently high that the implanter can accommodate the pressure inaccuracies. At the outside chamber location, the ion gauge can accurately sense pressure changes produced by implanting high energy impurities, such as arsenic. That is, the high energy of arsenic causes a lot of resist outgassing, and hence a large pressure increases. By positioning the pressure sensor away from the chamber, near the cryogenic pump, pressure changes are reduced to a range where the ion gauge operates efficiently.  
         [0012]     While the conventional position is acceptable for high energy impurities, more recent technologies require lower energy implants that work well with the present system. For example, when nitrogen is implanted, the ion gauge detects very little pressure increase. However, nitrogen implantation causes a substantial amount of resist outgassing and beam neutralization, which are not detected by the ion gauge in its current location. One way to address this deficiency is to use a large K value in calculating pressure compensation. The large K value effectively amplifies the pressure readings of the ion gauge. For example, typical P-COMP values for a high energy impurities such as arsenic range from 8 to 60, depending on the beam current, species and energy. For low energy nitrogen beams, a significantly higher P-COMP value is necessary. Perhaps P-COMP would be in excess of 150.  
         [0013]     The above high P-COMP values make ion gauge inaccuracies critical in terms of dose accuracy. A high pressure compensation value exposes the process to a non-acceptable risk of dose error. The risk stems from the pressure read out variability and instability that are inherent in any ion gauge. Unstable pressure readings lead to varying dose, even when all other variables are perfectly stable. Experience has shown that for very high P-COMP values, it is difficult to repeatedly produce wafers with the same dosing. High P-COMP values introduce wafer-to-wafer variations.  
         [0014]     Another approach to addressing the problem is to reduce the beam current until resist outgassing does not significantly affect the process. Specifically, the beam current is reduced to a point that free electrons produced by resist outgassing are consumed by the cryogenic pump(s) as soon as they are released. In this manner, there are not enough electrons to appreciably neutralize the ion beam. However, reducing the beam current lowers the throughput of the implanter and reduces the tool capacity.  
       SUMMARY OF THE INVENTION  
       [0015]     To address these and/or different concerns, the inventors propose a device to implant impurities into a semiconductor wafer. The device has a process chamber having a wall, a pressure compensation unit, a disk to support a plurality of semiconductor wafers within the process chamber. The disk has a radialy extending slot arranged among the wafers. A beam gun is positioned within the process chamber to shoot an ion beam at the semiconductor wafers. A cryo pump minimizes the pressure within the process chamber. A first ion gauge is positioned between the process chamber and the cryo pump. A second ion gauge extends through the wall of the process chamber. A switching device selectively connects the first or second ion gauge to the pressure compensation unit. A faraday receives ions from the ion gun after the ions travel through the slot in the disk. A current meter counts the number of electrons flowing to the disk faraday to neutralize the ions. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which:  
         [0017]      FIG. 1A  is a schematic side view of an ion implanter;  
         [0018]      FIG. 1B  is a schematic side view of the pressure sensor controller;  
         [0019]      FIG. 2A  is a schematic side view of a disk faraday used to determine beam current, which is in turn used to count the number of implanted ions;  
         [0020]      FIG. 2B  is a top view of a disk holding a plurality of wafers for implantation;  
         [0021]      FIG. 2C  is an enlarged top view of the disk shown in  FIG. 2B ;  
         [0022]      FIG. 3  is a side view of the disk faraday shown in  FIG. 2A  and the disk shown in  FIGS. 2B and 2C ;  
         [0023]      FIG. 4  is a schematic representation of the time spent on the wafer during the beam travel path;  
         [0024]      FIG. 5  shows the correlation between the time the beam spends on the wafer at various disk positions, the amount of outgassing, the number of neutral species implanted and the amount of overdose if the neutral species are ignored in the calculations;  
         [0025]      FIG. 6  is a plot of mean sheet resistance versus P-COMP values for a conventional method of determining a P-COMP value;  
         [0026]      FIG. 7  shows an X-Y plot of the uncompensated beam current I DISK  as a function of disk radius;  
         [0027]      FIG. 8A  is a plot of vertical disk speed versus disk position;  
         [0028]      FIG. 8B  is a plot of vertical disk speed versus disk position when outgassing causes a drop in beam current;  
         [0029]      FIG. 9  is a sheet resistance map for an a non-uniformly dosed wafer;  
         [0030]      FIGS. 10A and 10B  are plots of beam current and pressure versus disk position for a higher energy implant;  
         [0031]      FIGS. 11A and 11B  are plots of uncorrected beam current and pressure versus disk position for a lower power implants such as nitrogen;  
         [0032]      FIG. 11C  is a plot of pressure versus disk radius using an ion gauge located within the process chamber; and  
         [0033]      FIG. 12  is a schematic view of a switching circuit to switch between the first and second ion gauges (pressure sensors) shown in  FIG. 1A . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0034]     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout.  
         [0035]      FIG. 1A  is a schematic side view of an ion implanter  10 . The implanter has a chamber  105 , which is kept at a very low pressure by cryo pumps, one of which is represented by reference numeral  115 . Within the chamber  105 , a beam gun  125  produces an ion beam  120  which is focused on wafers  210 . The wafers  210  are placed around a disk  200 . A faraday  100  is provided under the disk  200  to sense beam current. A first pressure sensor  135  is provided outside of the chamber  105  in the vicinity of the cryo pump  115 . This location corresponds with a conventional location and is useful for implanting ions that produce a large pressure response during resist outgassing. A second pressure sensor  145  is provided within the chamber  105 . The second pressure sensor (an ion gauge)  145  is useful for the implanting impurities that exhibit a smaller pressure response with resist outgassing. Both the first and second pressure sensors  135 ,  145  are connected to a pressure sensor controller  155 . The pressure sensor controller  155  is in turn connected to a user interface  165 . On the user interface  165 , the pressure from one of the pressure sensors  135 ,  145  is displayed together with an indication of which pressure sensor or gauge is being used.  
         [0036]      FIG. 1B  is a schematic side view of the pressure sensor controller  155 . Connectors  175 ,  185  are provided respectively for the first and second pressure sensors  135 ,  145 . Through these connectors  175 ,  185 , the pressure sensors  135 ,  145  communicate with the pressure sensor controller  155 . A user interface input  190  receives information regarding the recipe from the user interface  165 . A connector  195  allows various signals within the pressure sensor controller  155  to be manipulated. An output  198  supplies information regarding the detected pressure (or detected number of ions) and information regarding which pressure sensor is active. The information regarding the detected pressure may be provided as an analog output. The information from the output  198  may be provided to the user interface  165  or to a separate display on the implanter  10 .  
         [0037]     In  FIG. 1A  an additional ion gauge  145  is added to the implanter  10 . This ion gauge  145  is located to close to the wafer  210  such that it can more accurately determine pressure changes. In the process chamber  105 , the ion gauge  145  is closer to where resist outgassing, pressure increases and beam neutralization occurs. This placement works well for recipes having a high ratio of beam neutralization to pressure increase.  
         [0038]     In order to place the ion gauge  145  within the process chamber  105 , a hole is drilled in the process chamber  105 . The ion gauge mounting hardware is installed. The ion gauge  145  is installed and connected to the ion gauge controller  155 .  
         [0039]     Under resist outgassing conditions, the beam current drops due to recombination. Recombination is when an ion is combined with an electron and becomes a neutral atom. The neutral atom is still implanted into the wafer but not counted by the disk faraday of the implanter. Therefore, for every neutralized ion at a specific wafer area, the implanter focuses the beam on the wafer for an additional time long enough to implant one additional ion. This causes an overdose of one ion. The amount of resist exposed to the beam is greatest at the vertical center of the wafer. If pressure compensation is not used, the vertical center of the wafer is overdosed relative to the top and bottom areas of the wafer.  
         [0040]      FIG. 2A  is a schematic side view of a disk faraday used to determine beam current, which is in turn used to count the number of implanted ions.  FIG. 2B  is a top view of a disk holding a plurality of wafers for implantation. In  FIG. 2B , a plurality of wafers  210  are arranged on the disk  200 . A disk slot  220  is positioned among the wafers  210 . An ion gun focuses a beam spot at one point on the wafers  210  as the disk  200  rotates.  
         [0041]      FIG. 2C  is an enlarged top view of the disk shown in  FIG. 2B . In  FIG. 2C , reference numeral  230  represents a beam spot, where the beam is currently being focused. If the disk  200  is being rotated in a clockwise direction with the beam spot  230  focused as shown, the beam spot  230  will move from wafer to wafer, implanting impurities toward the top of each wafer  210 . According to one embodiment, the disk rotates at approximately 1200 rpm. As the disk rotates, a portion of the beam will extend through the disk via the disk slot  220 . At this point, the beam spot  230  is directed through the disk  200  to the disk faraday  100  shown in  FIG. 2A . The beam  120  contains ions. The disk faraday  100  is grounded. Electrons flow into the disk faraday  100  through a current meter  110  to thereby neutralize each of the ions. The current meter  110  counts the number of electrons, producing a current reading.  
         [0042]      FIG. 3  is a side view of the disk faraday  100  shown in  FIG. 2A  and the disk  200  shown in  FIGS. 2B and 2C . The disk  200  moves vertically up and down with respect to a horizontally traveling beam  120 . This ensures that the beam  120  strikes the complete area of each wafer  210  mounted on the disk. Referring to  FIG. 2C , the vertical travel path of the beam is such that the beam travels slightly past the top of the wafer  212  (where it is shown in  FIG. 2C ). When the beam is slightly above wafer  212 , the beam changes direction to travel toward the bottom of the wafer  212 . After traveling slightly paste the bottom of the wafer  212 , the beam again changes direction so as to head toward the top of the wafer  212 . The beam therefore stops and changes directions when the beam is not on the wafer.  
         [0043]     The amount of outgassing is proportional to the amount of time that the beam is on the resist, which can be translated to the time the beam spends on the wafer. The beam spends time on the wafer, on the disk slot and the on the disk between, above and below the wafers.  
         [0044]      FIG. 4  is a schematic representation of the time spent on the wafers during the beam travel path. When the beam is above or below the wafer, there is no time spent on the wafers. All of the time during the disk rotation is spent on the disk or possibly on the disk slot. When the beam is at the top and bottom of the wafer, there is a minimum time spent on the wafer. During each revolution of the disk  200 , the beam  220  spends most of its time on the disk  200 , rather than on a wafer  210 . At the middle of the wafer, the beam spends the maximum time on the wafer.  
         [0045]      FIG. 5  shows the correlation between the time the beam spends on the wafer at various disk positions ( FIG. 4 ), the amount of outgassing, the number of neutral species implanted and the amount of overdose if the neutral species are ignored in the calculations. When the beam is at the center of the wafer, there is more outgassing and beam neutralization. The current meter  110  detects fewer implanted ions. Without accounting for the neutralized atoms, the disk moves more slowly when the beam is focused at the center of the wafer. This allows the beam to implant more ions on the center of the wafer. Because the implanted neutralized atoms are ignored, this results an overdose in the center of the wafer.  
         [0046]     At the beginning of the implantation process, there is more resist to be burnt. There is therefore more resist outgassing at the beginning of the implantation process than at the end of the implantation process. Pressure is proportional to the amount of outgassing.  FIG. 6  is an X-Y plot of outgassing as a function of disk radius. As can be seen, the largest pressure increase happens on the first pass of the ion beam over the wafer. There is less of a pressure increase with each succeeding pass. For each pass, the pressure increase is greatest at the center of the wafer. It should be noted that the beam may be traveling in opposite directions for each succeeding pass. For example, the first pass may be a downward pass, the second pass may be an upward pass, the third pass may be a downward pass, and so on. The outgassing is reduced with the each pass due to the decreasing availability of hydrogen and resist solvents in the organic resist. This is known as resist conditioning.  FIG. 7  shows an X-Y plot of the uncompensated beam current I DISK  as a function of disk radius.  FIG. 7  represents the current detected by the disk faraway. Comparing  FIGS. 6 and 7 , it can be seen that when there is maximum outgassing, the disk faraday detects the minimum current. This is because the outgassing causes the ions to recombine before implantation.  
         [0047]     The dose system controls the vertical disk speed. It attempts to minimize vertical dose non-uniformity by changing the vertical disk speed in response to changes in the measured beam current I DISK . A drop in measured beam current causes the vertical disk speed to decrease at that vertical position so that an underdosing situation does not occur. The vertical disk speed may be varied while maintaining the same disk rotational speed. If the beam current drop is “legitimate” meaning it is caused by something that effects the number of implanted impurities (ions plus atoms), then dose uniformity is optimized by the vertical speed reduction. If the beam current drops due to beam neutralization, then overdosing occurs at the vertical position on the wafer that is being hit by the beam when the speed reduction occurs. Pressure changes are considered in order to differentiate between legitimate current drops and neutralization current drops. With the following equation, I DISK  to control vertical disk speed: 
 
 I   DISK   =I   DOSE   ·e   −KP  
 
         [0048]     For one revolution of the disk, the relative area struck by the beam spot is related to the circumference. That is, the closer the beam is to the center of the disk (not center of the wafer), the less area (wafers plus exposed disk) that is covered by the beam spot during each revolution. As the beam spot gets further towards the outer periphery of the disk, more area (wafers plus exposed disk) is covered during each revolution.  
         [0049]     Since the area changes for a given disk position and the disk rotational speed does not change, the horizontal speed of the beam over the outer areas disk is greater. Toward the outer portions of the disk, fewer impurities are implanted per area for each rotation. To compensate for this and to ensure dose uniformity, the vertical speed of the disk changes.  
         [0050]      FIG. 8A  is a plot of vertical disk speed versus disk position. The vertical speed is slowest towards the top of the wafer, where the horizontal disk speed is greatest. A slower vertical disk speed effectively allows the beam to implant on a given area for more revolutions the disk.  
         [0051]      FIG. 8B  is a plot of vertical disk speed versus disk position when outgassing causes a drop in beam current. The flat portion of the curve shown in  FIG. 8B  demonstrates that when there is outgassing toward the middle of the wafer, the vertical disk speed is slower than would otherwise be necessary.  
         [0052]      FIG. 9  is a sheet resistance map for a non-uniformly dosed wafer. Towards the middle of the wafer, the sheet resistance is less than the average sheet resistance. This indicates that there is overdosing toward the middle of the wafer. The sheet resistance of  FIG. 4  can be obtained through testing, by placing probes at different positions on the wafer.  
         [0053]     Some implanters minimize dose non-uniformity by ignoring beam current fluctuations. This can be done, for example, by not monitoring the beam current during “beam-on-wafer” time periods. That is, a disk faraday is not used during implanting. However, if there is a problem upstream from the wafer during implanting, which cases fewer impurities to enter the wafer, then this problem cannot be recognized. Alternatively, non-uniformity can be minimized by increasing the capacity of the cryo pumps. With more powerful pumps, the resist outgassing electrons are sucked into the cryo pumps as they are released. The inventors, however, are avoiding non-uniformities with pressure compensation and the formula below. 
 
 I   MEASURED   =I   DOSE   ·e   −KP  
 
 I MEASURED  is also referred as I DISK , and is current detected by the disk faraday. I DOSE  is the corrected dose current. The K-factor, which is input by the operator, either magnifies or diminishes the effect of pressure on the corrected dose current I DOSE . If K is large, then an inaccuracy in I MEASURED  produces a large inaccuracy in the compensated beam current I DOSE  While K should be high enough to assure cross wafer dose uniformity, a small K is better in terms of dealing with noise from the pressure sensor and avoiding inaccuracies. 
 
         [0054]      FIGS. 10A and 10B  are plots of beam current and pressure versus disk position for a higher energy implant. The pressure readings for  FIG. 10B  were produced by an ion gauge at the conventional position. In  FIG. 10A , the uncorrected beam current is plotted versus disk position. The uncorrected beam current is the current that is detected through the disk faraday. The bottom curve in  FIG. 10A  shows that for the first pass across the wafer, there is the greatest drop in beam current. This can be explained by reviewing  FIG. 10B  which shows that the pressure increase is greatest on the first pass over the wafer. On the first pass, there is maximum resist outgassing, which causes the pressure increase and introduces free electrons into the chamber in the vicinity of the wafer. The electrons cause ion beam neutralization and hence a drop in the detected current. Because of the significant pressure response detected, a smaller P-COMP value is sufficient to compensate for beam neutralization.  
         [0055]      FIGS. 11A and 11B  are plots of uncorrected beam current and pressure versus disk position for a lower power implant such as nitrogen. Again, the beam current is the beam current detected by a disk faraday. Like  FIG. 10B , the pressure readings for  FIG. 11B  were produced by an ion gauge at the conventional position. As can be seen, significant beam neutralization (current drop) only produces small changes in the pressure reading. A very high P-COMP value is necessary in order to flatten out the beam current (I DOSE ) versus disk position curve.  
         [0056]     The reason for this phenomenon is not yet fully understood. It could be that, under similar process conditions other than impurity species, low energy implants, such as N + , are much more likely to accept an electron (neutralization) than BF 2   + , P + , or AS + . This would result in excessive/greater beam current drop, as a function of pressure. Conditional P-COMP systems and ion gauges, while being very accurate for other (high energy) implant processes, are not sufficient for N + . One solution to this problem is to increase the ion gauge response in order to bring it up to the magnitude with which the implanter works. In order to achieve this, pressure must be monitored close to where the resist outgassing occurs. Conventional ion gauges resides upstream of the beam gate, outside of the process chamber, next to the cryo pump. This location is too far from the outgassing source (and too close to the cryo) to detect more subtle pressure fluctuations. Placing the pressure monitor in the process chamber results in the pressure response shown in  FIG. 11C , which is another plot of pressure versus disk radius. Unlike  FIGS. 10B and 11B , the pressure readings for  FIG. 11C  were produced by an ion gauge located within the process chamber. Comparing  FIG. 11C  with  FIG. 11B , it can be seen that if the ion gauge is located within the process chamber for low energy implants, a sufficient pressure response is produced.  
         [0057]     Placing an ion gauge in the process chamber results in much smaller P-COMP values than an ion gauge placed at the conventional location. For example, a P-COMP of 7 is possible instead of a P-COMP in excess of 150. This is a dramatic improvement as it drastically reduces dose inaccuracies/functuations and increases dose repeatability by reducing the affect of ion gauge inaccuracies.  
         [0058]     Placing an ion gauge in the process chamber, while desirable for low energy impurities, such as N + , makes the pressure readings unacceptable for the other processes such as high energy implants. Therefore, the ion gauge within the process chamber does not replace the ion gauge located downstream, toward a cryo pump. Both ion 1  gauges should be installed. Depending on the process/recipe, the operator should switch between the ion gauges.  
         [0059]     An alternate solution is to place two ion gauges within the process chamber. The first ion gauges should be a highly sensitive ion gauge, which accurately detects pressures below a given threshold, perhaps 1×10 −4  Torr. A second ion gauge is positioned within the process chamber, perhaps at the same general location as the first pressure gauge. The second ion gauge is less sensitive than the first ion gauge. For example, the second ion gauge can detect pressures above the given threshold, perhaps 1×10 −4  Torr. The first ion gauge is used for low energy implants, and the second ion gauge is used for high energy implants.  
         [0060]      FIG. 12  is a schematic view of a switching circuit to switch between the first and second ion gauge (pressure sensors) shown in  FIG. 1A .  FIG. 12  shows the connector  195  of the pressure sensor controller  155  shown in  FIGS. 1 and 2 . Pin number  9  is a beam line gas signal. Beam line gas is a parameter that serves to focus the beam on the wafer. The beam line gas is a parameter that may not be used for either high energy implants, such as BF 2   + , P + , As + , and low energy implants such as nitrogen. Although beam line gas is shown in  FIG. 12 , any recipe parameter that is unused for both high and low energy implants could be used for the circuit shown in  FIG. 12 . The beam line gas is part of the recipe, which may be developed through the user interface shown in  FIG. 1A . Because beam line gas is not used for either high or low energy implants, the recipes would normally call for beam line gas to be off. However, the inventors propose using the beam line gas parameter to select an ion gauge. Thus, the beam line gas recipe parameter now serves an important function.  
         [0061]     Pin number  8  is a digital interface signal. The digital interface signal is low when one of the pressure sensors is to be on and high when both of the pressure sensors are to be off. Pin number  7  is the controller input for pressure sensor  2 . Pin number  6  is the controller input for pressure sensor  1 . If only one of the two pressure sensors (ion gauges) were to be used, the digital interface signal could simply be jumped from pin number  8  to either pin number  7  or pin number  6 .  
         [0062]     The circuit shown in  FIG. 12  allows the user to select between either the first or second pressure sensor and allows the operator to select the appropriate pressure sensor automatically. The circuit shown in  FIG. 12  has two delay circuits and two relays. Delay circuit  1210  is connected to relay circuit  1215 , and delay circuit  1220  is connected to relay circuit  1225 . Both of the delay circuits  1210  and  1220  have two inputs, for a total of four inputs. Each of these four inputs is connected to the beam line gas signal. The delay circuits  1210  and  1220  are substantially the same. However, inverters U 3 A and U 3 B cause the delay circuits to operate at different times. Transistor T 2  is similar to transistor T 5 . However, transistor T 2  is connected to a resistor R 8 , and transistor T 5  is connected to an inverter U 3 A. Transistor T 3  is similar to transistor T 6 . However, transistor T 3  is connected to an inverter U 3 B, and transistor T 6  is connected to a resistor R 15 . With these connections, transistor T 2  is on when transistor T 5  is off. Transistor T 3  is off when transistor T 6  is on. Because of the similarities between the two delay circuits  1210 ,  1220 , only one will be described in detail.  
         [0063]     Transistor T 2  through the capacitor C 2  and resistor R 5  form a delay circuit together with the associated components. The RC time constant of C 2  and R 5  determines how long transistor T 2  must be on before a signal is received at U 5 B. U 5 B together with R 9  and T 4  assure that an accurate voltage signal is output even if the input to U 5 B fluctuates. They also serve to control the timing of the delay circuit such that the delay does not change with changing temperature, for example.  
         [0064]     When transistor T 2  is on and a signal is eventually output through U 5 B, and R 9  and T 4 , the second pressure sensor is turned on through pin  7  of the ion gauge controller. When transistor T 2  is on, transistor T 3  is off. Thus, transistor T 3  has no effect when the delay circuit is working to activate the second pressure sensor. However, when transistor T 2  is off, transistor T 3  is on. The purpose of transistor T 3  is to discharge the capacitor C 2 . In this manner, when transistor T 2  is again turned on, the RC time constant will not be altered by any charge stored from a previous operation.  
         [0065]     The delay circuit  1220  operates substantially the same as the delay circuit  1210  with the exception that the components U 5 A, R 10  and T 7  produce an on signal when the components U 5 B, R 9  and T 4  produce an off signal.  
         [0066]     The delay circuits  1210  and  1220  are connected to the pressure sensor controller through respective relay circuits  1215  and  1225 . Both relay circuits  1215  and  1225  receive the digital input signal from pin  8 . When the relay circuit  1215  or  1225  receives a high signal from the delay circuit  1210  or  1220 , the relay circuit  1215  or  1225  effectively passes the digital input signal from pin  8  to pin  7  or  6 . The relay circuits  1215  and  1225  are used instead of a direct connection in order to provide insulation from the rest of the device. In some applications, it is certainly possible to eliminate the relay circuits  1215  and  1225 .  
         [0067]     The devices labeled U 5 A and U 5 B together form a hex buffer between each delay circuit and the associated relay circuits. The hex buffer is a logic device which outputs a predetermined voltage when an input voltage is greater than or equal to a certain voltage. For example, referring to hex buffer U 5 A, when pin  3  reaches a voltage greater than or equal to five volts, a five-voltage signal will be output on pin  2 .  FIG. 12  shows an indicator light LED 2 . This indicator light can be used to inform the operator which pressure sensor is operating. Alternatively, this can be done through the user interface shown in  FIG. 1A . Although the pressure gauge controller has inputs for two ion gauges, it is not designed to switch between the two ion gauges. Thus, an instantaneous digital switching signal is ineffective. With the circuit shown in  FIG. 12 , sufficient delay is provided. The delay emulates an operator mechanically changing a jumper between pins  6  and  7 . This jumper would connect one of pins  6  and  7  to pin  8 .  
         [0068]     When the pressure sensor/ion gauge controller is instructed to operate either the first or second pressure sensor, the pressure sensor controller communicates with the appropriate pressure sensor through the connector  175  or the connector  185  (see  FIG. 1B ), as described previously.  
         [0069]     The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.