Abstract:
A combination retainer and electrical contact mechanism for a deposition monitor sensor includes a sensor body and a removable flexible electrical contact spanning between a fixed electrical (contact) element in the sensor&#39;s body and one face of an associated monitor crystal. A retainer insulates or insures electrical isolation of the spanning electrical contact from unwanted contact to electrically grounded components in which at least one of the retainer and crystal holder include features that maintain the electrical contact with the retainer in order to provide a single mechanism.

Description:
TECHNICAL FIELD 
     This application relates generally to the field of material processing and more specifically to a combined monitor crystal retainer and electrical contact system, as well as a related method for retaining a monitor crystal while providing electrical contact therewith for use in deposition monitor sensors. 
     BACKGROUND AND SUMMARY 
     A quartz crystal microbalance is often used as part of a control system in order to optimize the production of specialty thin film coatings that may be used for a wide variety of functional and decorative uses. Exemplary uses of these film coatings cover a fairly wide spectrum that may include those aimed at increasing a machine component&#39;s wear resistance, narrow band optical filtering for increasing optical communication channels or spectral based chemical analysis, enhancement of light transmission through lenses and windows, and enhancing the reflection of mirrors and reducing the light reflected from a transparent surface or simply to apply a color; e.g. create a low cost substitute for gold color, among a myriad of other possible uses. The quartz crystal microbalance that is used for optimizing these processes is commonly known as a deposition process controller. A deposition process controller is used for real time sensing of the precise amount of material that is incident onto and subsequently adheres to a face of a resonantly vibrating piezoelectric crystal. The deposition parameters commonly sensed with this technology are the rate of deposition and total thickness. The crystal microbalance&#39;s sensor function relates to the frequency reduction of the composite resonator (monitor crystal plus deposited material) caused by the mass added to the vibrating piezoelectric plate that is partially exposed to the deposition. The foregoing application has a generally well-accepted formulation and is clearly described in chapter 2 of “Applications of Piezoelectric Quartz Crystal Microbalances” by Lu and Czanderna (Elsevier, first edition pp. 19-57) the entire contents of which are herein incorporated. In practice, the quartz crystal microbalance is located among or nearby the substrates that are being coated and is used as a surrogate substrate, allowing for precise inference of the amount of material added to the substrates by careful calculation based on the directional distribution of the material leaving a deposition source and use of the geometric relationships between the deposition source, substrates and the monitor crystal. 
     The precise and accurate measurement of the monitor crystal&#39;s frequency is an essential component of deposition control. The measurement of frequency is commonly accomplished according to two (2) basic techniques or methods. The first technique is referred to as the so called “active” method in which the monitor crystal and its adlayer becomes part of an oscillator circuit and the resulting frequency is measured by one of many possible ways of measuring frequency. One commonly employed method used to measure frequency employs an independent precision reference oscillator of known frequency to establish a precise and repetitive period of time by counting this oscillator&#39;s pulses to a predetermined fixed number. This process of repeatedly counting a fixed number of the precision oscillator&#39;s pulses establishes an exactly recurring period of time. A second counter is started and stopped by this recurring period that similarly measures the pulses from the resonating monitor crystal. This method of counting the number of measurement crystal pulses over a fixed period of time permits a very accurate measurement of the monitor crystal&#39;s frequency and is commonly known as the “period measurement technique”. The change of the monitor crystal&#39;s frequency is related to the mass added, so therefore knowing the density of the added material, the thickness may be inferred. Noting the change of thickness between successive recurring measurement periods, the rate of material deposition may then be calculated. This measured deposition rate is often used as the measured variable in a control system that manipulates the power to the deposition source such that the deposition rate can be stabilized or in some cases changed and manipulated in desired and advantageous ways. 
     The second common technique of determining the monitor crystal&#39;s frequency is considered a “passive” method that is described in detail, for example, in U.S. Pat. No. 5,117,192 to Hurd, the contents of which are herein incorporated. In the Hurd method, the monitor crystal is excited with a voltage of specific frequency and the related current response of the piezoelectric monitor crystal to this specific frequency is detected as being either capacitive, inductive or in-phase, the latter which is indicative of zero phase shift and infers the composite resonator (piezoelectric crystal plus coating(s) is operating at the desired series resonance point. Using the teachings of Hurd, the result of the phase measurement can be very rapidly converted to an intelligently calculated new applied interrogation frequency. Knowledge of the nature (i.e., sign and magnitude) of the monitor crystal&#39;s phase error response is used to calculate the next interrogation frequency, so that in only a few interrogation cycles the series resonance of the monitor crystal can be determined with very low error, even if material deposition is taking place and the frequency is simultaneously rapidly changing in response to the mass of material that is being added. 
     The measurement of frequency by each of the above methods are improved by minimizing sources of noise and insuring the circuits to and from the monitor crystal are low resistance, thereby further insuring the effective Q (i.e., the quality factor of the monitor crystal) can be maintained at a high level, during which as much deposition material may be added as is possible. When the effective crystal Q deteriorates, the measurement circuits are substantially less able to make a consistent frequency measurement and the control system is compromised by this source of noise. The adherence and growth of the deposition material on the monitor crystal is, by its nature, a dissipative process due to its lack of piezoelectric contribution, acoustic dissipation due to crystalline defects, and in many cases the introduction of tensile or compressive stresses to the composite resonator. 
     Essential requirements of a deposition monitor sensor constructed for commercial use include highly repeatable low noise and low resistance electrical connections, easy replacement of the monitor crystal, and product design of the sensor to insure that the deposition material being monitored and controlled is excluded from those areas that might compromise electrical isolation and integrity. The sharpness of the monitor crystal&#39;s resonance, which is related to the quality factor Q, is known to be reduced as the amount of deposition material is increased on the face of the monitor crystal and this loss of Q is known to increase the perceived noise of the measurement. When the noise reaches a level sufficient to make the measurement noise larger than that which can be tolerated by the process, replacement of the monitor crystal is necessitated. Any improvements taken to reduce the electrical resistance or shield the circuit elements from deposition or from other deterioration mechanisms, such as surface or interface corrosion, will have a positive effect on the measurement including lower noise and sometimes increased life of the monitor crystal. 
     To further clarify the needs outlined above, it must be understood that the electrical elements in the circuit used to apply the interrogation voltage waveform stimulating the monitor crystal and the subsequent sensing of the resulting current&#39;s phase relationship to that applied voltage waveform should have low resistance and the contacts and wires should be shielded from being coated by the material being applied. This is clearly required in the passive measurement technique and a necessary, but less obvious, requirement for any active measurement scheme. Loss of signal strength due to high resistance from loose or corroded connections or a parasitic electrical leak caused by conductive or capacitive leakage through deposited material from the applied radio frequency voltage and the return path allows a portion of the voltage to bypass the monitor crystal and is thereby detrimental to the optimal function of the measurement circuit. 
     In the prior art, the most common means of making electrical contact with a supported monitor crystal is to employ two separate spring contact assemblies or systems, often of the leaf type. Using two contact systems in series allows the user to have a convenient crystal holder package that can be simply and entirely removed from the deposition sensor for subsequent cleaning and monitor crystal replenishment without concern for the monitor crystal either falling out of a receiving cavity of the holder package, or otherwise tilting or hanging up and becoming broken during insertion into the receiving cavity. 
     In a known and typical dual contact scheme, a first leaf spring contact is used to make direct contact with one face (electrode) of a monitor crystal retained within a crystal holder and in which the first spring contact simultaneously pushes the monitor crystal onto an annular seat of the crystal holder. The first leaf spring contact is electrically connected to a conductive plate, allowing a second leaf spring contact, which is physically and electrically fixed to the deposition sensor body to be electrically connected to the first leaf spring contact when the holder assembly is physically inserted into the sensor body. Another intermediate conductive element then completes the electrical circuit to the detection/driving system of the deposition controller. 
     Clearly, a single leaf spring contact system having fewer pieces and contact junctions would be electrically superior, but without a retainer to hold the monitor crystal in proper relationship to its desired position in the crystal holder package all of the aforementioned practical problems associated with monitor crystal replacement are strongly manifested. When the positioning of the monitor crystal&#39;s face with the holder&#39;s crystal seat is not automatically aided by the local gravity field and instead the local gravity field tends to tilt or dislodge the monitor crystal, the installation or removal of the holder package becomes extremely difficult. One known early design that was successful in eliminating contacts, but was problematic regarding the replacement of the monitor crystal in hard to reach or gravity challenged positions, is typified in Lu&#39;s  FIGS. 17 a    &amp;  b  of the aforementioned book by Lu and Czanderna at page 53 thereof. 
     It is logical to assume that when any process is difficult and unpleasant, it is more likely to be performed improperly than when the process is easy and simple. For these reasons, it is common for most crystal sensor contact systems to include a retainer and accept the drawbacks of higher cost and slightly diminished electrical conduction properties in order to ease the above-referred to replacement task. The basic two contact system discussed herein has been employed successfully for more than 40 years. 
     The present invention minimizes the detrimental resistive effects of having numerous electrical contacts in series with the monitor crystal but without losing the convenient, orientation independent crystal holder package that is provided by using a retainer component. This desired electrical conduction benefit is manifested in a way that limits the potential for monitor crystal damage, while still providing secure placement of the monitor crystal within the holder package for insertion into the sensor that is orientation independent. The disclosed invention also provides a means of quickly and easily renewing the electrical contact system without hand tools or a need to solder in-situ. While it may be possible to add a retainer component to a design, such as that disclosed by Lu et al., it is found in practice that the spring contact used to make contact with the monitor crystal is susceptible to damage during routine cleaning; for example, a vacuum cleaning nozzle wiping across the sensor&#39;s holder cavity and reaching the contact spring causing distortion or breakage. The replacement of the contact spring requires tools, and or soldering and has to be performed in a position that is often difficult to reach or in an orientation that makes replacement and removal times long and frustrating. If soldering is required to repair the spring, the associated use of flux is a further complication because the applied flux must be thoroughly and meticulously removed after soldering and before processing can resume in order to avoid flux-caused contamination of the coating process and apparatus. 
     As a result, it can now be clearly seen that an invention that incorporates a retainer&#39;s function along with a means of reducing the number of electrical contacts, while fostering easy and quick in-situ replacement of any electrical contact system without tools or soldering is a very desirable improvement. 
     Therefore and according to one version, there is provided a combination retainer and electrical contact mechanism for a deposition monitor sensor, said mechanism comprising a sensor body and a monitor crystal retained within a crystal holder package. A removable flexible electrical contact spans between a fixed electrical contact element in the sensor body and a face of the retained monitor crystal. The mechanism further includes at least one insulating/isolating element in which the removable flexible electrical contact is associated with the at least one insulating/isolating element to provide a single mechanism. 
     In one version, the flexible electrical contact is defined by a coiled conductive spring having a first diameter section extending over a portion of its length and a second diameter section, which is larger than the first diameter section, defined over a separate portion. According to at least one version, the second diameter section is sized to engage a retention feature, such as an annular groove, formed within the insulating/isolating element. 
     The coiled conductive wire spring can be made from an electrically conductive wire. For example, the electrically conductive wire can be selected from the group consisting of stainless steel, piano wire, Inconel, beryllium copper, nickel copper and molybdenum, and in which each may be coated with gold or other contact enhancing material. 
     According to another embodiment, the removable flexible electrical contact can comprise a tubular body having leaf springs attached at respective ends thereof. The tubular body can include a first diameter section over a portion of its length and a second diameter section over another portion of its length, the second diameter section being sized for retention within an internal groove of the retainer. In one version, a split ring is disposed to engage the internal groove with the second diameter section. 
     In some versions, the retainer can be defined by a hollow cylindrical member having a split gap over its circumference, enabling the retainer to reduce its effective diameter when compressed. When the compressive force is removed, the retainer is configured to releasably engage an inner wall defining an axial bore of the crystal holder. 
     Alternatively, the retainer according to at least one embodiment can include a set of externally disposed ears that are configured to engage receiving slots defined in the crystal holder. The receiving slots include circumferential groove portions, enabling the retainer to be releasably secured to the crystal holder by rotating the retainer as engaged with the slot(s). As such, the retainer maintains a relatively light friction or interference fit within the machined bore of the crystal holder and provides a modest level of retention for the monitor crystal as the contained spanning electrical contact (e.g. spring) engages the crystal and creates friction. 
     The retainer can be made from an insulating material such as ceramic or a rigid plastic with acceptable process temperature and outgassing qualities, such as PEEK. 
     According to another version, there is provided a method for retaining a monitor crystal and providing electrical contact therewith for use in a deposition control monitor, said method comprising the step of providing a crystal holder having an axial bore and an annular seat sized for receiving a monitor crystal. According to this method, a retainer is disposed between the crystal holder and a sensor body, the retainer being at least partially disposed in the axial bore of the crystal bore and the sensor body including a fixed electrical contact engaged with an electrical source of the deposition control monitor. A releasable flexible spanning electrical contact is engaged between a fixed electrical contact of the sensor body and one face of a retained monitor crystal, wherein at least one of the crystal holder and the retainer includes at least one feature for maintaining the spanning electrical contact in a fixed orientation. 
     In one embodiment, the removable flexible spanning electrical contact includes a first diameter section over a portion of its length and a second diameter portion over another portion of its length. The second diameter section is configured to engage an internal groove provided in the retainer. 
     In some embodiments, the flexible electrical contact is a coiled conductive spring having respective ends configured to engage a fixed electrical contact of the sensor body and the face of the retained monitor crystal, respectively. In another version, the flexible electrical contact is defined by a tubular conductive body having leaf springs attached at opposing ends of the tubular body. 
     In some versions, the retainer can be defined by a hollow cylindrical member having a split gap over its circumference, enabling the retainer to reduce its effective diameter when compressed. When the compressive force is removed, the retainer is configured to releasably engage an inner wall defining an axial bore of the crystal holder. 
     Alternatively, the retainer according to at least one embodiment can include a set of externally disposed ears that are configured to engage receiving slots defined in the crystal holder. The receiving slots include arcuate or circumferential grooved portions, enabling the retainer to be releasably secured to the crystal holder by means of a twisting action once engaged. As such, the retainer maintains a relatively light friction or interference fit within the machined bore of the crystal holder and provides a modest level of retention for the monitor crystal. 
     One advantage provided by the herein described combination contact/retainer system is that fewer components are required, reducing the overall number of components as well as related costs in manufacture and replacement. The reduction in the total number of parts and at least some machined features are eliminated, thereby creating a much simpler and more reliable apparatus. 
     Another related advantage is an overall reduction in the number of electrical interfaces in the monitor crystal sensor, which reduces the voltage drop due to contact resistance. 
     In addition, maintenance of sensors is simplified herein by eliminating the need to solder at least some replacement items, eliminate removal and replacement of screws, and avoiding the necessity to remove the entire sensor assembly from the vacuum coating tool to do this. 
     Still further, convenience of positive retention of the monitor crystal within the crystal holder assembly is provided, thereby easing the process of removal and replacement of monitor crystals by eliminating concern for having to make contact with the monitor crystal&#39;s surface and minimizing the possibility of breakage of the monitor crystal while performing these routine operations. 
     In addition, a crystal retainer is introduced that can provide the function of restraining the monitor crystal in the crystal holder, while allowing a contact spring device to pass through the holder unimpeded. 
     These and other features and advantages will be readily apparent from the following Detailed Description, which should be read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded elevational view of a prior or known dual contact monitor crystal retention and electrical contact system for use in a material deposition sensor; 
         FIG. 2( a )  is an exploded side elevational view of a combination monitor crystal retention and electrical contact apparatus in accordance with an exemplary embodiment; 
         FIG. 2( b )  is an assembled side elevational view of the combination monitor crystal retention and electrical contact apparatus of  FIG. 2( a ) ; 
         FIG. 3( a )  is a side elevational view of a retainer used in the combination system and in accordance with one exemplary embodiment; 
         FIG. 3( b )  is a plan view of the retainer of  FIG. 3( a ) ; 
         FIG. 4  is a side elevational view of a spanning electrical contact of the combination system and in accordance with an alternative construction; 
         FIG. 5( a )  is a plan view of a retainer in accordance with another exemplary embodiment; 
         FIG. 5( b )  is a side elevational view of the retainer of  FIG. 5( a ) ; 
         FIG. 6( a )  is a plan view of a crystal holder body made in accordance with an exemplary embodiment for use with the retainer of  FIGS. 5( a ) and 5( b ) ; and 
         FIG. 6( b )  is a side elevational view of the crystal holder body of  FIG. 6( a ) . 
     
    
    
     DETAILED DESCRIPTION 
     The following description relates to several embodiments of a system for retaining a monitor crystal that is used in conjunction with a deposition process monitor, the system further insuring electrical contact between an electrical source and a face of a retained monitor crystal, such as a piezoelectric crystal. Throughout the course of discussion, several terms are used in order to describe the invention in accordance with the accompanying drawings. These terms, such as “front”, “rear”, “lateral”, “upper”, “lower”, “proximal”, “distal” and the like are merely intended to provide a suitable frame of reference with regard to the accompanying drawings. These terms are not intended to otherwise inhibit the scope of the present invention, including the claims. In addition, the included drawings are not necessarily to scale and are simply intended to clearly illustrate the salient features of the invention. 
     A typical prior art version of a deposition sensor that incorporates a retainer is shown in  FIG. 1  and depicted as a cross-sectional view of a typical sensor body  1  with an integral water passage  3  that is used to provide a fluid passage for the purposes of cooling, heating or temperature control depending on the nature and temperature of the fluid flowing within. The fluid is typically brought to the sensor body  1  and removed after flowing through the integral passage  3  by a suitable number of small tubes (not shown) that are interconnected to the passage  3  and sealed. A male/female coaxial cable connector  5  disposed in relation to a retaining cavity  4  of the sensor body  1  provides a means of easy electrical connection/disconnection to the deposition controller&#39;s exciting and detection circuitry (not shown). In this exemplary illustration, the male/female coaxial electrical connector  5  is locked into position with a set screw  10  to avoid back out when a mating coaxial electrical connector (not shown) is removed. A leaf spring with tail  25  is mounted to the sensor body  1  with a non-conducting screw  20  and is further electrically isolated by an intervening insulator disk  30 . Electrical connection to the leaf spring with tail  25  is made by routing the tail to the center pin  6  of the male/female coaxial cable connector  5  and soldering, or alternatively, a spring contact (not shown) or spot welding can be used. Depending on the physical clearance to the sensor body  1 , an insulator over the tail of the leaf spring  25  may or may not be required. 
     Still referring to  FIG. 1 , a crystal holder and retainer assembly  75  includes two distinct subassemblies; namely, a crystal holder subassembly  63  and a retainer subassembly  80  that combine to maintain a monitor crystal  70 , such as a piezoelectric quartz crystal, seated in proper position and in good electrical contact within an axial machined bore  66  of a substantially cylindrical crystal holder body  60  of the crystal holder subassembly  63 . Each subassembly is ultimately configured to provide flat and intimate contact with an annular seat  67  provided at the end of the axial machined bore  66  of the crystal holder body  60  and against which the monitor crystal  70  is retained. The crystal holder body  60  is made from a material such as stainless steel and includes a pair of opposing ends connected by the axial bore  66 . The monitor crystal  70  and retainer subassembly  80  engage one end of the crystal holder body  60 , while the opposite end defines a material aperture  65  that enables exposure of the remaining side of the monitor crystal to the deposition chamber (not shown). 
     The retainer subassembly  80  provides all-orientation mechanical retention of the monitor crystal  70  and also transfers the electrical connection from the leaf spring  25  to the monitor crystal  70 . The retainer subassembly  80  consists of an insulating retainer body  45  that can be made from a ceramic or rigid machinable plastic such as PEEK, that is designed to slide into the axial bore  66  machined within the crystal holder body  60  while an expanding wire retainer  55  is sized for insertion within an annular retainer groove  61  formed within an inner wall of the axial bore  66  and is hereafter firmly restrained until intentional removal. The retainer subassembly  80  is constructed by inserting a conducting rivet  35  through a center hole formed in a contact disk  40 , through the hole  46  in the center of the insulating retainer body  45  and through the center of a leaf spring  50 . Finally, the conducting rivet  35  is carefully deformed to insure good electrical contact and long term mechanical integrity. In place of mechanical deformation, the conducting rivet  35  is sometimes soldered. 
     In the prior art crystal holder/retainer assembly  75  herein described, the electrical circuit to and from the male/female coaxial cable connector  5  encounters numerous joints and connections. More specifically and from the coaxial connector&#39;s center post  27 , a solder joint  26  connects to the leaf spring with tail  25 , which makes physical contact with the contact disk  40 , which in turn makes contact to the conducting rivet  35 , and which then makes contact with the leaf spring  50 . Finally, the leaf spring  50  makes contact with the piezoelectric monitor crystal  70 , thereby requiring a total of (5) five separate electrical contacts and junctions. 
     With the foregoing background, a design in accordance with an exemplary embodiment is herein depicted in  FIGS. 2( a ) and 2( b )  in a manner that parallels the description of the known assembly  75  shown above. Many features and functions are identical to those depicted in  FIG. 1 , or are at least functionally equivalent, with some small variation and therefore similar parts are herein labeled with the same reference numerals for the sake of clarity. 
       FIG. 2( a )  provides a cross-sectional view of a typical sensor body  1  having an integral water passage  3  that is used to provide a fluid passage for the purposes of cooling, heating or temperature control depending on the nature and temperature of the fluid flowing within. The fluid is typically brought to the sensor body  1  and removed after flowing through the integral passage  3  by small tubes (not shown) connecting to the sensor body  1 . A male/female coaxial cable electrical connector  5  is used to provide a means of easy electrical connection/disconnection to the exciting and detection circuitry (not shown). In this exemplary embodiment, the coaxial cable electrical connector  5  is locked into position using a set screw  10  to avoid back out when the mating coaxial connector (not shown) is removed. In lieu of the first leaf spring  25 ,  FIG. 1 , a flat contact plate with tail  25   a  is mounted to the sensor body  1  with a non-conducting screw  20  and is further electrically isolated by an intervening insulator disk  30 . Electrical connection of the flat contact plate  25   a  is made by routing the integral tail of the plate  25   a  to the coaxial connector&#39;s center pin  6  and soldering or in some cases a spring contact or spot welding is used. Depending on physical clearance to the sensor body  1 , an insulator over the tail may or may not be necessary. 
     A crystal holder and retainer assembly  75   a  is defined by a pair of subassemblies, namely a crystal holder subassembly  63   a  and a retainer subassembly  80   a , respectively, used to maintain a monitor crystal  70 , such as a piezoelectric quartz crystal, in proper position/orientation and in good electrical contact within a substantially cylindrical crystal holder body  60  by maintaining flat and intimate contact against an annular seat  67  defined at the end of a machined axial bore  66  of the cylindrical crystal holder body  60 . The holder and retainer assembly  75   a  provides mechanical retention of the monitor crystal  70  and also transfers the electrical connection from the flat contact plate with tail  25   a  to the monitor crystal  70 . 
     More specifically and according to this exemplary embodiment, the retainer subassembly  80   a  comprises an insulating retainer body  45   a  having a hollow substantially cylindrical configuration including a pair of opposing ends and a coiled spring contact  90 . The coiled spring contact  90  is defined by respective ends and is preferably made from an electrically conductive wire made from, but not limited to stainless steel, Inconel, beryllium copper, nickel copper, piano wire or molybdenum. The coiled spring contact  90  is defined by a first diameter region  91  extending over a substantial portion of its overall length and a second expanded diameter region  92  extending over an intermediate portion of the length of the spring contact. The second diameter region  92  has a slightly larger diameter designed to protrude into an annular retention groove  47  which is formed within an inner wall of the hollow insulating retainer body  45   a . For purposes of operation, the expanded second diameter region  92  needs only to be sufficiently enlarged to provide retention within the annular retention groove  47  of the retainer  45   a  to avoid the casual separation of the coiled spring contact  90  by gravity or normal movement during use. In this way, the coiled spring contact  90  may be easily removed by applying modest separation forces and can be easily replaced, as needed, within the retention groove  47 . According to this exemplary embodiment, the insulating retainer body  45   a  is made from a resilient, low outgassing and machineable plastic or other insulator that is compatible with high purity process vacuums. As discussed herein and briefly referring to  FIGS. 3( a ) and 3( b ) , the retainer body  45   a  is designed according to one version with a split gap  49 ,  FIG. 3( b )  along its circumference, enabling the retainer body  45   a  to be slightly compressed radially to permit insertion into the machined axial bore  66  of the crystal holder body  60  and in which removal of the compressive force on the retainer body  45   a  restores the retainer body  45   a  to attempt to assume its original size by radial expansion and in compressive contact with the crystal bore  68  of the crystal holder  60 . In this way, the function of the holder&#39;s wire spring retainer  55  of the prior known version can be directly incorporated into the retainer body  45   a , thereby eliminating the need for a wire spring retainer and its necessary groove  68 . 
     The herein described retainer subassembly  80   a  can be assembled by inserting the coiled conducting spring contact  90  so that the second diameter region  92  is intentionally made to a larger diameter than the outermost turns and is positively retained in the annular retention groove  47  of the retainer body  45   a . This expanded region  92  according to this version extends over approximately two turns of the spring contact  90 , but providing this region over a single turn or more than two turns would also be sufficient to meet the intended purpose. The retention of the coil spring contact  90  within the insulating retainer body  45   a  is not essential to function, but is a user convenience that reduces the number of individual pieces that typically have to be accounted for when replacing the monitor crystal  70 . 
     In the improved assembly described herein, the electrical circuit to and from the coaxial cable connector encounters fewer joints and connections. Summarily and from the coaxial cable connector&#39;s center post  27 , a solder joint  26  connects to the integral tail of the flat contact plate  25   a , which makes physical contact to one circular face of the coil spring contact  90 , while the other circular face of the coiled conductive spring  90  makes contact with a face of the monitor crystal  70  retained against the annular seat  67  of the crystal holder body  60  for a total of three (3) separate contacts, or two (2) fewer than that of known retention/contact system versions including those depicted in  FIG. 1 . 
     The electrical return path from the opposite side (face) of the monitor crystal  70  is through physical contact with the annular seat  67 , which is an integral portion of the crystal holder body  60  at the end of the axial bore  66   a , and then being forced against a deformable spring  15  that is provided in the inner wall of the receiving cavity  4  of the sensor body  1 , wherein the deformable spring makes intimate contact with the sensor body  1  and then finally by contact to the shell of coaxial cable connector  5  being aided by the set screw  10 . The return electrical path described in this embodiment is literally identical in both the previously known and improved designs. Similarly, the signal is connected to the excitation and measurement circuits (not shown) of the deposition controller (not shown) through a mating coaxial cable connector and coaxial cable in both the current and improved designs and in which the monitor crystal can be excited and resonated by known techniques, as previously described. 
     As noted,  FIGS. 3( a ) and 3( b )  depict respective cross sectional and plan views of the retainer body  45   a  in accordance with an exemplary embodiment. The retainer body  45   a  is made from an insulating material such as a ceramic, rigid plastic or other suitable material and is defined by a substantially cylindrical configuration with a pair of open ends and an inner diameter  48 , which is manufactured slightly larger than the axial bore  66  machined in the crystal holder body  60 . According to this embodiment, the outer diameter  48  is approximately 0.005 to 0.010 inches larger when machined, but any diameter slightly larger than the axial bore  66  but still capable of entering the axial bore  66  will work in a way that although not ideal, can be considered functional. Even if the outer diameter  48  is slightly smaller, it will in practice exhibit some small retention capability due to misalignment induced friction with the bore. The cylindrical retainer body  45   a  according to this embodiment is normally completely machined in all other ways before the retainer&#39;s split gap  49  is added, insuring good roundness and finish on all features. The width of the split gap  49  is typically about 0.020″ to about 0.032″, and chosen so standard and readily available cutters might be used. A slightly smaller gap might also be used, as long as the cylindrical retainer body  45   a  can be squeezed sufficiently circumferentially to enter the axial machined bore  66  of the crystal holder body  60 . A larger gap might also be used with a limitation to not make it overly large to the point that the cylindrical retainer body  45   a  would lose its ability to self-center within the axial bore  66  of the crystal holder body  60  and consequently allow spring contact  90  to contact the inner wall of the axial bore  66 , shorting the electrical circuit. 
       FIG. 4  shows a cross sectional view of an alternate construction for an electrical contact  99  that uses essentially the same cylindrical retainer body  45   a , as previously described. According to this alternate version, two leaf springs  50   a  are captured by two screws  110  that are threaded into respective ends of an electrically conductive contact body  100  and thereby completing electrical continuity between the two individual leaf springs  50   a . The contact body  100  must be electrically conductive and is preferably hollow. While not essential that the entire length of the tubular contact body  100  be hollow, the feature must extend sufficiently on each side to be conveniently threaded in extent to engage the screws  110  completely and insure tightness of the leaf springs  50   a  for good electrical contact. At least one vent hole  103  is drilled into the contact body  100 . It is easier to provide the entire length of the contact body  100  as hollow in order to cross-drill into the formed volume only once rather than having to provide one vent hole  103  for each end. The purpose of the drilled vent hole  103  is to allow easy passage of retained atmospheric or process gasses into the vacuum vessel and avoid a condition known as virtual leaking, which is well known and can create contamination. This contamination delays the process of complete evacuation of the processing chamber to high vacuum, thereby further delaying the start of processing. An external retainer groove  102  is provided as a second diameter section of the tubular body  100 , this retainer groove  102  being designed to loosely engage one of many types of snap or split rings  105  in order to provide and maintain at least a loose fastening between the insulating retainer groove  47  and the contact body  100 . As noted and in use, this alternative electrical contact  99  can be provided in lieu of the coiled spring contact  90 . In use, one of the leaf springs  50   b  will make contact with the fixed electrical contact of the sensor body (not shown in this view), while the remaining leaf spring  50   a  will contact one face of the monitor crystal  70 ,  FIG. 2( b ) , as retained within the defined axial bore  66  of the crystal holder body  60 . The return path using this electrical contact  99  is unchanged from that previously described. 
       FIGS. 5( a ), 5( b ) and 6( a ), 6( b ) , taken together, describe another alternative design for the crystal holder and retainer subassemblies, respectively. These alternative subassembly designs do not necessarily rely on the resiliency of the retainer material or a separate spring for retention of same within the machined axial bore  66  extending through a substantially cylindrical crystal holder body  60   b . In this specific version, it is necessary for the inner diameter  48   a  of the crystal holder body  60   b  to be somewhat less than the axial machined bore  66  thereof in order to freely pass. More specifically, the retainer body  45   b , which is a substantially cylindrical and hollow member does not include a split gap  49 , but is defined by at least one radially outwardly projecting retainer ear  52  that is configured and sized to engage a crystal holder body  60   b  through at least one receiving slot  54  defined therein for retaining an ear  54 . 
     As shown in  FIGS. 5( a ) and 5( b ) , the cylindrical hollow retainer body  45  includes a pair of diametrically opposed retainer ears  52  that project radially from the exterior of the retainer body  45   b  and are sized and configured for engagement within a corresponding pair of receiving slots  54  provided in the inner wall of the axial bore  66  of the crystal holder body  60   b . Each of the receiving slots  54  extend from the end of the crystal holder body  60  opposite the material aperture  65 , the receiving slots  54  further including an enclosed arcuate or circumferential grooved portion  57 . When assembled, the receiving slots  54  permit retention of the retainer body  45   b  when a small angular twist of the retainer body  45   b  is made about its primary axis in a predetermined (e.g., counterclockwise) direction, releasably securing the retainer ears  52  within corresponding arcuate grooved portions  57  of the receiving slots  54 . Disassembly can be easily performed by rotating the retainer body  45   b  in the opposite (e.g., clockwise) direction so as to align the retainer ears  52  with the receiving slots  54  and then axially withdrawing the insulating retainer body  45   b . There is no imperative for a tight or close fit of the retainer ears  54  within the defined arcuate grooved portion  57  as the tension that the coil spring contact  90 ,  FIG. 2( b ) , generates from contacting the retained monitor crystal  70 ,  FIG. 2( b ) , will have a natural tendency to provide a reaction force so that sufficient friction will be present to avoid rotation during the activity necessary to insert the combined assembly into the sensor body  1 ,  FIG. 2( b ) , to engage the deformable spring  15 ,  FIG. 2( b ) . Once the retainer body  45   b  is fully inserted in the sensor body  1  and even though the retainer and holder ears  54  are coincidently aligned, there will be no loss of electrical function as the coiled contact spring  90  is compressed between the piezoelectric monitor crystal  70 ,  FIG. 2( b ) , and the flat contact plate  24 ,  FIG. 2( b ) , respectively, the monitor crystal  70  being firmly engaged against the defined annular seat  67  of the crystal holder body  60   b.    
     PARTS LIST FOR FIGS.  1 - 6 ( b ) 
     
         
           1  sensor body 
           1   a  sensor body with holder threads 
           3  integral fluid passage 
           4  retaining cavity 
           5  male/female coaxial cable connector 
           6  center pin, connector 
           10  set screw 
           15  deformable spring 
           20  non-conducting screw 
           24  flat contact plate with tail 
           25  leaf spring with tail 
           26  solder joint 
           27  center conductor 
           30  Insulator disk 
           35  conducting rivet 
           40  contact disk 
           45  insulating retainer body 
           45   a  insulating retainer body 
           45   b  insulating retainer body 
           46  hole 
           47  insulating retainer groove 
           48  insulating retainer outer diameter 
           48   a  retainer outer diameter 
           49  retainer split gap 
           50  leaf spring 
           50   a  leaf spring 
           52  retainer ear 
           53  retainer lip 
           54  receiving slot for retainer ear(s) 
           55  retainer wire spring 
           57  arcuate or circumferential grooved portions, receiving slot 
           60  crystal holder 
           60   b  crystal holder 
           60   a  crystal holder 
           60   c  crystal holder 
           63  crystal holder subassembly 
           63   a  crystal holder subassembly 
           65  material aperture 
           66  crystal bore 
           67  annular seat 
           68  retention groove, retainer 
           70  monitor crystal 
           74  crystal holder assembly 
           75  crystal holder and retainer assembly 
           75   a  crystal holder and retainer assembly 
           75   b  crystal holder assembly 
           80  retainer subassembly 
           80   a  retainer subassembly 
           90  coil spring contact 
           91  first diameter portion 
           92  second diameter portion 
           99  contact, electrical 
           102  external retainer groove 
           103  vent 
           105  split ring 
           110  screw 
       
    
     It will be readily apparent that other modifications and variations will be readily apparent based on the teachings of this disclosure, and in accordance with the following claims.