Patent Publication Number: US-10777393-B2

Title: Process condition sensing device and method for plasma chamber

Description:
CLAIM OF PRIORITY 
     This Application is a divisional of commonly-assigned U.S. patent application Ser. No. 14/505,289, filed Oct. 2, 2014, the entire contents of which are incorporated herein by reference. 
     Application Ser. No. 14/505,289 is a divisional of commonly-assigned U.S. patent application Ser. No. 12/691,695, filed Jan. 21, 2010, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to plasma processing systems, and more particularly to apparatus for making in-situ measurements of plasma properties in plasma processing systems. 
     BACKGROUND OF THE INVENTION 
     Plasma processes are frequently used to modify or treat the surfaces of workpieces such as semiconductor wafers, flat-panel display substrates, and lithography masks. Conditions within a plasma process are designed to produce a complex mixture of ions, reactive chemical species (free radicals), and energetic neutral species. The interaction of these materials then produces the desired effect on the surfaces of workpieces. For example, plasma processes are used to etch materials from the surfaces of semiconductor wafers so as to form complex electrical elements and circuits. The conditions within the plasma process are carefully controlled to produce the desired etch directionality and selectivity. 
     The surface modifications produced by a specific plasma are sensitive to a number of basic parameters within the plasma. These parameters include such variables as: chemical concentrations (partial pressures), temperatures (both surface and gas phase), and electrical parameters (ion fluxes, ion energy distribution functions). A number of these parameters (e.g. gas concentrations and pressure) can generally be easily controlled using external actuators such as Mass Flow Controllers (MFCs) and servo driven throttle valves. Other important parameters (e.g. temperatures and free radical concentrations) can often be observed or measured via sensor systems (e.g. thermocouples and Optical Emission Spectrometers (OES)) installed on the process tool. A last set of important parameters such as ion fluxes and ion energies are more difficult to either directly control or monitor. 
     US publication No. 2005-0151544 discloses a plasma processing system with diagnostic apparatus for making in-situ measurements of plasma properties. The diagnostic apparatus generally comprises a non-invasive sensor array disposed within a plasma processing chamber, an electrical circuit for stimulating the sensors, and means for recording and communicating sensor measurements for monitoring or control of the plasma process. In one form, the sensors are dynamically pulsed dual floating Langmuir probes that measure I-V characteristic, displacement RF current into or through the wafer and self-bias due to electrons piling up on the surface, which can be used to determined the charge on the wafer. 
     Wafer charges are formed due to different flux rates for ions and electrons (due to their very different masses). Wafer charging can lead to damage to the devices. One type of tool that is conventionally used for characterizing wafer charging during wafer processing in ion-based and plasma-based IC processing equipment includes EEPROM-based peak potential sensors and current sensors to characterize the I-V relationship of charging transients. The gate of the transistors is coupled to the antenna structures on the wafer. The device measures the cumulative charge, not charge as a function of time. Furthermore, the wafer has to be taken out of the plasma chamber to read the charge measurement. 
     US publication No. 2006-0249729 discloses a sensor wafer that uses a triple capacitor stack to measure apparent alternating current (AC) at the surface of the wafer. This rectification (detection) device has a minimum bias requirement and a strong frequency dependency on the range of interest. The measurement is purely AC and the center capacitor, formed by a polyimide substrate is the shunt impedance that generates the AC potential to be measured. The sensor responds in a confounded way to a number of electrical parameters in the plasma chamber and is unable to relate specifically to any one parameter. This makes it difficult to find the right “knobs” to tune the chamber when problems are encountered. 
     In addition, many prior art sensor wafers include a module atop of the wafer that houses electronics for the sensor array. This module can cause severe disturbance in the plasma or can be a point of discharge damages and can also be a source for contamination. 
     Another problem with prior art sensor wafers is that sensor pads in the array and electrical connections between these pads and associated electronics are often made of metal traces, e.g., Aluminum, that is deposited on the surface of the wafer. Exposure to plasma, e.g., Argon plasma, eventually erodes aluminum traces on the surface of the wafer. In some sensor wafers, entire surface of the wafer is covered by polyimide to protect the traces and sensor pads. However, the polyimide coating can have a very short life time in certain plasma environments and may also be a source of contamination. In addition, the use of certain metals, such as copper, is strongly avoided in many process steps. 
     It is within this context that embodiments of the present invention arise. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Objects and advantages of embodiments of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which: 
         FIG. 1  is a perspective view schematic diagram illustrating a wafer having plugs for sensing devices according to an embodiment of the present invention. 
         FIG. 2A  is a cross-sectional view of a sensing device according to an embodiment of the present invention. 
         FIG. 2B  is a cross-sectional view of a sensing device according to an alternative embodiment of the present invention. 
         FIG. 2C  is an equivalent electrical diagram of the sensing devices shown in  FIGS. 2A-2B . 
         FIG. 3  is a top view of a sensing device according to an embodiment of the present invention. 
         FIG. 4A  is a perspective view illustrating measurement of current across the surface of a plasma measurement device according to an embodiment of the present invention. 
         FIG. 4B  is a combined cross-section and electrical schematic diagram of an equivalent circuit of the device shown in  FIG. 4A . 
         FIG. 4C  is an equivalent electrical circuit diagram of the device shown in  FIG. 4B . 
     
    
    
     DESCRIPTION OF THE SPECIFIC EMBODIMENTS 
     Although the following detailed description contains many specific details for the purposes of illustration, anyone of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope of the invention. Accordingly, the exemplary embodiments of the invention described below are set forth without any loss of generality to, and without imposing limitations upon, the claimed invention. 
     According to one embodiment of the present invention, a plasma sensing device may include sensors and electronics, embedded in the surface of a wafer substrate and directly visible to the plasma sheath. The surface of the wafer substrate and portion of the sensing device that is exposed to the plasma can both be made of a material that is conventionally processed by the type of plasma that the device is designed to sense. By way of example, in some embodiments, a shunt impedance for one or more of the sensors can be formed by a “slug” of material that is substantially the same as the material the wafer substrate and that is also directly visible to the plasma sheath. As used herein, the term “substantially the same material” means that the one material is chemically similar if not physically identical to another material. For example, the substrate and slug materials can be substantially the same if the wafer substrate and slug are both made of single crystal silicon but with different crystalline orientations. Alternatively, the substrate and slug materials can be substantially the same if the substrate is made of single crystal silicon and the slug is made of polycrystalline silicon. The term “substantially the same” also encompasses slight variations in the chemical composition the two materials, e.g., due to different but otherwise acceptable levels of impurities. 
     Forming the shunt impedance in this manner provides a durable and non contaminating surface for the plasma and can also provide a DC connection to the surface of the substrate. Hence, surface DC potential may be measured. This structure can give valuable information such as surface charge, self-bias, load-line characteristics or I-V characteristics is a DC bias is simultaneously imposed upon the sensor. 
       FIG. 1  is a perspective view of a sensing device  100  that includes a substrate  102  several slugs  104  as collectors for sensors. The substrate  102  and slugs  104  can be made of the same material. By way of example, and not by way of limitation, the substrate  102  and slugs  104  can both be made of silicon, or any other conductive or semiconductor material that is compatible with the processing conditions in the processing environment in which the device  100  is intended to operate. By way of example, and not by way of limitation, the substrate may include a silicon wafer with a diameter of between 100 mm and 450 mm. The sensing device  100  may have an overall thickness of 0.3 mm to 10 mm 
     The substrate  102  and slugs  104  can provide a durable and non contaminating surface presented to the plasma. For example, if the plasma diagnosed by the sensing device is conventionally used for processing, e.g., etching of or deposition on silicon wafers, the substrate and a collector pad that is part of a sensor may be made of silicon so that the plasma “sees” a silicon surface. In some implementations, if it is desired to present a dielectric surface to the plasma, a plasma-compatible polymer coating, such as photoresist, can cover the surface of the wafer substrate. Many plasma process chambers are designed to process substrates that are covered with photoresist. Consequently, the presence of photoresist on the surface of the substrate  102  and slugs  104  poses no additional contamination hazard beyond those already taken into account in the design of the plasma process diagnosed by the device  100  and the chamber in which such a plasma process takes place. Contamination of a plasma chamber resulting from exposure of the surface of the substrate  102  and slugs  104  to the plasma may be rectified by whatever conventional process is used following normal treatment of production substrates in the chamber. For example, after using the device  100  to diagnose a plasma in a chamber used to etch a silicon substrate covered with a patterned photoresist, the chamber may be cleaned by a conventional process normally used to clean such a chamber after such plasma etching. 
     The slugs  104  may be electrically coupled to sensor electronics  106  located beneath the surface of the substrate  102 . There may be more than one sensor electronics  106  below the surface of the substrate  102  for the purpose of processing the sensor signals. By way of example, the slugs may serve as collector pads for sensing a flux of electrons or ions or energetic radiation from a plasma. In some implementations, the substrate  102  and slugs  104  may provide a DC connection from sensor electronics to the surface. Such a configuration can be used measure surface DC potential. Such a structure can also be used to obtain valuable information such as surface charge and self-bias. In some implementations, AC signal detection may be performed by a temperature compensated, DC biased high-frequency, diode bridge circuit. This method can eliminate the disadvantages of poor linearity and low power levels, frequency effects as well as temperature drift associated with prior art methods. 
     In some embodiments, the electronics  106  may apply an excitation voltage at one sensor slug  104  and measure a signal at one or more different slugs to measure the plasma impedance. 
     According to some embodiments of the present invention, active circuitry and a power-source may be embedded into and/or in-between layers of a device like that shown in  FIG. 1 , thereby effectively forming a Faraday cage around such components. By way of example, and not by way of limitation,  FIGS. 2A-2B  are cross-sectional views illustrating different implementations of sensing devices  200 A and  200 B. By way of example, and not by way of limitation, the sensing devices  200 A and  200 B can be used to measure electrical parameters such as self bias voltage, saturation current, charge and polymer build up in process chamber with process chemistry and under high plasma power over a wide range of RF and Microwave frequencies. 
     As shown in  FIG. 2A , the sensing device  200 A includes a first and second layers  202  and  204  respectively separated by an insulating layer  212 , which can be an oxide or other dielectric layer. The first layer  202  can be made from the same material as a production wafer that is normally processed by the type of plasma the device  200 A is designed to sense. In the device  200 A a cavity  216  can be formed at the surface of the first layer  202 . A plug made of the same material as the first layer  202  sensor can be positioned in the cavity  216  with the top surface of the plug  206  coplanar with the top surface of the first layer  202 . The underside of the plug  206  can have a metallic coating  222  that is suitable for making an ohmic contact connection. By way of example, the metallic coating can be diffused into the silicon and thus does not create a diode junction to the silicon. Examples of suitable materials for the metallic coating  222  include platinum and palladium. If wire bonding is performed, aluminum can also be used. Therefore, the top surface of the device  200  can be made planar and have a profile substantially the same as that of a standard production substrate processed in the type of chamber that the device  200 A is designed for. A dielectric layer  220  (e.g., polyimide) can be deposited between the silicon plug  206  and the first layer  202  to provide electrical isolation. The dielectric layer  220  can also form a shunt capacitance that can be used to measure an RF current which may be impinging on the sensor surface formed by plug  206 . 
     The plug  206  can be exposed to plasma and act as a signal collector by collecting a raw flux, e.g., charge (in the form of electrons or ions), from the plasma that can be detected as a raw signal, e.g., in the form of a current or voltage. Alternatively, the plug  206  may be subject to radiation from the plasma in the form of energetic photons. To detect such photons, the plug  206  may be made of a semiconductor material having electrical properties that change upon exposure to such photons. The plug  206  can be coupled to suitable sensor electronics  205 . The combination of a signal collector, such as the plug  206 , and sensor electronics  205  is sometimes referred to herein as a sensor. Depending on the nature of the electronics, the sensor may be used to detect AC or DC signals. 
     Examples of suitable sensor electronics  205  include signal conditioning electronics and signal processing electronics. In addition, the electronics may include a power source, such as a battery, which may be used to apply a bias voltage to the plug  206 . As used herein, signal conditioning includes, but is not limited to, filtering, noise rejection, or amplification of the raw signal to make it more suitable for analysis. Examples of signal conditioning components include, but are not limited to, rectifiers, oscillators, or amplifiers. Signal processing refers to analysis of a signal, e.g., using analog or digital circuitry. Signal processing may include, but is not limited to, analog-to-digital conversion, arithmetic and/or logical operations, Fourier transforms or other mathematical transforms, spectral analysis, and the like. Examples of signal processing components include, but are not limited to a microprocessor or an application-specific integrated circuit (ASIC). By way of example, and not by way of limitation, the electronics  205  could include data acquisition (DAQ) electronics that produce a DC signal that is coupled to processing electronics. 
     The plug  206  may be electrically connected to the electronics  205 , e.g., by circuitry, which may be embedded in the second silicon layer  204  or in the insulating layer  212 , e.g., in the form of a flex circuit  208 . In such a case, the components (e.g., transistors, resistors, capacitors, logic devices, etc, that make up the electronics  205  can be connected by conductive (e.g., copper) traces patterned onto a polyimide flexible substrate. Alternatively the electronic components that make up the sensor electronics  205  and associated circuits may be formed directly on the surface of the second layer  204  or the insulating layer  212 , e.g., using standard semiconductor processing techniques. The electronics  205  may be electrically connected to the first layer  202  e.g., by an ohmic contact  214  coupled to the flex circuit  208 . An ohmic contact  214  may also be formed between the first layer  202  and the second layer  204 . The ohmic contact  214  can provide an electrical connection to the substrate  204  and  206 , e.g., by means of a suitable metallization  222  as described above. Furthermore, the ohmic contact  214 , may be expanded to essentially create a contact that would cover most of the available surface area of silicon layers  202  and  204 . 
     It is noted that multiple slugs  206  may be used to provide multiple collectors that may be coupled to appropriate sensor electronics. The collectors and sensor electronics may be configured in many different ways to measure plasma parameters. For example, with appropriate electronics and additional circuits, pairs of collectors and corresponding sensor electronics may be configured to operate as dual differential Langmuir probes. In such a configuration, the sensor electronics can apply a bias voltage between two collectors and measure currents to the two collectors. By varying the bias voltage, ion saturation current and electron temperature may be determined from an I-V curve. Furthermore, by simplification of the control circuitry, triple differential Langmuir probe operation is possible or a differential Langmuir probe operated in a time division configuration. 
     A triple differential probe with appropriate circuitry can provide a response with minimal processing. In a triple probe configuration, two collectors may be biased positive and negative with a fixed voltage (V + -V − ) between them while a voltage on a third collector is allowed to float to the plasma floating potential V B . If the bias voltage is sufficiently large compared to the electron temperature (i.e., e(V + −V − )&gt;&gt;k B T e , where e is the charge on the electron and k B  is Boltzmann&#39;s constant) the negative biased collector can be expected to draw the ion saturation current, which, like the floating potential V fl , can be directly measured. If the biased collector configuration is floating, the current to the positive-biased collector is approximately equal in magnitude to the ion saturation current drawn by the negative-biased collector and the floating collector can be expected to effectively draw no current. 
     Under these conditions the electron temperature is approximately proportional to the measured voltages as follows:
 
( V   +   −V   fl )=ln 2( k   B   T   e   /e ).
 
     More sophisticated analysis of triple probe data can take into account such factors as incomplete saturation, non-saturation, unequal areas. Triple probes also can be symmetrically, asymmetrically or highly asymmetrically depending on the region the probe is expected to operate under. Triple probes have the advantage of simple biasing electronics (no sweeping of the bias voltage is required), simple data analysis, excellent time resolution, and insensitivity to potential fluctuations (whether imposed by an RF source or inherent fluctuations). The disadvantage is that they require three probe areas which may not be possible to implement in all cases. 
     Alternatively, by time division multiplexing, four bias voltage conditions may be set and used to derive an electron temperature T e  and ion density n i  with a dual Langmuir probe. The Ion Saturation current, I sat , may also be similarly estimated. The result is very similar to the triple probe but using only two probe areas. 
     Sensing device  200 B shown in  FIG. 2B  is similar to the sensing device  200  described in  FIG. 2A , except that the silicon plug  206  is positioned on the insulating layer  212  like the first layer  202 . By way of example, and not by way of limitation, the plug may be formed from a silicon-on-insulator (SOI) wafer in which the insulator is disposed between two layers of silicon. By selectively etching one of the two silicon layers the plug  206  may be electrically separated from a remaining portion of that layer. The plug may be further insulated from the silicon layer by additional insulating material  217 . By forming the plug  206  from the same initial material as the first layer  202  the surface of the plug  206  can be made almost perfectly flush with the surface of the rest of the first layer  202 . Furthermore, the first layer  202  and the plug  206  can be assured to have almost identical material properties. 
     It is noted that in the devices  200 A and  200 B, the first layer  202  and second layer  204  may be made sufficiently electrically conducting (e.g., by suitable doping) so that they form a Faraday cage that is suitable for protecting the electronics  204  from electromagnetic interference during operation within a Plasma procession environment. 
       FIG. 2C  is an electrical schematic diagram of an equivalent circuit of the sensing devices illustrated in  FIGS. 2A-2B . As indicated in  FIGS. 2A-2B , a polymer layer  218  may build up on the surface of the first layer  202  as a result of a polymerization reaction that takes place in the plasma chamber. Such polymerization is a fairly common occurrence in many types of plasma processing. The polymerization layer  218  can affect the measurements made with the devices  200 A,  200 B. 
     In some embodiments, the surface of a collector portion, e.g., plug  206  can be modified, e.g., by etching a three-dimensional pattern into the surface or by deliberately coating it with one or more materials  219  such as a photoresist and/or metal to modify the interaction between the collector and a plasma. By way of example, and not by way of limitation, the material  219  can be deposited or otherwise formed in pattern, e.g., a grid or series of stripes. The material  219  can be different from the material that makes up the bulk of the plug  206 . 
     Specifically, as shown in  FIG. 2C , the polymer layer  218  can act as a variable capacitor C 1 . Referring simultaneously to  FIG. 2A  and  FIG. 2C , a measuring capacitor C 2  may be formed in series with the variable capacitor C 1  due to the polymer layer with the silicon plug  206  as the upper plate and the electronics, first and second silicon layers  202  and  204  and the ohmic contacts  214  forming the lower plate of the capacitor. The polyimide layer  220  can act as an insulating material between two plates of the measuring capacitor C 2 . The capacitor C 2  can be connected to a power source, e.g., a battery, B. If a switch SW is added across the plates of the measuring capacitor C 2 , a rate of the electron built up on the surface of the substrate could be measured by shorting the plates of the measuring capacitor C 2  by closing the switch and then measuring the voltage change when the switch is opened. 
       FIG. 3  is a top view of a sensing device  300  according to an embodiment of the present invention. As shown in  FIG. 3 , the sensing device  300  includes a substrate  302  with several sensors  304 , which may be similar to the sensing devices  200  and  201  as described in  FIGS. 2A-2B . Specifically, each sensor  304  may include a collector  306  that is formed from the same material as the surface of the substrate  302 . A surface of the collector may be flush with the surface of the substrate  302 . The collector  306  may be coupled through sensor  304  to local sensor electronics  305  which may be embedded into the substrate  302 . The collector  306  may include such features as patterned film-stacks, micro-machined surface features or simply a metal film such as a layer of aluminum. In addition, the sensing device  300  may include a centralized processing and/or communication electronics unit  308  that is coupled to each of the sensors  304  and collector  306  if any. The centralized electronics  308  can provide centralized component for transmitting and storing data from the sensors  304  out of device  300  to a remote receiver. The electronics can also provide a centralized component for receiving data from an external transmitter and relaying such data to one or more selected individual sensors  304 . The electronics  300  may include a wireless or wired transceiver unit that converts the data into signals that can be transmitted wirelessly, e.g., by electromagnetic induction or radiation. Alternatively, the electronics  308  may transmit the signals over a medium, such as a signal cable or fiber optic link. 
     Examples of suitable sensor electronics  305  include signal conditioning electronics and signal processing electronics. In addition, the electronics may include a power source, such as a battery, which may be used to apply a bias voltage to the sensor  304  and collector  306 . As noted above, signal conditioning includes, but is not limited to, filtering, noise rejection, or amplification of the raw signal to make it more suitable for analysis. Examples of signal conditioning components include, but are not limited to, rectifiers, oscillators, or amplifiers. Signal processing refers to analysis of a signal, e.g., using analog or digital circuitry. Signal processing may include, but is not limited to, analog-to-digital conversion, arithmetic and/or logical operations, Fourier transforms or other mathematical transforms, spectral analysis, and the like. Examples of signal processing components include, but are not limited to a microprocessor or an application-specific integrated circuit (ASIC). 
     By way of example, and not by way of limitation, the electronics  305  could include a localized processor unit that is specific to the sensor  304  with which it is associated. By using a localized processor unit at each sensor  304 , part of the burden of processing the raw signals from the sensor  304  and collectors  306  may be distributed. In this way, the processing burden may be made to scale with the number of sensors on the device  300  without having to add processing capacity to the centralized electronics  308 . For example, in addition to rectification, amplification, and A/D conversion the sensor electronics  305  could perform certain digital signal filtering functions on the raw data and then transmit the resulting filtered data to the centralized electronics. This frees up processing resources on the centralized electronics for collating the filtered data from the different sensors  304 . 
     It is noted that embodiments of the present invention encompass many different configurations in which a sensor device may include a collector may be formed from the same material as a production substrate. These embodiments include versions in which the collector pad comprises an entire continuous surface of the sensor device. By way of example, and not by way of limitation, a single continuous surface of silicon substrate may be used as a collector pad. By measuring voltage differences between different locations on the surface of the substrate it is possible to determine differences in surface charge buildup. 
       FIG. 4A  is a schematic perspective view illustrating a plasma sensing device  400  according to an alternative embodiment of the invention. In the device  400 , a collector, is made from a substrate  402  that is made of resistive material that the same as or similar to the material used in production substrates that are subject to plasma  401  in a process chamber. The substrate  402  may have a planar top surface that has substantially the same profile as a production substrate that is processed in the process chamber. If charge builds up non-uniformly on the surface of the substrate  401 , a sheet current I s  may flow across the surface of the substrate  401 . As shown in  FIG. 4A , a voltage may be measured between different locations A and B on the surface of the substrate  401 . If the substrate material has a sufficiently large sheet resistance, the sheet current I s  can be used as measure of the charge imbalance across the substrate surface. Specifically, the voltage between points A and B may be determined from the current, which depends on sheet resistivity of the substrate material  401  and the charge imbalance between the two points. 
       FIG. 4B  is a combination equivalent electrical circuit schematic diagram and cross-sectional view of the sensing device  400 . The sensing device  400  generally includes an upper semiconductor substrate  402  and a lower semiconductor substrate  404 . In some embodiments, an intermediate semiconductor substrate  416  may be sandwiched between the upper and lower substrates. For convenience the substrates  402 ,  404 ,  416  are sometimes referred to below simply as the upper, middle, and lower substrates. By way of example, upper substrate  402  may be a first silicon layer  402  doped P− to make it resistive, and the bottom substrate  404  may be a second silicon layer doped P+ to make it conductive. An insulating layer  428  is sandwiched between the upper and lower substrates  402  and  404 . In the example shown in  FIG. 4B , the insulating layer is more specifically sandwiched between the upper substrate  402  and the intermediate substrate  416 . The insulating layer  428  may be relatively, thick, e.g., 25 microns or more in thickness. There are a number of ways to form the insulating layer  428 . For example, oxide may be implanted into a silicon wafer. Alternatively, oxide may be grown on the surface of a silicon wafer and polycrystalline silicon (polysilicon) may be grown or deposited on the oxide. In addition, a polymer layer may be laminated between two silicon wafers. 
     Similarly, an insulating layer  430  may be formed between substrates  404  and  416 . 
     Suitable electronics  418  may be positioned in cavities formed in the lower substrate  404 . Each electronics unit may be electrically coupled to a corresponding sensor contact  403 , by a corresponding pattered metallization or a pattered flexible circuit  408  formed on, or inlaid into, the lower substrate  404 . The electronics can be configured to measure direct current (DC) or alternating current (AC) electrical quantities such as voltage, current, charge, capacitance, and the like at the sensor locations  403 . Also the electronics  418  may excite one or more sensor contacts  403  and/or a reference contact  405  with DC levels or AC waveforms and simultaneously measure AC and DC parameters at other sensor contact locations  403  and/or reference contact  405  to determine various plasma parameters. By way of example, the electronics  418  may apply an excitation voltage at one sensor contact  403  and measure a signal at one or more different sensor contacts and/or the reference contact  405  to measure the plasma impedance. Alternatively, the excitation voltage may be applied to the reference contact  405  and signals may be measured at the sensor contacts  403 . 
     As seen in  FIG. 4B  the collector portions of the sensors can be embedded in a substrate by forming them as integral parts of the upper semiconductor layer  402 . By way of example one or more sensor contacts can be formed by depositing a suitable conductive material such as platinum, palladium or aluminum in the areas of the sensor contacts  403  and reference contact  405 , in locations on an underside of the upper semiconductor layer  402  where a sensor is desired. The can be diffused into the upper semiconductor layer  402  thus creating a ohmic connection. Since the vertical dimension of the upper semiconductor layer  402  is much smaller that the horizontal dimension, the effect is that the metallic sensor contacts  403  at the bottom of the upper layer  402  is mirrored onto the top surface as a virtual sensor  420 . In a like manner the effect of the central reference contact  405  is mirrored as a virtual sensor  422 . Any voltage present at the top surface would be translated to the bottom surface and there for can be sensed and driven by appropriate electronics  418 . The reference contact  405  at a center position may be connected to the middle substrate  416  via a electronic switch in the electronics module  418 . Such ohmic contact to substrate  416  can effectively provide for a DC reference potential of the middle and lower substrates  416  and  404 . This DC reference can be important, e.g., when measuring DC voltages with the electronics  418 , but can be turned of when measuring AC voltages. 
     In some embodiments, one or more portions of the surface of one or more collector portions, e.g., selected portions of the upper surface of the upper layer  402  can be modified, e.g., by etching a three-dimensional pattern into the surface or by deliberately coating them with a material  419  such as a photoresist and/or metal to modify the interaction between the collector and a plasma. The material  419  can be deposited or otherwise formed in pattern, e.g., a grid or series of stripes. The material can be different from the material that makes up the bulk of the upper layer  402 . 
     The electronics  418  can sense sheet currents I e  flowing through the upper substrate  402  between the sensor contacts  403  and a reference contact  405  at a center location thereby providing a measure of the self-bias and charge build-up on the surface of the upper layer  402 . Voltage differences can be measured due to the sheet resistance of upper layer  402  and the sheet currents I e  flowing between different sensor contacts  403  and/or between the sensor contacts  403  and the reference contact  405 . In  FIG. 4C , the sheet resistance of the substrate is represented by resistors designated R sub . By placing several such sensor contacts at different locations within the device  400  and sensing the sheet currents between the image  422  of the central reference contact  405  and each images  420  of the sensor contacts  403  the charge imbalance over the surface of the upper silicon layer  402  can be measured in real time. The voltage or charge measurements can be converted to digital data and stored at a central electronics unit (not shown) for later transmission to an external receiver without having to remove the device  400  from the plasma chamber. 
     The middle and lower substrates  416  and  404  may also act as a Faraday cage for shielding of the electronics  418  and the associated metallization  408 . By way of example, the middle substrate  416  and the lower  404  can be ohmically connected to each other through contacts  426 , and  413 , which may be in the form of metallization regions created in a manner similar to that described above, and/or the ohmic contact  432 . Furthermore, the upper substrate  402  together with insulating layer  428  and base substrate  416  can act as a distributed capacitor. This capacitance can serve as a test load to sense an RF current by the RF voltage generated at each sensing point. The amplitudes of such voltages may be rectified and measured by the electronics  418 . Schematically, the local part of the distributed capacitance is equivalent to the capacitor C 2  shown in  FIG. 2C . The equivalent capacitances between the sensor contacts  403  are indicated as C and C′ in  FIG. 4C . The equivalent capacitance between the reference contact  405  and the plasma is indicated as C″ in  FIG. 4C . The DC part of the signal, as measured by the electronics  418 , at the sensor contact  403 , is related to the charge distribution. The sensor contact  403  and reference contact  405  may also be excited by an applied AC or DC voltage for measurement of parameters such as ion saturation current, electron temperature and plasma impedance. 
     As in the embodiments shown in  FIGS. 2A and 2B , a collector area  424  constructed with pattered film stacks can be placed upon the first surface of substrate  402  to modify the electrical response of the substrate  402  to the plasma. 
     Embodiments of the present invention provide a tool for measurement of ion and electron characteristics of a plasma and, optionally, for measurement of surface charging in a wafer production system. The ability to measure these quantities in a wafer production system can provide additional insight to plasma processing surface parameters. Also surface related topology may radically alter the behavior of the sensing locations and this phenomenon may be taken advantage of to measure such parameters such as polymer re-deposition, ion angles and charge damage with higher sensitivity and a wider dynamic range. Embodiments of the present invention can help more easily identify key elements that affect plasma chamber performance and subsequently adjust the relevant parameter to optimize performance. 
     While the above is a complete description of the preferred embodiment of the present invention, it is possible to use various alternatives, modifications and equivalents. Therefore, the scope of the present invention should be determined not with reference to the above description but should, instead, be determined with reference to the appended claims, along with their full scope of equivalents. Any feature, whether preferred or not, may be combined with any other feature, whether preferred or not. In the claims that follow, the indefinite article “A”, or “An” refers to a quantity of one or more of the item following the article, except where expressly stated otherwise. The appended claims are not to be interpreted as including means-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase “means for.”