Abstract:
The invention relates to the use of the metrology methods and the related apparatus disclosed herein that incorporate thermal treatment devices and methods that improve defect detection. Specifically, in one aspect the invention relates to method of thermally treating a semiconductor wafer such that an acceleration of interstitial defect migration is achieved while leaving vacancy defects substantially unaltered.

Description:
RELATED APPLICATION  
       [0001]     This application claims priority to United States Provisional Application 60/661,557 filed on Mar. 14, 2005, the disclosure of which is herein incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to the testing of semiconductor wafers during manufacturing and specifically to the real-time in-line testing of semiconductor wafers during integrated circuit-fabrication.  
       BACKGROUND OF THE INVENTION  
       [0003]     There are numerous individual operations, or processing steps, performed, in a strictly followed sequence, on a silicon wafer in the course of manufacturing a complex integrated circuit (IC). Each such operation must be precisely controlled in order to assure that the entire fabrication process yields integrated circuits displaying the required electrical characteristics.  
         [0004]     Frequently, failure of an individual operation is detected only after the completion of the entire, very expensive, process of IC fabrication. Due to the very high cost of advanced IC fabrication processes, such failures result in the severe financial losses to the integrated circuit manufacturer. Therefore detection of errors in the manufacturing process, immediately after their occurrence, could prevent the unnecessary continuation of the fabrication of devices which are destined to malfunction, and hence, could substantially reduce the financial losses resulting from such errors.  
         [0005]     Process monitoring in semiconductor device manufacturing relies upon the examination of the changes which occur in certain physical and/or chemical properties of the silicon wafer upon which the semiconductor devices are fabricated. These changes may occur following the various processing steps to which the silicon wafer is subjected and are reflected by changes in the electrical properties of the wafer. Therefore, by monitoring selected electrical properties of the silicon wafer in the course of IC fabrication, an effective control over the manufacturing process can be accomplished.  
         [0006]     Not all of the electrical characteristics of a completed integrated circuit can be predicted based on the measurements performed on a partially processed wafer. Most of the characteristics however, can be predicted directly or indirectly based on the investigation of the condition of the surface of the silicon wafer (substrate) in the course of IC manufacture. The electrical condition of the silicon surface is very sensitive to the outcome of the individual processing steps that are applied during IC manufacturing. Hence, the measurement of the electrical properties of the substrate surface can be an effective tool by which the monitoring of the outcome of the individual processing steps can be accomplished.  
         [0007]     The determination of the electrical characteristics of the wafer surface typically requires physical contact with the wafer surface, or the placement of a contactless probe over a stationary wafer. In the latter case an optical signal or a high electric field is used to disturb equilibrium distribution of the electrons in the surface and near-surface region of semiconductor. Typically, the degree of departure from equilibrium is driven by variations of one or more electrical characteristics of the surface region, the near-surface region, and the bulk of the semiconductor. To obtain a more complete picture of the entire surface of the wafer, several measurements at various points on the surface can be made. Such a procedure, known as “mapping” performs a measurement at each location before the measuring device moves on to the next location. The substrate, in this procedure, typically does not remain in continuous motion, so consequently the applicability of such a method for use in real-time in-line process monitoring is limited.  
       SUMMARY OF THE INVENTION  
       [0008]     In part, the invention relates to a multi-mode, electro-optical metrology system that improves the sensitivity of currently available instruments. Improvements include a combination of non-contact, real-time, inline, measurements over an extended process space. In particular, a low temperature thermal treatment is described that can be added to an existing metrology system. This thermal treatment accelerates the migration of interstitial defects before measurement commences; however, the process does not remove the vacancy defects that are of interest to the process engineer.  
         [0009]     Furthermore, the low temperature heat treatment has special advantages when applied to multiple wafers. Because of the heat exposure, the interstitial defects in each wafer are stabilized to approximately the same level. Application of the heat treatment prevents each wafer from exhibiting skewed individual capacitive measurements due to interstitial variations. Thus, by using the disclosed low temperature technique on a group of wafers, a relative comparison of the individual measurements is possible. This comparison is feasible due to the standardized interstitial defect baseline that results from the heating process.  
         [0010]     The general metrology technique disclosed herein relies on a capacitive measure of modulated photo-carriers generated from absorption of over-band-gap-light with varying wavelengths, interacting with silicon wafers (up to 300 mm diameter) at different stages of the device fabrication process. Thus, the devices and techniques disclosed herein are applicable to, but not limited to Epitaxial Layer Doping, Ion Implant, Rapid Thermal Processing (RTP), and Trace Metal Contamination monitoring.  
         [0011]     The interaction of high frequency, chopped light, with single crystal silicon has been treated theoretically through the modulation of surface potential. Low-level intense light slightly varies the surface potential through electron-hole production, without changing electrical or optical properties of the semiconductor. If the surface potential is enough to deplete the surface of charge carriers, accurate calculations of doping density can be made for a uniform doping profile. Charge potentials associated with other physical conditions throughout the crystal depth (including polished backside) can be varied through drift and diffusion of photo carriers, in turn causing a measurable surface potential modulation. The dynamic photo-carrier equations are normally reduced to a steady state, that can readily be analyzed with an equivalent electrical circuit can be used to relate the capacitance and conductance to physical quantities of the semiconductor, such as; doping density, carrier lifetimes, and defect density.  
         [0012]     A first aspect of the invention relates to a method of measuring defects of an ion implanted semiconductor wafer. The method includes the steps of heating the wafer to a treatment temperature for a treatment time; conveying the wafer such that a surface of the wafer is substantially parallel to a surface photovoltage electrode of a head assembly; exposing at least a portion of the wafer to light having a wavelength and an intensity; detecting with the surface photovoltage electrode a photovoltage induced at the surface of the wafer in response to the modulated light intensity; and calculating an electrical property of the wafer from the photovoltage induced at the surface of the wafer. In variations of the first aspect, the following additional steps may also be included: modulating the intensity of light at a frequency; applying a hydrogen fluoride wash to the wafer; inducing an inversion layer at the surface of the wafer, which may be accomplished by applying a corona or a chemical to the wafer; and comparing an electrical property against a standard. One of more of the steps in the first aspect may be repeated. In addition, the light intensity modulation frequency may be varied from 0.1 through 100 kHz, and the electrical property may comprise net carrier concentration, which may be determined by a specific formula.  
         [0013]     In a second aspect, an apparatus for measuring defects in an ion implanted semiconductor wafer is provided. The apparatus includes a heating element adapted for heating each wafer to a specified temperature for a specified time period such that interstitial defect migration is stabilized; a head assembly comprising a surface photovoltage electrode; a conveyer for conveying the wafer such that a surface of the wafer is substantially parallel to the surface photovoltage electrode of the head assembly during the semiconductor processing; a light source generating light having a wavelength, and having an intensity, the light source being modulated over a plurality of frequencies; a detector for detecting with the surface photovoltage electrode a photovoltage induced at the surface of the wafer in response to the light; and a processor in electrical communication with the detector for calculating an electrical property of the wafer from the photovoltage induced at the surface of the wafer. In variations of the second aspect, the apparatus may further comprise the following items: a grower, such as a rapid thermal furnace, for growing an oxide layer on the wafer; a washer for applying a hydrogen fluoride wash to the wafer; and a corona. In addition, the processor calculating the electrical property of the wafer may do so by at least comparing an electrical property against a standard, and the electrical property may be net carrier concentration determined by a specific formula.  
         [0014]     In a third aspect, a method of measuring damage of an ion implanted semiconductor wafer during semiconductor processing is provided. The method includes the steps of growing an oxide layer on the semiconductor wafer; performing ion implantation on the wafer, heating the wafer to a temperature T 1  for a time period P 1 , such that the selection of T 1  and P 1  substantially stabilize interstitial defect migration; conveying the wafer such that a surface of the wafer is substantially parallel to a surface photovoltage electrode of a head assembly during the semiconductor processing; exposing at least a portion of the wafer to light having a wavelength and having an intensity, the light intensity being modulated at a frequency; detecting with the surface photovoltage electrode a photovoltage induced at the surface of the wafer in response to the light; calculating an electrical property of the wafer from the photovoltage induced at the surface of the wafer; and repeating some of the aforementioned steps for a plurality of light intensity modulation frequencies. In variations of the third aspect, the electrical property may comprise net carrier concentration, which may be determined by a specific formula.  
         [0015]     The invention also relates to an apparatus for measuring damage to an ion implanted semiconductor wafer during semiconductor processing. In one embodiment, the apparatus includes a head assembly including a surface photovoltage electrode; a conveyer for conveying the wafer such that a surface of the wafer is substantially parallel to the surface photovoltage electrode of the head assembly during the semiconductor processing; and a light source generating light having a wavelength and an intensity. The intensity of the light is modulated at a frequency, and the modulation frequency is varied. The apparatus also includes a detector for detecting a photovoltage induced at the surface of the wafer in response to the varying of the modulation frequency of the light and a processor in electrical communication with the detector for calculating an electrical property of the wafer from the photovoltage induced at the surface of the wafer at each of the light intensity modulation frequencies. A heat source for modulating the temperature of the wafer can also be include in accordance with the teachings provided herein. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     This invention is pointed out with particularity in the appended claims. The above and further advantages of this invention may be better understood by referring to the following description taken in conjunction with the accompanying drawings, in which:  
         [0017]      FIG. 1A  is a plot depicting a decay study according to an illustrative embodiment of the invention;  
         [0018]      FIG. 1B  is a plot depicting a series of repeated daily measurements for a semiconductor wafer that was not subjected to heat treatment prior to measurement;  
         [0019]      FIG. 1C  is a plot depicting a series of repeated daily measurements for a semiconductor wafer that was subjected to heat treatment prior to measurement according to an illustrative embodiment of the invention;  
         [0020]      FIG. 1D  is a block diagram of an embodiment of an apparatus for the real-time, in-line, electrical characterization of a semiconductor during manufacturing;  
         [0021]      FIG. 2  is a perspective view of an embodiment of the probe assembly of the apparatus of  FIG. 1  in position above a wafer transfer system;  
         [0022]      FIG. 3  is a top perspective cutaway view of the probe assembly of  FIG. 2 ;  
         [0023]      FIG. 4  is a bottom perspective view of an embodiment of the sensor plate of the probe assembly of  FIG. 3 ;  
         [0024]      FIG. 5  is a schematic diagram of an embodiment of an electrical circuit for measuring the surface photovoltage using front wafer surface coupling;  
         [0025]      FIG. 6A  depicts a block diagram of a corona control circuit used to charge a wafer so as to generate an inversion layer at the wafer surface;  
         [0026]      FIG. 6B  depicts a block diagram of the corona control circuit of  FIG. 6A  used to discharge a wafer;  
         [0027]      FIG. 7  is a bottom perspective cutaway view of an embodiment of the coated sensor plate of  FIG. 4  with a polyimide coating, used with sensor charging and high voltage biasing;  
         [0028]      FIG. 8  is a schematic diagram of an embodiment of a preamplifier circuit used for the high voltage biasing of the wafer using the sensor electrodes;  
         [0029]      FIG. 9  is a graph of front and back surface charge measurements of a silicon wafer undergoing cleaning;  
         [0030]      FIG. 10  is a flowchart showing the steps of a system for monitoring as-implanted (before annealing) wafer parameters and of implanted/annealed (after annealing) wafer parameters, according to an illustrative embodiment of the invention;  
         [0031]      FIG. 11  is a schematic of the equivalent circuit of the measured AC-SPV in ion implanted silicon;  
         [0032]      FIG. 12  is an example of illustrative roll-off frequency sweep curves for as-implanted silicon using the embodiments of the invention; and  
         [0033]      FIG. 13  is an example of a calibration curve derived from frequency sweep curves for as-implanted silicon according to the teaching of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0034]     Embodiments of the present invention will be more completely understood through the following detailed description, which should be read in conjunction with the attached drawings. In this description, like numbers refer to similar elements within various embodiments of the present invention. Within this detailed description, the claimed invention will be explained with respect to embodiments. However, the skilled artisan will readily appreciate that the embodiments described herein are merely exemplary and that variations can be made without departing from the spirit and scope of the invention.  
         [0035]     Given the high costs associated with semiconductor processing, techniques and devices that help eliminate defective wafers before and during wafer processing are of great value. Specifically, the ability to detect defects non-destructively via electrical characterization of wafers is valuable because such an approach is easily integrated into existing fabrication systems.  
         [0036]     The apparatus and techniques disclosed herein use a non-destructive approach for semiconductor defect detection at the individual wafer level. One aspect of the invention uses a heat treatment of the wafer in order to control for a certain class of defects. However, prior to considering this specific thermal processing improvement, an overview of the supporting assumptions and relationships that support an exemplary form of capacitive measurement will be useful.  
         [0037]     One apparatus suitable for performing various electrical characterizations uses the method for measuring the photo-induced voltage at the surface of semiconductor materials, termed the surface photovoltage (SPV), disclosed in the U.S. Pat. No. 4,544,887. In this method, a beam of light is directed at a region of the surface of a specimen of semiconductor material and the photo-induced change in electrical potential at the surface is measured. The wavelength of the illuminating light beam is selected to be shorter than the wavelength of light corresponding to the energy gap of the semiconductor material undergoing testing. The intensity of the light beam is modulated, with both the intensity of the light and the frequency of modulation being selected such that the resulting AC component of the induced photovoltage is directly proportional to the intensity of light and inversely proportional to the frequency of modulation.  
         [0038]     When measured under these conditions, the AC component of the surface photovoltage (SPV), designated δV s , is proportional to the reciprocal of the semiconductor space-charge capacitance, C sc . When the surface of the specimen is illuminated uniformly, the relationship between the surface photovoltage (SPV) and the space-charge capacitance is given, at sufficiently high frequencies of light modulation, by the relation:  
         δ   ⁢           ⁢     V   s       =         ϕ   ⁡     (     1   -   R     )         K   ⁢           ⁢   f       ⁢   q   ⁢           ⁢     C   SC     -   1             
 
         [0039]     where φ is the incident photon flux, R is the reflection coefficient of the semiconductor specimen, f is the frequency at which the light is modulated, and q is the elementary charge. The constant K is equal to 4 for a square wave modulation of the light intensity and is equal to 2π for sinusoidal modulation.  
         [0040]     In the above referenced patent, only a uniform configuration is considered in which the area of the sensor is at least the same size as the semiconductor wafer and the entire area of the specimen is uniformly illuminated. When only a portion of the semiconductor specimen surface is coupled to the sensor, that is, when the sensor is smaller than the wafer, and when the semiconductor surface uniformly illuminated in that area is coupled to the sensor, the surface photovoltage, δV s , may be determined from the measured signal, δV m , according to the relationships:
 
 Re (δ V   s )= Re (δ V   m )−(1 +C   L   /C   p )+ Im (δ V   m )·(ω· C   p   ·R   L ) −1 
 
 IM (δ V   s )= Im (δ V   m )·(1 +C   L   /C   p )− Re (δ V   m )·(ω· C   p   ·R   L ) −1 
 
 where Re(δV s ) and Im(δV s ) are the real and imaginary components of the voltage, ω is an angular frequency of light modulation, C p  is the capacitance between sensor and the wafer, and C L  and R L  are the input capacitance and resistance, respectively, of the electronic detection system. 
 
         [0041]     From the sign of the imaginary component, the conductivity type may be determined. If the measurement is calibrated for a p-type material, then the sign of the imaginary component will change if the material is n-type.  
         [0042]     Using above relationships, the depletion layer width, W d , is given by equation:  
         W   d     =         ɛ   s     q     ⁢         ω   ⁢           ⁢          Im   ⁡     (     δ   ⁢           ⁢     V   s       )                ϕ   ⁡     (     1   -   R     )         ·     (     1   +       [       Re   ⁡     (     δ   ⁢           ⁢     V   s       )         Im   ⁡     (     δ   ⁢           ⁢     V   s       )         ]     2       )             
 
 where φ(1−R) is the intensity of light absorbed in the semiconductor, q is the elementary charge, and ∈ s  is the semiconductor permittivity. 
 
         [0043]     In addition to the space-charge capacitance, C sc , the measurement of the surface photovoltage can be used to determine the surface charge density, Q ss , the doping concentration, N sc , and the surface recombination lifetime, τ, using the following relationships. The space charge capacitance, C sc , is proportional to the reciprocal of the semiconductor depletion layer width, W d , according to the relationship:  
         C   sc     =       ɛ   s       W   d           
 
 where ∈ s  is the semiconductor permittivity. The density of space charge, Q sc , is in turn described by equation:
 
Q sc =qN sc W d 
 
 where q is an elementary charge and the net doping concentration in the space-charge region, N sc , is positive in an n-type material and negative in a p-type material. In addition, since the surface charge density, Q sc , is given by the expression:
 
 Q   sc   =−Q   ss 
 
 the surface charge density is easily determined from the space charge density. 
 
         [0044]     Further, if an inversion layer can be created at the wafer surface, the depletion layer width, W d , under inversion conditions is related to the net doping concentration, N sc , according to the relationship:  
         W   d     =         4   ⁢     ɛ   s     ⁢   kT   ⁢           ⁢     ln   ⁡     (            N   sc     /     n   i            )             q   2     ⁢          N   sc                    
 
 where kT is the thermal energy and n i  is the intrinsic concentration of free carriers in the semiconductor. Several methods of forming such an inversion layer at the semiconductor surface are disclosed below. 
 
         [0045]     In addition, the surface recombination rate may also be determined from the SPV. The recombination lifetime of the minority carriers at the surface, τ, is given by the expression:  
         1     ω   ⁢           ⁢   τ       =              Re   ⁡     (     δ   ⁢           ⁢     V   s       )         Im   ⁡     (     δ   ⁢           ⁢     V   s       )              .         
 
         [0046]     In general, the ac photovoltaic signal might be presented as  
         V   SPV     =         I   eh       G   +     j   ⁢           ⁢   ω   ⁢           ⁢   C         ∝         I   eh       C   sc       ⁢       f   ⁡     (     ω   ⁢           ⁢     τ   s       )       .             
 
         [0047]     Here I eh  is an electron-hole generation rate, G and C are total conductance and capacitance of the system, ω is a light modulation frequency and τ s  is a carrier lifetime at the near surface region. The electron-hole generation rate is given by  
           I   eh     =     q   ⁢           ⁢     Φ   ⁡     (     1   -   R     )       ⁢     (     1   -       exp   ⁡     (       -   α     ⁢           ⁢     W   d       )         1   +     α   ⁢           ⁢   L           )         ,       
 
 where Φ is a photon flux, R and α are reflectivity and absorption coefficients, L is a carrier diffusion length and W d  is a depletion layer width. High defect density conditions αW d &lt;&lt;1 and αL&lt;&lt;1 give us I eh ∝qΦ(1−R)αL. 
 
         [0048]     The diffusion length  
         L   =         (     D   ⁢           ⁢   τ     )       ∝       〈   τ   〉     ⁢       kT   m         ∝         f   ⁡     (   E   )         N   D       ⁢       kT   m             ,       
 
 where D is a diffusion coefficient, N D  is a number of defects/recombination centers, f(E) is a function of charge carrier energy, which depends on prevailing energy scattering mechanism, m is a charge carrier effective mass, k is a Boltzman constant. 
 
         [0049]     Combining the last two expressions we get  
         I   eh     =           q   ⁢           ⁢     Φ   ⁡     (     1   -   R     )           N   D   *       ⁢           ⁢   with   ⁢           ⁢     N   D   *       =         N   D     /   α     ⁢           ⁢     f   ⁡     (   E   )       ⁢       kT   m               
 
 representing an effective defect density. 
 
         [0050]     Finally, in the case of ion implanted silicon wafers, it is found that, especially in as-implanted conditions, carrier lifetime is inversely proportional to implant damage. In very low dose implanted cases, free carrier concentration is reduced. With heavy dose implant application, increased crystal damage gives a photovoltage signal dominated by photo carrier lifetimes rather than free carrier concentration. In some cases carrier lifetime is the dominant factor in the measured SPV signal. After the wafers are annealed, the substitutional site implanted dopant contributes to the net carrier concentration, N sc  which is derived from SPV. For as-implanted p or n-type wafers the charged defects density is a measure of implant dose/energy. For implanted/annealed silicon wafers the measured quantities give the doping concentration, which is directly correlated to implanted dose/energy.  
         [0051]     The overview introduced above describes one approach for semiconductor wafer defect analysis. Other methods and relationships known in the art can be used to detect defects in a given semiconductor wafer. Additional detail describing specific apparatus and methods are described in more detail below. However, prior to describing general apparatus and device embodiments, it will be useful to introduce the thermal treatment processes that can be incorporated in all aspects of the invention disclosed herein.  
         [0052]     Semiconductor manufacturing processes, such as ion implantation, introduce a high level of crystal defects that act as recombination centers for photo-generated electron hole pairs. This defect density affects the surface photo-voltage by reducing the photo-carrier lifetime. As the number and types of defects can limit the functionality of an electronic device made with the implanted semiconductor material, manufacturers are interested in the detection of various defects.  
         [0053]     The implantation of single crystalline silicon creates multiple types of radiation defects. Some of them are stable at room temperature, and high temperature annealing (500-1000° C.) is required to remove them. The most stable defect type at room temperature is a di-vacancy—a complex composed of two nearest neighbor vacancies. Distribution of these defects plus several other types of stable vacancy based complexes may be considered an “implant signature” that can be used to monitor implanter uniformity performance.  
         [0054]     Other post-implant defects existing in single crystal silicon at room temperature are unstable. They can migrate, change their configurations and distributions until they reach “sinks”, for example, wafer surfaces, where they can stabilize or disappear. The existence of these defects makes any damage related measurement of implanted silicon wafers unstable at room temperature. Most of these types of defects can be annealed out at temperatures less than 200° C. At room temperature the most common unstable defects are single and double self-interstitials (di-interstitials) and their complexes with doping impurities. There are some evidences to show that single self-interstitials are annealed out at around room temperature. Silicon di-interstitials, at room temperature, are nearly as mobile as single interstitials, and are annealed at approximately 170° C. Two major interstitial-impurity complexes, positively charged Si-P6 and neutral Si-A5, are annealed out at 120° C. and 160° C. respectively. The first defect complex is usually associated with a Boron implant, the second with a Phosphorus.  
         [0055]     The parameters of thermal treatment such as temperature and time required to stabilize an implanted wafer depend on specific implant conditions including implant species, dose and energy, and implant temperature. Heating/annealing temperature is a function of the activation energy for migration of given defects and defect complexes. Therefore, it is the type of implant species that determines the type of these complexes. The time of annealing depends on both defect activation energy and on defect density, hence, it is a function of all above listed implant conditions, with implant dose being a major factor. The dependence of heating time on implant species is straightforward. The larger the ion the greater the number of elastic scattering events per ion and the greater is the total number of displaced atoms. The effect of implant energy is less but still a factor. Up to some energy value, the elastic scattering grows and dominates. Beyond this threshold energy, the inelastic scattering prevails and there is no significant growth in the number of defects.  
         [0056]      FIG. 1A  shows the decay study of three different implant species. To analyze the mobile defect migration we can write a first order rate equation  
             ⅆ   n       ⅆ   t       =     A   ⁢           ⁢     exp   ⁡     (     -       E   A     kT       )           ,         
 where n is a concentration of one type of defects, E A  is the activation energy for migration of these defects, T is a temperature, pre-exponential frequency factor A. Integrating the equation, and letting t/t 0 &gt;&gt;1, we obtain  
         n     n   ⁡     (     t   0     )         =     A   -       kT     E   A       ⁢       ln   ⁡     (     t     t   0       )       .               
 Fitting the measurement data to this formula we calculate the activation energy characteristic to different implant species. Errors due to Interstitial Migration can be controlled for by thermal pre-treatment. 
 
         [0057]     Two types of crystalline defects are of particular interest to semiconductor metrologists, vacancy defects and interstitial defects. A vacancy defect occurs if an atom is missing from one of the positions in the crystal lattice that should be occupied. In contrast, if an atom is ejected to a non-lattice position within the crystal, an interstitial defect results.  
         [0058]     Implant related interstitials are not stable at room temperature. As a result, a thermal pre-measurement treatment has been developed. Heating the semiconductor material for a prescribed time period P 1  for a prescribed temperature T 1  stabilizes the interstitials by accelerating the responsible atoms to “sinks” along the wafer surface. This pretreatment removes the instability of freshly implanted wafers and allows accurate repeatable measurements to be collected.  
         [0059]     Applicants have discovered, in the context of semiconductor defect metrology, that a low temperature thermal pre-measurement treatment, accelerates migration of interstitial defects without annealing the vacancies. As the vacancy defects are the main implant effects of interest, this discovery is important to the semiconductor industry.  
         [0060]     Thus, one aspect of the invention relates to applying heat over time to stabilize interstitial migration before initiating a semiconductor photo-voltage measurement. This general concept can be used with existing semiconductor metrology techniques. In one embodiment, the heating time P 1  for the treatment step can range from about 3 minutes to about 15 minutes. Similarly, the temperature range T 1  for heating the semiconductor wafer or wafer portion prior to surface photo-voltage measurement can range from about 70° C. to about 180° C. However, additional heating times and temperatures can be determined without undue experimentation that falls within the general scope of the invention. It is also desirable to maintain substantially uniform heating of the wafer as part of the thermal pre-treatment process. Thus, in one embodiment, variations in the temperature across the surface of the wafer at or below about 2% are desirable.  
         [0061]     In addition, various heating elements, such as hot zones, convection heaters, ROST, ovens, hot plates, fab stations and other sources of thermal energy can be used to heat the wafer prior to measurement without limitation. In one embodiment, a heating station is incorporated in an in-line measurement system for a semiconductor fabrication or metrology facility. In another embodiment, localized heating is performed using a laser or focused optical device. Localized heating embodiments allow a portion of the wafer to be heated. In turn, by measuring the localized area, the thermal treatment method of the invention can be used without heating the entire wafer.  
         [0062]     Moreover, by consistently performing a thermal pretreatment step for a set of wafers, variations in defects can be controlled for among the different wafers. This occurs because the heating step uniformly standardizes the level of interstitial migration in each wafer. As a result, only the effect of other defects, i.e. vacancies, influences the final measurement. An example of the benefits of the thermal pre-treatment can be seen by comparing  FIG. 1A  with  FIG. 1B .  
         [0063]      FIG. 1B  shows the characteristic decay for recently implanted wafers and the resultant steady state over several days. The dynamic charge is plotted against the time in days. The decay level shown in  FIG. 1B  shows the gradual change in interstitial migration. As such, the results in  FIG. 1B  are indicative of normal decay without using the disclosed thermal pre-treatment.  
         [0064]     In contrast,  FIG. 1C  is plotted on the same scale as  FIG. 1B , but reveals a very different result.  FIG. 1C  shows a plot that depicts daily repeatability measurement on a semiconductor wafer with 150° C. thermal pre-treatment before each measurement. The advantages of the disclosed method are clear as essentially zero decay is present with a, standard deviation 0.2%.  
         [0065]     The low temperature heating process disclosed herein is unrelated to high temperature annealing. High temperature annealing is performed at temperatures several times higher than the ranges disclosed herein. Further, while the disclosed pre-treatment method is used to stabilize interstitial migrations, a high temperature anneal is used to restructure the crystal lattice and remove defects or to reposition dopants. Thus, while the low temperature approach designed enhances wafer measurement, high temperature annealing is only used to permanently change the semiconductor crystal. As such, a high temperature anneal would not be used at the defect measurement stage following ion implantation.  
         [0066]     Different species exhibited different rates of decay and hence different energies. Maximum performance was obtained when the following parameters Substrate Type: Oxide thickness  30 A, Corona application for low Dose Thermal pre-treatment, Implant chuck temperature, Beam current, and Tilt Angle, were controlled. Control over these parameters allowed control of the implant process to be one percent or better, for three sigma control limits.  
         [0067]     Different species exhibited different rates of decay and hence different activation energies. This in-turn influenced the actual pre-measurement stabilization temperature used. In general, higher temperatures of approximately 150° C. are used for higher damage implants; temperatures of approximately 130° C. are used for lower damage implants. Very deep implants require higher temperatures around 180° C. Since the measurement techniques disclosed herein are repeatable, an alternative approach is to model the decay accurately using a first order rate equation and compensate the measurement through software algorithms.  
         [0068]     In brief overview, and referring to  FIG. 1D , an embodiment of such an apparatus  10  for the real-time, in-line, electrical characterization of a semiconductor during manufacturing using induced surface photovoltage includes a sensor head assembly  14 , supporting electronics  18 , and a wafer conveying device  22 . In operation, the wafer conveying device  22 , such as a conveyor belt, a robotic arm, a wafer chuck or similar device, moves wafers  28 ,  28 ′ through the manufacturing process and, in one embodiment, beneath the sensor head assembly  14 .  
         [0069]     Referring to  FIG. 2 , the sensor head assembly  14  includes a probe head  32  mounted in a bracket  36  on a motorized stage  40 . The motorized stage  40  moves the probe head  32  in a vertical direction (arrow z) to adjust vertical position of the probe head  32  with respect to the wafer  28  to within a 0.2 μm accuracy. The mechanical stage  40  is attached to a probe arm  44 .  
         [0070]     The longitudinal axis L-L′ of the probe head  32  is adjusted to be perpendicular to the plane of the wafer  28 , by adjusting the tilt of the probe arm  44 , either manually (using set screws  46 ) or mechanically (using for example piezoelectric actuators  48 ). The vertical position of the probe head  32  with respect to the wafer  28  is controlled by feedback signal from capacitive-position sensing electrodes described in detail below.  
         [0071]     Briefly, three capacitive-position sensing electrodes are located on the periphery of the sensor. To measure capacitance between each of these electrodes and the wafer, a 70 kHz 1V signal is applied through a respective 10 kohm resistor connected to each of these electrodes. The AC current flowing through these resistors is measured using a preamplifier and a lock-in amplifier. The lock-in signal is further processed by a computer and supplied to the motion control board that, in turn, positions the probe at a predetermined distance from the wafer surface using vertical (z-axis) motorized stage.  
         [0072]     Referring to  FIG. 3 , the probe head  32  includes a sensor mount assembly  50  which provides support for a sensor  54  that is connected to a preamplifier board  58  by a plurality of flexible connectors  60 . Light emitted by a light emitting diode (LED)  64  is collimated by lens  68  prior to passing through a beam splitter  72 .  
         [0073]     LED  64  is mounted on a LED driver board  74  which controls the intensity of the LED  64 , in response to a signal from a reference photodiode  78 , (through a preamplifier  79 ) at an intensity level determined by the computer  160 . Light from the LED  64  reaches the reference photodiode  78  by being partially reflected by the beam splitter  72 . The light which passes through the beam splitter  72  passes through openings  80 ,  82  in the circuit board  86  and the preamplifier board  58 , respectively, prior to passing through the sensor mount assembly  50  and impinging on the wafer  28  undergoing testing.  
         [0074]     Light reflected by the wafer  28  passes back along the light path just described before being reflected by the beam splitter  72  to a measuring photodiode  92 . The light reflected by the wafer  28 , Φ R , is used to detect edge of the wafer passing beneath the probe head  32  and trigger measurements. The reflected light is also used to measure light absorbed in the wafer  28  according to the relationship:
 
Φ=Φ 0   −ΦhdR 
 
         [0075]     where Φ 0  is the incident light which can be determined by measuring the light reflected from an aluminum mirror replacing the wafer  28 . In this way, the reflection coefficient of the wafer  28  can be determined. Although the above embodiment describes the splitting of light by a beam splitter, other embodiments are possible in which light is split using optical fibers.  
         [0076]     Referring again to  FIG. 1 , the LED  64  is controlled by signals from, and the probe head  32  returns signals to, supporting electronics  18 . The supporting electronics  18  include an oscillator  100  which supplies a 40 kHz modulation control signal  104  that is used as a reference signal by an LED control  62  to control an LED driver  63  which powers the LED  64 . Oscillator  100  also provides a reference signal  108  to a lock-in amplifier  112 . The output signals  116  from the surface photovoltage sensor and the measurement photodiode  92  (through a preamplifier  93 ) of the probe head  32  are input signals to multiplexer  120  that alternately connects each signal to the input of the lock-in amplifier  112 . The lock-in amplifier  112  demodulates the input signal and supplies the demodulated signal to another multiplexer  150 . Multiplexer  150  switches between the two input signals from lock-in amplifiers  112  and  140  connecting them to a data acquisition (DAQ) board  156  that in turn digitizes the input signals making them available for further processing in the computer  160 . In an alternate embodiment, multiplexer  150  is part of the data acquisition board  156 .  
         [0077]      FIG. 4  is a bottom perspective view showing the sensor plate of the sensor head  32 . A plurality of electrodes are formed on a rigid and insulating substrate  200 . In one embodiment, a 10 mm diameter fused quartz disc is used. A central surface photovoltage electrode  204  detects the signal from the wafer  28 . The central surface photovoltage electrode  204  is partially transmissive, thereby permitting the light from the LED or laser  64  to reach the wafer  28 . Three other electrodes  208  located on the periphery of the substrate are used both for sensing the position of the sensor head  32  above the wafer  28  and for measuring the parallelism of the sensor with respect to the surface of the wafer  28 . All electrodes  204 ,  208  are formed by the deposition of an indium-tin-oxide film through a shadow mask.  
         [0078]     Similarly, a plurality of electrodes  212 , for connecting the sensors with the preamplifier circuit board  58  through the flexible connectors  60 , are formed on the surface of the substrate  200  which is opposite the electrodes  204 ,  208 . Thin conductive electrodes  218 , on the side walls of the substrate  200 , which connect the electrodes  204 ,  208  on the first surface with their respective electrodes  212  on the second surface, are also deposited using a shadow mask. This deposition avoids the use of vias through the substrate and thereby retains the flatness of the sensor to better than 0.2 μm. Both front  204 ,  208  and side electrodes  218 , may be protected with a thin insulating coating, such as polyimide, formed by spinning so as to maintain the flatness of the sensor.  
         [0079]     The electrodes  208  are used for capacitively sensing the position of the sensor above the wafer  28 . Referring again to  FIG. 1 , a 70 kHz input signal  124  for measuring the distance from a wafer  28  is supplied by an oscillator  128  to the position electrodes  208 . The same signal is also supplied as a reference signal  132  for a lock-in amplifier  140 . A position signal  146  from each of the three position sensing electrodes  208  is supplied as the input signal to a multiplexer  148  through a preamplifier  149 . The multiplexer  148  in turn, switching between each of these signals, connects each alternately to a lock-in amplifier  140 . The demodulated output signals from the lock-in amplifiers  112  and  140  are input signals to a multiplexer  150  which connects each signal alternately to a data acquisition board  156  located in a computer  160 , including a CPU  164 . Again, in an alternative embodiment, multiplexer  150  is part of the data acquisition board  156 .  
         [0080]     The position signal  146  is compared by the CPU  164  with the reference value corresponding to a desired distance (established by calibration and stored in the computer) between the sensor  54  and the wafer  28 . The difference between these two values, corresponds to the deviation of the sensor-wafer distance from the desired value, is supplied to a motion control board  170  that positions the probe head  32  at a predetermined distance from the wafer  28  using the motorized stage  40 .  
         [0081]     In operation, when an edge of the continuously moving wafer  28  crosses the beam of the intensity modulated light from LED or laser  64 , the intensity of the reflected light increases, thereby increasing the signal from the photodiode  92 . This measurement of the reflected light is repeated and the new value compared with the previous value. The light intensity measurements are repeated until the difference between sequential values decreases to below 5% indicating that the entire light beam is within the flat portion of the wafer.  
         [0082]     This decrease in deviation triggers acquisition of the SPV signal by the surface photovoltage electrode  204 , followed by acquisition of the capacitance signals by the position electrodes  208 . If capacitance signals from different electrodes ( 208 ) differ by more than 5%, the SPV signal is stored but not recalculated. The sequence of all measurements is then repeated until capacitances from different position electrodes ( 208 ) fall within 5% limit indicating that the electrodes are not near the edge of the wafer  28 . The average of the capacitances from the three positioning electrodes  208  at this point is used to recalculate all previous values of the SPV signal.  
         [0083]     The SPV measurement cycle is repeated, sequentially measuring light intensity, SPV signal and capacitance of positioning electrodes, until capacitances from the three positioning electrodes ( 208 ) differ by more than 5%, indicating the approach of the opposite edge of the wafer  28 . After reaching this point of the wafer  28 , the SPV measurements are made using the previously measured values of capacitance. The measurements of each value (reflected light, SPV signal, capacitance), in each cycle, are repeated for 10 msec and averaged by CPU  164 .  
         [0084]     The wafer  28 , in one embodiment, is placed on the grounded chuck (conveyor belt, robotic arm, or other similar device)  178 , coated with an insulating material, that is used to carry the wafer  28  beneath, above, or otherwise, such that the surface of the sensor of the probe head  32  and the surface of the wafer are parallel. Alternatively, the conveying device may be biased by a DC voltage. In one embodiment the DC bias voltage is selected to be between −1000 and 1000 volts. Although  FIG. 1D  illustrates the use of a grounded, insulated chuck  22  to move the wafer  28  beneath the probe assembly  14 , it is possible to provide all the necessary measurements without grounding the chuck using only the electrodes provided by the sensor  54 . Referring to  FIG. 5 , the SPV signal is, as described previously, received by the central surface photovoltage electrode  204  which is connected to the input terminal of an operational amplifier  250  located on the preamplifier circuit board  58 . The other input terminal of the operational amplifier  250  is connected to ground and to the output terminal of the operational amplifier  250  through one or more resistors. What was previously a back capacitive contact, supplied by the chuck, is now provided by the three positioning electrodes  208  located on the periphery of the sensor and which, during the SPV measurements, are connected to the ground  252  rather than to the input terminal of the capacitance (current measuring) preamplifier located on the preamplifier circuit board  58 .  
         [0085]     To measure capacitance, the electrodes  208  are alternatively switched between the ground  252  and input of the capacitance preamplifier located on preamplifier circuit board  58 . This arrangement makes possible non-contact measurements with any type of wafer support. Thus, the wafer support does not need to be connected to ground and could be made of insulating material.  
         [0086]     As discussed above, measurements of the surface doping concentration require the formation of an inversion layer at the wafer surface. In one embodiment this is accomplished by charging the wafer  28  using a corona generator and subsequently performing surface photovoltage measurement on the wafer  28 . Specifically, the wafer  28  is first charged to inversion with a corona generator. N-type wafers require a negative surface charge and p-type wafers require a positive surface charge. In one embodiment, the corona generator includes a single metal tip, for example tungsten, located 5 mm above the wafer  28  and biased to 3.5 kV for 2 to 3 sec. After charging, the wafer  28  is moved beneath the probe assembly  14  and the measurements performed. After the measurement, the wafer  28  is either moved beneath a neutral charge corona generator or returned to the original corona generator operated in a neutral discharge mode in order to discharge the wafer.  
         [0087]     The simple corona generator with the metal tip or wire does not allow for the controlled charging of the wafer surface. The control of charging is important because while there is a minimum charge required to induce an inversion layer at the wafer  28  surface, overcharging may damage the wafer surface, and even cause electrical breakdown of the insulating coating formed on the wafer surface. To avoid overcharging the wafer  28 , a closed loop controlled corona charging arrangement, disclosed in  FIGS. 6A and 6B , controls the charge deposited on the surface of the wafer and thereby prevents surface damage.  
         [0088]     Referring to  FIG. 6A , the wafer  28  on the grounded, insulated chuck  22  is moved beneath an ionized air source  260  located about 10 mm above the wafer  28 . A mesh, stainless-steel, reference electrode  264  is placed in a distance of about 0.5 mm to 1 mm from the wafer  28 . The difference between the potential on the reference electrode  264 , V el , and a user defined and computer generated reference voltage, V ref ,  268 , termed the differential potential, V diff , is amplified and its polarity is reversed within the corona control module  270 . This voltage, V corr , is applied to the ionized air source  260 . Thus, the polarity of the potential applied to the ionized air source  260 , V corr , by the corona control module  270  is opposite to the polarity of differential voltage and is given by the expression:
 
 V   corr   =V   ref   −V   el 
 
         [0089]     Control of the corona charging during the charging process allows not only for real-time control but allows also simpler electronic circuitry to be used. The presence of the ions between ionized air source  260 , reference electrode  264 , and the wafer  28  lowers the equivalent impedances in the circuitry and permits amplifiers to be used (in the control module  270 ) which have an input impedance of 10 9 -10 10  ohms. This input impedance is several orders of magnitude lower than in the amplifiers utilized in previous approaches (typically 10 13 -10 15  ohms) when a potential of the wafer surface is measured not during charging but after the turning off of the corona.  
         [0090]     Referring to  FIG. 6B , the wafer  28  may be discharged by setting the reference voltage  268  to zero, i.e., connecting it to ground. Alternatively, if separate corona units are used for charging and discharging of the wafers, the discharging corona reference voltage can be permanently attached to the ground.  
         [0091]     Referring to  FIG. 7 , an alternative approach to inducing a surface inversion layer is to bias the sensor with a high voltage. Such an approach requires formation of the insulating film  230  such as polyimide over the central electrode  204  and positioning electrodes  208  of the sensor.  FIG. 8  depicts this alternative approach to inducing an inversion layer at the surface of the wafer  28  by voltage biasing.  FIG. 8  shows a schematic diagram of an electronic circuit that includes a preamplifier for measuring AC surface photovoltage and a connection to a biasing high voltage source used with the sensor having a polyimide coating  230  as just described. The insulating coating  230  of the sensor  54  allows the application of a high enough voltage (500-1000 V) to induce a surface inversion layer in typical wafers used in manufacturing. The arrangement in which a rigid sensor electrode  204  is separated by an air gap from the semiconductor surface requires high degree of flatness of the electrode surface. When such a high DC voltage is used, any edges or surface roughness will increase the local electrical field and enhance ionization of the air resulting in electrical breakdown. Therefore electrical connections between the electrode and the detection electronics are constructed so as to have a minimal effect on the surface flatness. Thus, the use of the side connections  218  eliminates the need to form via holes in the sensor and maintains the high flatness of the sensor. The current in the space charge region of the wafer  28  (indicated in phantom) which is generated by the illumination of the wafer  28  by the LED  64  is depicted as an equivalent current source, J h . An equivalent resistor, R R , which represents the carrier recombination at the surface of the wafer  28  and an equivalent capacitor, C sc , which represents the space charge capacitance are also depicted. C G  represents capacitance between the wafer  28  and the chuck  22 , while C P  represents capacitance between the sensor electrode  204  and the wafer  28 . A computer controllable high voltage  300  is applied through a 10 Mohm resistor, R HV , to the sensor electrode  204 . The sensor electrode  204  is also connected to the input of the operational amplifier  250  (described previously) through a high voltage capacitor, C HV . The capacitance, C OA  (also shown in phantom) represents input capacitance of the operational amplifier  250 . C HV  is selected to be about 10 times larger than C OA  so that C L  used in calculating IM(δV s ) and Re(δV s ) is close to C OA . Similarly R L  used in calculating IM(δV s ) and Re(δV s ) is close to R HV .  
         [0092]     In addition to the methods just described to form an inversion layer, an inversion layer at the surface of the wafer  28  can be also formed using a chemical treatment. This approach is especially useful for p-type silicon wafers. Since HF introduces positive surface charge, HF treatment will produce a negative inversion layer at the surface of p-type silicon wafers. In one embodiment, the silicon wafer to be tested is subjected to a mixture of hydrofluoric acid and water (1:100 HF:H 2 O) in a liquid or vapor form. The wafer is then placed beneath the probe assembly  14 . In number of processes, HF treatment is already part of the production sequence so that probe assembly  14  needs only to be placed after HF processing location.  
         [0093]     In another embodiment, SPV is used to measure electrical characteristics of as-implanted (before annealing) silicon wafer parameters and of implanted/annealed (after annealing) silicon wafer parameters as shown in the flowchart depicted in  FIG. 10 . The SPV measures crystal lattice damage in the as-implanted cases and measures free carrier concentration in the annealed implant state. Implant dose sensitivities, as determined by the ratio of the relative percent change in SPV signal to the percent change in implant dose/energy, in the range of 0.5 to 3.0, along with high density measurement maps, provide improved analysis of ion implant uniformity.  
         [0094]     In one illustrative embodiment a silicon wafer is presented which either has undergone ion implantation or is implanted with ions as part of the monitoring process  400 . Ion implantation may performed by doping the substrate with a dopant  402  using any common species, such as boron (B), phosphorus (P), arsenic (As), flourine (F), argon (Ar), indium, (In), or boron difluoride (BF 2 ). The doping may performed using ultra-low energy ions, such as less than 10 keV. After ion implantation, the wafer typically has ion implant dose/energy range. By way of example, low dose may be in the range of 0.1E11-5E12 ions/cm 2 ; high dose may be in the range of 5E12-2E15 ions/cm 2 ; low energy may be in the range of 0.1 keV-20 keV; and high energy may be in the range of 20 keV-10 MeV. The wafer may then be monitored either as-implanted (before annealing) or the wafer may be annealed before undergoing the monitoring process, to measure the free carrier concentration.  
         [0095]     In an illustrative embodiment, when measuring an as-implanted wafer, after the wafer has undergone the optional ion implantation  402 , an optional hydrogen fluoride wash  414 ,  416  may be applied to the wafer to remove oxide. For example, the optional hydrogen fluoride wash may be applied in cases where there is low energy/low dose. Alternatively, when the wafer is characterized as low energy/high dose or high energy/low dose, a hydrogen fluoride wash may be applied in cases wherein oxide was applied to the wafer before ion implantation. As in the illustrative embodiments described above, a corona may then be optionally applied  418 ,  420 , before the wafer is moved beneath the probe assembly  14  and measurements performed  422 . According to one illustrative embodiment, the corona is applied in cases wherein the wafer is characterized as low dose/low energy. The corona application enables the calculation of the surface recombination time, whereas if no corona is applied to the substrate, the minority and majority lifetime may be calculated. After the optional corona application, the wafer is moved beneath the probe assembly  14  and the measurements performed to assess damage, such as to the crystal lattice  422 . The electrical properties of the as-implanted wafer may be characterized by comparing the measurements taken against a known standard of typical wafer damage.  
         [0096]     If the wafer is alternatively to be measured after annealing, after the optional ion implantation  402 , the wafer is annealed  412  using any standard annealing process, such rapid thermal processing (RTP) or furnace annealing  412 . After annealing, an optional hydrogen fluoride wash may be applied to the wafer  414 ′,  416 ′. According to one illustrative embodiment, after applying the optional hydrogen fluoride wash, an inversion layer may be created on the surface of the wafer  424 . By was of example, the inversion layer may be created  426  by applying a corona of appropriate polarity, or by applying a chemical treatment as described above. After the optional creation of the inversion layer, the wafer is moved beneath the probe assembly  14  and the measurements performed  422 .  
         [0097]     In an alternative embodiment of the method for using SPV to measure electrical characteristics of as-implanted (before annealing) silicon wafer parameters and of implanted/annealed (after annealing) silicon wafer parameters as, a thin thermal oxide layer is grown on the wafer before the species is implanted at step  402 . The oxide layer may be grown on the wafer in an rapid thermal furnace, RTF. By way of example, the thickness may be between 20 and 80 angstroms, and is calibrated to insure stable surface charge on the wafer.  
         [0098]     In yet another alternative embodiment, the low and varying electron-hole lifetimes associated with ion implant damage (i.e. before annealing) influence results obtained by the standard SPV analysis. When measuring as-implanted (i.e. before annealing) wafers, an optimum frequency for implant damaged silicon wafers may be calculated by varying the frequency at which the light is modulated. In addition, the wavelength of the light used for irradiation may be optimized to account for a wide spectrum of implant conditions. Therefore it is possible to vary the wavelength of light, the light intensity, and light intensity modulation frequency to optimize the measurement of the dopant impurities. For example, the light wavelength may be selected to specifically fall within a given spectrum, such as in the infrared or ultraviolet spectrum.  
         [0099]     In this embodiment, the frequency of modulation of light intensity sweeps across a given range of frequencies while the wafer is under the probe head. An optimum frequency of modulation for an implant damaged substrate, such as silicon, can be calculated by sweeping the frequency of modulation at a selected maximum light intensity and light wavelength. A wafer with known dose and damage measurements is used as a calibration standard for the substrates being tested.  
         [0100]     By way of example, the frequency of modulation may sweep from 0.1 through 100 kHz for a given maximum light intensity and light wavelength. An SPV measurement is taken and compared to a known ion implanted substrate that has been exposed to a defined energy, beam current, and tilt angle. Thus the system can be accurately calibrated and used to produce wafers with measured contour maps to control industrial implant processes using standard statistical methods.  
         [0101]     As described above, the probe head is capacitively coupled to the wafer surface during measurement. The capacitive coupling of the probe head to the wafer limits the frequency roll of analysis rather than being dominated by a depletion region capacitance or charged defect capacitance.  
         [0102]     Using this method and apparatus, the imaginary part of the SPV signal may be measured for any given frequency.  FIG. 11  shows the equivalent circuit model  500  of the substrate typically used with ion implanted analysis. The SPV voltage  502  is measured at the surface of the wafer and allows various electrical characteristics of the wafer to be calculated. The photo-current source  514 , which is swept across a given frequency range as described above. The conductance of the quasi neutral region (G 0 )  504 , capacitance of the quasi neutral region (C 0 )  506 , recombination capacitance in the space charge region (C 1 )  508 , the recombination capacitance (C d ) in the implant damaged region  510 , the recombination conductance in the space charge region (G 1 )  512 , wafer oxide capacitance (C i )  516 , and series resistance (R s ) in the implant defect region  518  may be calculated. Furthermore, by measuring the imaginary part of the signal as a function of the light modulation frequency, a maximum at the roll-off frequency due to wafer associated capacitances may be calculated.  
         [0103]      FIG. 12  shows a typical swept frequency plot  600  for implanted silicon. The curves show two different implant conditions; lightly implanted  604  and heavily implanted  602 , as well as the curve for a standard  606 . By way of example, the light damage dose implant curve  604  may represent a species such as boron having been implanted into the wafer at 5 E12 ions/cm 2 . The high damage implanted curve  602  may represent a species such as arsenic having been implanted into the wafer at 5 E13 ions/cm 2 . The curves shift up or down relative to the SPV  608  axis depending on the type of species being plotted for a given modulation frequency sweep  610 , the wavelength of light used and maximum intensity of the modulated light. Thus light dose implant  612 , heavy dose implant  618  and standard  614  may move up the SPV axis due to the difference in the implant species, maximum intensity of light and light wavelength. The real portion of the SPV measurement enables a calculation of actual light intensity and the imaginary portion is used to measure the phase shift.  
         [0104]     The roll-off frequencies (ω) and the slopes of the curves  602  and  604  (α), can be used as calibration points. A combination of parameters can be used to establish a calibration curve for implant parameter on a multi-point curve as show in  FIG. 13 .  FIG. 13  shows an empirical calibration curve from frequency sweep curves for as-implanted silicon.  
         [0105]     One advantage of the frequency sweep analysis is that the low lifetimes associated with implanted silicon can be measured over the full range of frequencies. Generally for ultra shallow implants that are less than 5 keV, a low wavelength (e.g. wavelength 0.4 μm) ultraviolet light gives highest signal. For implants with energies greater than 5 keV (e.g. wavelength 0.375 μm), blue light is more efficient. In general the more intense light source the higher the SPV signal with implanted wafers. Longer wavelength light allows SPV measurements for amorphized surface silicon from heavy ion damage. In addition, light intensities up to those associated with laser diodes will provide improved signals.  
         [0106]     Additionally, the present apparatus is particularly adaptable for use in a sealed chamber environment, such as a reduced pressure chamber, a chamber for chemically reactive gasses or a chamber for an inert environment. The entire probe assembly  14  may be positioned within the sealed chamber, with the connections to the electronics passing through the walls of the sealed chamber through pressure fittings. Alternatively, the probe assembly may be mounted in a wall of a sealed chamber such that the sensor is positioned within the chamber but the remainder of the probe assembly is positioned outside of the sealed chamber.  
         [0107]     The approach to process monitoring methodology using an AC-SPV method emphasizes determination of variations of the measured parameters from wafer to wafer rather than value of the specific parameter itself. Typically, measurements of the electrical parameters of the back surface of the wafer are not possible without altering the front surface, which has to be contacted in order to complete a measuring circuit. Hence, measurements performed on the back surface of the wafer are not typically used in process monitoring. The non-contact AC-SPV measurements allow process monitoring by measurement of the surface characteristics on the back surface of the wafer as well as the front surface. As described before, the probe head can be installed underneath the wafer, above the wafer, or otherwise, such that the sensor surface is parallel to the wafer back surface, depending on how the wafer conveying system conveys the wafer to the probe head. In addition, two probe heads can be used, one on each side of the wafer for simultaneous characterization of the front and back side of the wafer. As an illustration of such approach comparison of measurements of the surface charge on the front surface featuring mirror-like finish is shown in  FIG. 9 . The measurements were performed on the two halves of the same 100 mm, p-type, (100) silicon wafers that were simultaneously subjected to the wet cleaning treatments. At various stages of the cleaning process, the surface charge was measured on the front (polished) surface of one half, and on the back (unpolished) surface of the other half. The results shown in  FIG. 9  indicate identical behavior of surface charge on the front and back surfaces.  
         [0108]     In addition to measuring expitaxial silicon and ion implanted silicon, the disclosed apparatus and method may be used to measure silicon on insulator (SOI) material, strained silicon films, Si—Ge films (Si—Ge) and metallic contamination. For example, SPV can be used to measure the interface charge in SOI material.  
         [0109]     Having shown the preferred embodiment, those skilled in the art will realize many variations are possible which will still be within the scope and spirit of the claimed invention. Therefore, it is the intention to limit the invention only as indicated by the scope of the following claims.