CA1301954C - Noninvasive method and apparatus for characterization of semiconductors - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method and apparatus are described for characterizing a semiconductor using the surface photovoltage (SPV) effect. A region of the surface of the semiconductor is illuminated with an intensity modulated beam of light, the wavelength of the light being shorter than that corresponding to the energy gap of the semiconductor. The surface photovoltage (SPV) induced in the semiconductor is measured under bias voltage conditions. The intensity of the light beam and the frequency of modulation are selected such that the surface photovoltage (SPV) is directly proportional to the intensity and reciprocally proportional to the frequency of modulation. Using the surface photovoltage (SPV) and the bias voltage (Vg) measurements, the charge induced in the semiconductor space charge region (Qsc) and the charge induced in the semiconductor (Qind) are determined.
This information is used to determine various parameters of the semiconductor including surface state density and oxide/insulator charge. The technique is designed especially for use in characterizing semiconductor wafers, coated or uncoated. but may, if desired, also be used in characterizing MIS or MOS type semiconductor devices. The apparatus includes a reference electrode assembly which in one embodiment comprises a button made of insulating elastomeric material and attached to a rigid plate made of insulating material.
A film made of insulating material and having a conductive coating on one side which serves as a reference electrode is attached to the button. When SPV measurements are being taken, the film is pressed against the specimen with pressure sufficient to hold the reference electrode in close compliance with the specimen, with pressure being applied to the plate from an external source and being transmitted from the rigid plate to the film through the elastomeric button.

Description

~ONINVASIVE METI~OD AND APPARATUS FOR CHARACTERIZATION
OF S~MICONDUCTORS
BACKGRoliND OF THE INVENTION
The pre~sent invention relates generally to the cha~acteri~atior.
of semiconductors and more particularly to the characterization of semiconductor materials and devices using an ac surface photovolta~e (SPV) method to determine the surface space charge capacitance. The invention is particularly useful in determining parameters sucn as the surface state density of a semiconductor and/or an oxide/insulator cllarge in a dielectric film which may be formed on a semiconductor, either naturally or intentionally (i.e. by thermal oxidation). I~ut, as will hereinafter be pointed out, may ~)e used in determinillg other parameters of a semiconductor.
As is kllown the surface state density of a semiconductor is useful, for example, in indicating the quality and contaminatio!l of a semiconductor sureace or of the interface between the semiconduc~or and an o~ide coating which may be formed on a semiconductor while the oxide/illsula~or char~e ~s useful in indlcating the quality and contaminati.on of the oxide/.tnæulator coating itself.
The surface photovoltage effect as applied to semiconductors and tech~iques for measuring the (ac) surface photovoltage so as to determille characteristics such as the surface space charge capacitance in general are well known in the art.
Known patents of interest relatlng to the surface photovoltage effect include U.S, patent 4,544,887, issued on October 1, 1985 in the name of E. Kamieniecki, which discloses a method of measuring photo-induced voltage at the surface of semiconductor materials (i.e.
the sureace photovoltage); U.S. patent 4,286,215, issued on Aug. 25, ~

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~81 in the name of G.L. Miller, which discloses a method and apparatus for t!le contactless monltoring of the carrier lifet:i!ne in semicollductor materials; U.S. patent 4,333,051, issued on June 1, 1982 in the name of A.M. Goodman, which discloses a method and apparatus for determining minority carrier diffusion lengt}l in semiconductors; ~.S. patent 4,433,288, issued on February 21, 1984 in the name of A.R. Moore, which discloses a Inethod and apparatus for determinirlg minority carrier diffusion length in semicond-lctors: and U.S. patent 4,663,526, issued on May 5, 1987 in the nanle of E.
Kamieniecki, which discloses a method and apparatus for the nondestructive readout of a latent electrostatic image formed on an insu1ating material.
Knot~n pul~llcations of interest relating to the characterization of semiconductors and/or the surface photovoltage effect in general include Elnil Kamieniecki, "Surface Photovoltage Measured Capacitance:
Apl~lica~ion To Semlconductor/Electrolyte System", J. Appl. Pnys. Vol.
54, No. 11, November. 1983, pp. 64S1-648q; Emil Kamielliecki.
"Determinatloll of surface space charge capacitance using a light probe", J. Vac. Sci. Technol,, Vol. 20, No. 3, March, 1982, pp.
811-814; Hironliclli Shimi~u, Kan~i Kinameri, Noriaki Honma and Chusuke Munakata, "Determination of Surface Charge and Interface Trap Densities in Naturally Oxidized n-Type Si Waers Using ac Surface Photovoltages", Japanese Journal of Applied Physics, Vol. 26! No. 2, February, 1987, pp. 226-230; A. Sher, Y.H. Tsuo, John A. Moriarty, W.E. Miller and R.K. Crouch, "Si and GaAs Photocapacitive MIS
Infrared Detectors", J. Appl. Phys., Vol. 51, No. 4, April 1980, pp.
2137-2148; Olof Engstrom and Annelle Carlsson, "Scanned Light Pulse gL3~S4 iechnique For the Investigation of Insulator~semiconductor Interfaces", J. Appl. Phys. Vol. 54, No. 9, September, 1983, pp.
5245-5251; E. Thorngren and 0. Engstrom, "An Apparatus for the Determination of Ion Drift in MIS Structures", J. Phys. E: Sci.
Instrum., Vol. 17, 198~, printed in Great Britain, pp. 1114-1116; E.
Kamieniecki and G. Parsons, "Characterization of Semiconductor-Rlectrolyte System by Surface Photovoltage Measured Capacitance", 164th meeting of the Electrochemical Society, Washington, D. C. October, 1983; R.~. Chang, D.L. Lile and R. Gann, "Remote Gate Capacitance-Voltage Studies for Noninvasive Surface Characterization", Appl. Phys. Lett. Vol. 51, No. 13, September, 28, 1987. pp. 987-989; Chusuke Munakata, Shigeru ~'ishimatsu, Noriaki Honma and Kunihlro Yagi, "Ac Surface Photovoltages in Strollgly-Itlverted Oxidized p-Type Silicon Wafers", Japanese Journal of Applied Physics, Vol 23, No. 11, November 1984, pp. 1451-1461;
R.S. Nakhmallson, "Frequency Dependence of the Photo-EMF of Strongly Inverted Ge and Si MIS Structures - I. 1`heory", Solid State Electronics, 1975, Vol 18, pp. 617-626, Pergamon Press, Printed in Great Britain; R.L. Streaver, J.J. Winter and P. Rothwarf, "Photovoltage Characterizatlon of MOS Capacitors", Pro. Int. Symp.
Silicon Materials Sci & Tech., Philadelphia, May 1977 (Electrochem.
Soc. Prince~on, 1977) pp. 393-400; R.S. Nakhmanson, Z. Sh. Ovsyuk and L.K. Popov, "Frequency Dependence of Photo-EMF of Strongly Inverted Ge and Si M~S Structures - II Experiments", Solid State Electronics, 1975, Vol. 18, pp. 627-634 Pergamon Press, Printed in Great Britain;
Chusuke Munakata and Shigeru Nishimatsu, "Analysis of ac Surface Photovoltages in a Depleted Oxidized p-Type Silicon Wafer", Japanese Journal of applied Physics, Vol 25, No. 6, June, 1966, pp. 807-812:

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~husuke Munakata, Mitsuo Nanba and Sunao Matsubara, "Non-Destructive Method of Observing Inhomogeneities in p-n Junctions with a Chopped Photon ~eam", Japanese Journal of Applied Physics, Vol. 20, No. 2, ~ebruary, 1981, pp. L137-L140; Chusuke Munakata and Shigeru Nishimatsu, "Analysis of ac Surface Photovoltages in a Depleted Oxidized p-Type Silicon Wafer", Japanese Journal of Applied Physics, Vol 25, No. 6, June, 1986, pp. 807-812, S.M. Sze, "MrS Diode and Charge-Coupled Device", Physlcs of Semiconductor Devices, John Wiley & sons Inc. ~ew York 1981, second edition, pp. 362-394.
The front-end of a typical semiconductor device abrication line involves numerous steps after the initial scrubbing and cleaning of the raw wafer. These steps include oxidation, deposition, masking, diffusion, and implant operations. It can take several weeks from start to finish and testing of the final product. As can be appreciated, process variations which cause yield losses that are detected only at the end of the wafer fabrication cycle are an economic disaster for mallufacturers.
This invention is concerned wlth a method and apparatus for monitoring contamination and defects of a semiconductor surface tinterface) and/or of a dielectric film which may be coating a semiconductor and/or of a device, such as a metal~oxide-semiconductor or a metal-insulator-semiconductor, which includes a layer of semiconductor material. The invention is also applicable to determining the doping type and the doping concentration of a semiconductor in the region ad~iacent to the (frcnt) surface. One of the most important applications of the techni~ue described in this invention is in connection with silicon device fabrication and in ~3~ 4 ~articular monitorillg of the o~idation processes used in the fabrication of such devices. However, the technique may also find application in monitoring of other processes such as implantation and diffusion as well as in monitoring processing of semiconductor materials other than silicon, such as eor example, gallium arsenide or mercury cadmium te]luride.
As will hereinafter be explained, the present invention acldresses the use of the (ac) surface photovoltage effect developed under certain specific conditions for the characteri~ation of the bulk and surface (interface) properties of semiconductors. The ~emiconductor specimen being examined may be bare or may be coated with single layer of dielectric material such as a native o.Yide (e.g.,Si/SiO2) or a multi-layer dielectric coating (e.g.,Si/SiO2/polyimide, Si/SiO2/Si3N~,etc.) or may be an MIS
(metal-insulator~semiconductor) or MOS (metal-o~ide-semlconductor) device. More specifically, the present invention mak~s use of the known fact that the (ac) surface photovoltage sigllal ~the voltage photo-induced at the surface of a semiconductor) when measured under certain defined conditions is proportional to the reclprocal of the semiconductor space-charge capacitance.
The defined conditions of measurement are as follows: (1) the wavelength of the illuminating light is shorter than that corresponding to the energy gap of the semiconductor materlal, (2) the light is intensity modulated with the intensity of the light and the frequency of modulation being selected such that the induced (ac) voltage signal is directly proportional to the intensity of light and ; reciprocally proportional to the frequency of modulation.

When the surface of the specimen is illuminated unifor~ly this relatiorlship maybe e~pressed as ~ v~ = ~Cl-R) ~ C
where ~V5is the surface photovoltage. C is the space charge capacitance ~ is the incident photon flux, R is the reflection coefficient of the semiconductor material, f is the modulation frequency of the light, and q is the electron charge. K is equal to 4 for squarewave modulation of light intensity and is e~ual to 2~ for sinusoidal modulation. Details on the derivation of this relationship are presented in the above noted paper by Emil Kamieniecki entitled "Determination of Surface Space Charge Capacitance Using A 1ight Probe" published in the Journal of Vacuum Science Technology, Vol. 20, No. 3, Mar. 1982, pages 811-814. If the illumination of the semiconductor surface is local and not uniform, ~Vs is determined by using the equation SVm = (~ s where Vm is the output voltage s is the area of the illuminated portion (plus diffusion) and S is the total area of the semiconductor. ~sc is then determined using the equation noted above.
~ .S. Patent ~,64~,8~7, cited above, describes two specific arrangements for measuring the photo-induced voltage at the surface Or a specimen of semiconductor material under the conditions noted a~ove, namely, (i) for a specimen of semiconductor material placed in a suitable electrolyte, and (ii) for a specimen of semiconductor material spaced from the reference electrode by an insulating medium such as a gas or a vacuum. However, each arrangement has its shortcomings. The gas or vacuum arrangement is particularly unsatisfactory because of the electrostatic force of attraction between charges induced on opposing faces of the reference electrode and the senliconductor whicll tend to deflect the semicollductor towards the reference electrode resulting in nonlinearities in the system and the generation of spurious signals while t~le electrolyte arrangement will cause changes (contamination3 in the surface being tested. U.S.
patent 4,544,887 further suggest that the surface photovoltage so determined may be used to characteri~e properties of a scmiconductor material using "conventional" capacitance analysis. However, no method, conventional or nonconventional, which can be used for actually characterizing semiconductor materials once the ~urface photovoltage has been detected using the disclosed conditions is actually described in the patent. Similar equations establishing the proportionality between the surface photovoltage and the space charge capacitance along with the relation and conditions of measurement in connection with MIS devices are found in the Sher. etc. article noted above and the Nakhamson artlcle (1975), also noted above. P.quation 16 in the Nakhamson artiole deals with the imaginary romponent of the surface photovolta~e signal, 'I`he present invention, as will hereinafer be shown, describes an arrangement for measurlng the surface photovoltage in a way which is useful for characterization of semiconductors, especially, but not limited to, semiconductors in the form of wafers and, in addition, describes in detail a method of actually determining a number of parameters of the semiconductor once the surface photovoltage is so obtained, the method for determining the parameters being different ; from conventional and known capacitance analysis techniques.
~, q ~30~54 As will hereinafter be pointed out, one of the main features of the present technique for characterizillg semiconductors by using low intensity modulated light generated surface photovoltage involves the use of the dependence of the photovoltage signal so detected on a bias voltage. Another and very important feature is tlle way in which the parame.ters of the semiconductor specimen are derived from this dependence.
Measurements of surface photovoltage (generated due to low intensity illumination) versus bias voltage, in general, are very well known. R.L Streever, J.J. Winter and ~ othwarf in an article entitled, "Photovolta~e Characterization of MOS Capacitors" published in Proc~ Int. Symp. Silicon Materials Sci. & Tech.. Philadelphia, May 1977 (Electrochem. Soc., Princeton, lg77) pp. 393-400; ~. Sher, Y.H.
Tsuo, and John A, Moriarty in an article entitled, "Si and GaAs Photocapacitive MIS Infrared Detectors" publisiled in the Journal of applied Physics Vol. 51, No. 4, April 1980, pages 2137-2148; Olof Engstrom and Annelie Carlsson in an artlcle entitled, "Scanned Light Pulse Technique for the Investigation of Insulator-semiconductor Interfaces" published ln the Journal o~ Applied Physics Vol. 54, No.
9, September 1983, pages 5245-5251; and E. Thorngren and O. Engstrom in an article, "An Apparatus for the Determination of lon Drift in MIS Structures" published in J. Phys. E: Sci. Instrum., Vol. 17, 1984, pp. 1114-1116 all disclose such measurementx.
One of the shortcomings with the systems disclosed in the above articles is that they are all limited to MIS or MOS structures. The present invention, on the other hand, is not limited to such structures but rather ls applicahle (1) to arrangements in which a .
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~IL3~335~

semiconductor wafer (eventually llaving a dielectric coatingJ and an insulator which i9 used to separate the semiconductor from a conductive electrode for SPV testing are separate elements and (2) to MIS or MOS structures, in which the insulator and the semiconductor are a unitary structure (permanently integrated). From the point of view of the system characteristic and method of characterization, the main difference between the arran~ement where the semiconductor and the insulator are a unitary structure and the arrangement where the insulator and the semiconductor are separate elements is that the insulator in the unitary structure is much thinner than the insulator in the non-unitary structure. More specifically, while the insulator thickness in MIS/MOS structures is typically around 1000A or less, the typical thickness of the separately formed insulator arrangement is typically around 10 um (about 100 times thicker). Therefore to achieve similar changes in the semiconductor space-charge region using a separately formed (thicker) insulator requires about 100 times higher bias voltage (e.g. around 500 volts as opposed to about 5 volts). Because of this much higher bias voltage, the conventional analysis teohnique used for capacitance-voltage measurements and used for ac surface photovoltage in MIS/MOS structures cannot be used when a thick insulator is being used.
The conventional approach for capacitance-voltage measurements makes use of the distribution of the bias voltage (V ) between the insulator (Vi) and the semiconductor (Vs) i.e. (Vg=Vi+Vs), to evaluate the relation between the surface potential V and the appIied voltage Vg; for conventional capacitance analysis see Chapter 7 of the book by S.M. Sze, noted above, for surface g5~

photovoltage see page 5248 in the paper by En~strom et al. noted above. With a 10 ~m thick insulating spacer (such as a sheet of Mylar~ the bias voltage V is hundreds of times higher than surface potential Vs. Consequently, an error in evaluation of the voltage drop across the insulator (Vi) due to e.g., uncertainty in the thickness of the insulating spacer and hence its capacltance Ci(Vi=Qi"d/Ci1 where Qind is the charge induced in the semiconductor) makes evaluation of tile surface potential from the applied voltage impractical.
The measurement of the surface photovoltage versus the combination of the incident light and the modulating frequency of the light is shown in U.S. Patent ~,544,887 noted above.
It is also known to determine the capacitance in a semiconductor for eharaeterization purposes by measuring AC current rather than surfaee photovoltage.
Aeeordingly, it is an ob~ect of this invention to provide a new and improved method and apparatus for characterizing semieonduetor materials (either coated with an insulator or uncoated) and semieonduotor devlees using the surfaee photovoltage effeet.
It i8 a further object of this invention to provide a method and apparatus as deseribed above whieh Lnvolves determlnlng the surfaee space charge capacitance.
It is another ob~ect of this invention to provide a method and apparatus as deserlbed above whieh ls specifically suited for use with thick insulators but which ean also be used, if desired, with thin insulators.

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It is still another ob~ject of this invention to provide a method and apparatus as described above which may be used for determining surface state (interface trap) density.
It is yet still another object of this invention to provide a method and apparatus as described above which may be used for determining the oxide/insulator charge.
It is a further object of this invention to provide a method and apparatus as described above which may be used for determining doping type.
It is another object of this invention to provide a method and apparatus as described above which may be used for determiining doping concentration.
It is still another object of this invention to provide a method and apparatus as described above which is non-invasive.
It is a further object of this invention to provide a method and apparatus for use in characterizing a semiconductor wafer.
It is a still further object of this inventlon to provide a new and improved apparatus for making ac surface photovoltage measurements of a spec.imen of semiconductor material;
It is another object of this invention to provide a new and improved apparatus for making ac surface photovoltage measurements of a specimen of semiconductor material under dc bias voltage conditions.
It is still a further object of this invention to provide a new and improved capacitive type reference electrode for use in making ac - surface pbotovoltage measurements of a specimen of semiconductor material, 13~

It is yet a further objeet of this invention to provide a new and novel reference electrode assembly which is especially constructed for use in an apparatus for making ac surface photovolta~e measurements of a specimen o~ semiconductor material under high dc bias voltage conditions.

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SUMMARY OF THE INV~.NTION
The present invention makes use of the fact that the photovoltage at the surface of a semiconductor (SPV), measured with low intensity modulated light under certain defined conditions is proportional in a known way to the reciprocal of the space-char~e capacitance C
More specifically, whell the surface of the specimen is illuminated uniformly ~Vm= ~Vs and Csc is determined by the equation s - K P Q~ sc where~Vs is the surface phot.ovoltage, Csc is the space charge capacitance, ~ is the incident photon flux, R is the reflection coefficient of the semiconductor ma~erial, f iS the modulation frequency of the li~ht, and q is the electron charge. K is equal to 4 for squarewave mo~ulation of light intensity and is equal to 2 ~for sinusoidal modula~ion. Details on the derivation of the relationship are presented in the paper by Emil Kamieniecki entitled "Determination of Surface Space Charge Capacitance Using a Light Probe" published in the Journal of Vacuum Science Technology, Vol.
20, No. 3, Mar, 1982, pages 811-81~, If the illumination of the semiconductor surface is local and not uniform,~Vs is determined by using the equatlon ~Vm= (S/S )~Vs where &V m is the output voltage is the area Oe the llluminated portion (plus diffusion) and S
is the total area of the semiconductor. Csc is then determined using the equation noted above.
According to the present invention, the space charge capacitance (Cs~) when determined under the conditions noted above is used to determine both the surface potential (Vs) and the width of the depletion layer and hence the charge induced in the semiconductor 1~

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space-charge region (Q ). The applied or bias voltage (V ) is used to evaluate the charge induced in tlle semiconductor (Qi d) Using the charge induced in the semiconductor space charge region Qsc and the charge induced in the semiconductor Qind (which is comprised o~ the changes in the semiconductor space charge and surface charge~ various parameters such as surface state density and oxide/insulator charge can be easily and reliable determined.
According to another feature of this invention there is provided an apparatus for making ac surface photovoltage measurements of a specimen of semiconductor material under dc bias voltage conditions.
The apparatus includes a flexible reference electrode assembly. In one version, the ;flexible reference electrode assembly includes a reference electrode which is a coating on a film (i.e. a flexible thin sheet) of insulating material. When SPV measurements are being made, the reference electrode is held ln close compliance with the surface of the specimen by pressure which is transmitted to the reference electrode through a fluld. ~n a modlflcation, the pressure is transmitted to the reference electrode through a fluid. In a modifications, the pressure is transmitted to the reference electrode through an elastomeric button. Several embodiments of the button are dlsclosed. In another version the flexible reference electrode assembly includes a reference electrode which is a coating ormed on a rigid plate which in turn ls mounted on a flexible supportin~
frame. When the SPV measurements are being made, the reference electrode is brought into close compliance with the specimen throu~h a magnetic field produced by an electromagnet. In another version, the flexible reference electrode assembly includes a reference electrode which is a coating formed on a rigid plate wllich in turn is mounted on a flexible supporting frame, the frame having a plurality of piezoelectric actuators. When the SPV measurements are being made, the reference electrode is m~intained at a precise distance from the specinlen and locally parallel to the specimen by selectively energizing the piezoelectric actuators.
According to anotller aspect of the invention, the reference electrode is made much smaller in size than the specimen.
According to still another aspect of the lnvention, a guard electrode is included for limiting fringing filed problems in the application o~ the bias field, for defining more clearly the area of the specimen from which the SPV signals are received and for making easier the calibration of the surface space charge capacitance relationship to the photovoltage signals detected.
Various features and advantages will appear from the description to follow. In the description, reference is made to the accompanying drawlng which eorms a part thereof, and in which i8 shown by way of illustration, specific embodiments for practicing the invention.
These embodiment~ will be described ln sufeicient detall to enable those skilled in the art to practice the invention, and it is to be understood that otiler embodiments may be utilized and that structural changes may be made without departing from the scope of the invention. The following detailed description i9, therefore, not to be taken in a limiting sense, and the scope of the present invention is best deeined by the appended claims.

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BRIEF DESCRIPTION OF THE DR~WINGS
In the ~Irawings wherein like reference numerals represent like parts.
Fig. l is a schematic of an apparat~s constructed accor(iing t:o the teachings of the presen~ invention;
Fig. 2 is a schematic of a modification of the holder portion of the apparatus shown in Fig. 1;
Fig. 3 is a schematic illustration of an apparatus for making ac surface photovoltage measurements of a specimen of semiconductor material under bias voltage conditions constructed according to the teachings o~ the present invention;
Fig. 4 is a schematic of the light source, the back electrode and the reference electrode assembly in the apparatus shown in Fig. 3;
Fig. 5 is a perspective view of the diaphragm in the reference electrode assem~ly shown in Fig. 4;
Fig. 6 is a modiflcation of the diaphragm shown in ~ig. 3.
Fig. 7 is an electrical schematic useful in understand.ing how the guard electrode and the referenoe electrode in the diaphragm shown in Fig. 5 are interconnected;
Fig. 8 is a section view of a modification of the diaphragln shown ln Fig. 5:
Fig. 9 is an exploded perspective view of the diaphragm shown in Fig. 8;
Fig. 10 is a view of a modification of the diaphragm shown in Fig. 9;
Fig. 11 is a section view of another modification of the diaphragm shown in Fig. 8;

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Fig. 12 is a schennatic illustration of a modification of the embodiment ShOWtl in Fig. ~;
Fig. 13 is an exploded perspective view of the diaphragm and the elastomeric button in the reference electrode assembly shown in Fig.
l2.
Fig. 14 is a sec-tion view of a modification of the embodiment showl1 in Fig. 12;
Fig. 15 is an exploded perspective view of the button and diaphragm portions of the reference electrode probe assembly shown in Fig. l4;
Fig. 16 is a schematic representation of another modification of the arrangement shown in Fig. 3 for picking up the SPV signals and supporting the specimen;
Fig. 17 is a schematic representation of another arrangement for illuminatit1g the specimen, picking up the SPV signals and supporting the specimen:
FiF. l8 is a schematlc representation of another arrangenlent for illumil1ating the specimen, picking up tl1e SPV signals an-l supporting the specimen;
Fig. 19 is a schematic of the reference electrode assembly in the apparatus ln Fig. 18;
Fig. 20 is a schematic representation of another arrangement for illuminating the specimen, supporting the specimen and picking up the SPV signaIs;
Fig. 21 is a schematlc representation of another version of the reference electrode assembly according to this inventiotl;

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Fig. 22 is a plan view taken from the bottom of the glass plate stlowll in Fig. 21;
Fig. 23 is a simplified perspective view of an implementation of an apparatus constructed according to this inventioll;
Fig. 24 is a side elevation view of the apparatus shown in Fig.
23.
Fig. 25 is a section view of another modification of the embodiment shown in Fig. 12;
Fig. 26 is an exploded perspective view of por-tions of the reference electrode probe assembly ~hown in Fig. 25: and Fig. 27 is a plan view taken from the bottom of the support shown in Fig. 25.
Fig. 28 is a graph of the reciprocal of the space charge capacitance (1/Csc) versus induced charge density (q/cm2), for a sample semiconductor; and Fig. 29 is a graph of surface state density versus energy in the band gap for a sample semiconductor;

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~L3q~54 DETAILED DESCRIPTION OF PREI'~RRED IMHODIMENTS
Referring now to the drawings, there is ShOWIl in E~ig. 1 a simplified schematic illustration of an apparatus for use in the characteri~ation of a semiconductor constructed according to this invention, the apparatus being identified by reference numeral ll.
Apparatus ll includes a holder 13 for holding the specimen being examined, the specimen being identified ~y reference numeral 15. For illustrative purposes, the specimen 15 u1lder examination is a wafer 17 of silicon having an oxide coating :L9 (i.e~SiO2) on its front surface.
Holder 13 includes a substrate 21 a reference electrode 23, an insulator 25, a mechanical press 27. a rubbe~ pad 29 and a back contact 31. Substrate 21 which serves as a base is made of glass or other transparent material having good optical quality. Referenee electrode 23 is a conduetive and transparel-t coating, sueh as indium-tln-oxide, whieh is formed on substrate 21 by any suitable means sueh as deposition and annealing. Insulator 25 is a sheet of mylar or other equivalent material sucll as te10n~ Rubber pad 29 is fixed to meehanieal press 27 by glue, cemellt or other suitable means. Baek eontaet 31 is a sheet o conductive material, sueh as aluminum foil and is fixedly attached rubber pad 27 by glue, cement or other suitable means.
Substrate 21 is stationary while press 27 can be mo~ed baek and forth from substrate 21 in the direction shown by arrows A.
In use, insulator 25 is placed on reference electrode 23, and speelmen l5 positioned on insulator 25 as s!lown with its oxide ~3~S~

coating 19 in contact with insulator 25. Press 2~ is then moved toward specimen 15 so that back colltac~ 31 is pressed up against the back surface of specimen 15, as showll. As can be appreciated, insula-tor 25 prevents leakage betweel-l the specimen and reference electrode 23. In addition because specimen 15 is pressed up against reference electrode 23, but separated therefrom by insulator 25, reference electrode 23 and t~le specimen 15 are maintained parallel to each other.
If the specimen to be examined is an MIS device or an MOS device instead of a wafer, then insalator 25 alld reference electrode 23 are eliminated and the specimen placed in holder 13 with the metal layer portion facing substrate 21. rn this arrangement, the metal layer portion serves as the reference electrode.
If desired, an insulator made of mylar or other dielectric material may be placed between back contact 31 and the back surface 32 of the specimen 15, for protec~lve purposes.
In the operation oi` apparatus ll. a c:ollimated beam of light emitted from a source 33 is deflected off of a scanning galvanometer 35 and brought to foous by a len~ 37 at. the front surfaoe of specimen 15. Source 33 is driven by a modula~ed power supply 38 so that the output beam that is emitted Prom so!lrce 33 is inten~ity modulated.
Source 33 may comprise a light emittillg diode and a collimator. A
variable blas vol~.age is applied to back contact 31 from a DC voltage source 39, which may be a voltage ramp generator or an amplifier with the ramp supplied by a computer 40. The ac photovoltage signal developed across the surface of specimen 15 upon illuminatiion is capacitively picked up by reference electrode 23 and fed into an ~3~ SD~

amplifier 41. The output of amp]ifier 4] is fed into a phase sensitive detector ~3 which is set up so that at deep depletion or inversion one component (i.e. the real component) of the photovoltage signal (SPV) vanishes and the other component (i.e. the imaginary component) which is phase shifted by ninety degrees relative to the impinging light peams (see E. ~amieniecki paper noted above in the journal of applied Physics date Nov. 1983) reaches a maximum. The output of phase sensitive de~ec~or 43 is fed into an analog to digital converter (A/D) 45 whose output is fed into computer 40 for processing.
The polarity (i.e. positive or negative) of the surface photovoltage signal (SPV), and especially, the imaginary component thereof, depends on the doping type (p or n) o~ the specimen and can therePore be used to determin~ the doping type of the specimen. The change in polarity of the SPV signal results from the difference in the slgn of the surface potential barrier under depletion conditions for n and p type semiconductcrs.
ln Fi~. 2 there is shown anotller embodiment of the holder portion of the apparatus, the ilolder b~ing identified by reference numeral Si. In this arrangement the specimen does not actually come into contact with the insulator scparating the specimen from the reference electrode.
Holder 51 includes a support frame 53, which defines a chamber 54 a vacuum chuck 55, an insulator 57, a reference electrode 59, a substrate 61, a plurality of position sensors 63 and a plurality of positioning devices 65.
Support frame 53 includes an opening 67 for the illuminating light beam. Vacuum chuck 55 also serves as a back contact. Specimen .

~3~ S~

as can be seen is seated on vacuum chuck 55. Insulator 57 may either be sheet of mylar or teflon which is fixed to reference electrode 59 by any sui~al)le means or an insulative coating such as a polyimide which is formed by any suitable means on reference electrode 59. An e~am~ o!` a polyimide is Pyralin made by DuPont.
Reference electrode 59 is a conductive and transp~rtnt coating which is formed on substrate 61 by any suitable means. Substrate 61 is made of transparent material such as glass. The position sensors 63 which may be capacitive or optical are attached to substrate 61 and are used to determine the geometric relation between the reference electrode 59 and the semicollductor 15 being examined. The information from sensors 63 is fed into computer 40 and used to control the positionillK devices G5 which may be dc motors or piezoelectric transla~:ors. Positioning devices 65 are fixed to substrate 61 and are useci to an~ularly move substrate 61 so as to maintain reference electrodc 59 and thé (front surface of) æemiconductor 15 ill ~arallel relat1onship.
As can be seen thtre ix a gap between insulator 57 and semiconductor 15. The gap is about 1~. Since speclmen 15 is held on chuck 55 by a vacuum it will not bend or curl as a result of any electrostatic attraction with the reference electrode 59. Also by using positioning sensors 63 and positioning devices 65 reference electrode 59 and specimen 15 can be adjusted to the desired separation and maintained parallel. In addition insulator 57 prevents leakage between reference electrode 59 and the semiconductor 15.
Chamber 54 is preferably flashed with an inert gas such as argon or nitrogen to prevent contamination of the sample.

:
..

~3~54 Apparatus a~s sllowtl ,in Fig. 1 has been constructed in the laboratory. ~ecurrent problems with the apparatus as so constructed have been caused by the conflicting needs to (i) apply sueficient pressure to hold the insulator motionless during the sweep of the bias field voltage and maintain the reference electrode in close compliance (i.e. locally parallel relationship) with the semiconductor and (li) avoid hi~h voltage break down of the insulator which may become damaged by the physical holding forces and dust particles and/or breakage or contamination of the semiconductor itself. Insulator motion during the bias sweep is caused by the electrostatic forces produced by the bias field. The forces may cause the insulator to move, changing the distance between the reference electrode and the semiconductor during the bias sweep.
Changes in the dist:ance between the reference electrode and the semicondutor will cause local variations ln the coupling capacitance ~i.e. the capacitallce between the reference electrode and the specilnen) and therefore dlsturb output photovoltage signal. Sucll changes Inay also cause Inodulation of the optical interference fringes in ~he narrow gaps between the insulator, the specimen and reference electrode. This light intensity modulation can distort the results and render it uninterpretable.

~ , . .,~., .

g5~

Referrin~ now to Fig. 3 tllere is illustrated a simplified schematic of another apparatus cons~ructed accordin~ to this invention for making ac surface photovoltage measurements of a specimen of semlconductor material. In the illustration, the soecimen of semiconductor material is identified by reference.numeral 15 and the apparatus is identified by reference numeral 73. As can be seen, in the Fi~. 3 aDparatus s~ecimen 15 ls illurninated from the back rather than the front as in the ~igure l apparatus.
S~ecimen 15 has two major surfaces 75 and 77, res~ectively, surface 75 being the surface under testing. Specimen 15 may comprise a slab of silicon in a wafer configuration. An oxide coating (not shown) may be on surface q5.
Apparatus 73 includes a back electrode 79 and a caracitive pickup type reference electrode assembly 81. Back electrode 79, which also serves as a base or support for specimen 15, is a rigid member made of a conductive metal, such as aluminum. Aæ can be seen, specimen 15 is seated on back electrode 7'~ w.ith surface 75 facing upward (at the top in Fig. 3) and surface 77 in contact with back electrode 79 (at the bottom in Fig. 3). Back electrode 79 is connected to ground.
Back electrode 79 is statlonary while reference electrode assembly 81 is movable vertically relative to back electrode 79, as shown by arrows A, so that it can be lowered into contact with specimen 15 for testing and then raised after the measurements have been taken.
Alternatively, (not shown) reference electrode assembly 81 could be stationary and back electrode 79 movable vertically relative to reference electrode assembly 81.

: 24 , Reference electrode assembly 81. wllich is also shown in ~ig, fii, includes a flat plate 83 of rigid transparent material, such as glass. An annularly shaped spacer 85 made of a ri~id material is fixedly attached to the bottom side of flat plate 83, plate 83 and spacer 85 defin.in~ a frame. A flexible diaphragm 87 is fixedly attached to the bottom side of spacer 85, the area between diaphragm 87, spacer 85 and plate 83 constitutin~ an alr tight ehamber 89.
Chamber 89 is filled with a quantity of fluid 90, sueh as air, which is under pressure. A port 91, which is formed in spaeer 85 and which is covered by a removable plug 93 permits access to chamber 89, when desired.
Diaphragm 87, which is also shown in Fig. 5, eomprises a film 95 (i.e, a thin fle~ible sheet) of flexible transparent dieleetrie material, sueh as mylar. Sheet 9S, which serves as an insulator, is smaller in si~e than specimen 15. A pair of coatings 97 and 99 of eonduetive material are formed on tlle top surfaee 100 of insulator 95 by any suitable means, such as deposit.ion. Coating 97 is circular in shape and serves as a reference eleetrode. Coating 99 surrounds eoatin~ 97 but is separa~ed ~rom eoatin~ 97 b~ an annular shaped uneoated area 101 and serves as a guard eleetrode. Both eoatings 97 and 99 may be made of aluminum. The thickneæs of eoatin~ 37 is such that it is trans~arent. Reference eleetrode 97 is much smaller than speeimen 15, As ean be seen, insulator 95 electrically separates coatings 37 and 39 from specimen 15. A conductor 97-1 is connected to eleetrode 97 and a eonductor 99-1 is eonnected to eleetrode 99.
An alternate arrangement of diaphragm 87 is shown in Fig. 6 and is idenSified by reference numeral 187. Diaphra~m 187 ineludes .. . .

reference and guard electrodes 197 and 199, respectively whi C}l are formed on sheet 95. Reference electro(ie includes a thin st:ripe 197--i which extends to the edge of diaphragm 187 and coating 199 surrounds but is spaced fronl coating 1~7. Conductor 97-1 is attached to the outer end of strip 197-1.
Referring back to Fig. 3, apparatus 73 further includes a li~ht source 103. a light source driver 105, an oscillator 107. a variable dc bias voltage source 109, an amplifier 111, a demodulator 113, an analog to digital A/D converter llS and a computer 117.
Light source 103 is fixed in the x-y (i.e. horizontal) direction relative to reference electrode assembly 81 and positioned so that the light beam is vertically aligned with reference electrode 97.
Li~ht source 103 is separate from reference electrode assembly 81, but may if desired be attached to reference electrode assembly 81 by a frame (not shown~.
In using apparatus 73, specimen 15 is plaeed on back electrode 79 as shown. Referellce electrode assembly 81 is then moved vertically down so that reference eleotrode 97 is in close proximity to specimen 15. Reference electrode assemhly ~1 is then pressed down a~ainst specimen 15 by any suitable external means (not shown) with sufficient pressure so that reference electrode 97 is in close compliance (i.e. parallel relationship) with surface 75 of specimen 15, the pressure being transmitted from plate 83 to reference eleetrode 97 through the fluid in chamber 89. S~nee reference electrode 97 is formed on the top surface of insulator 95 it does not actually come into contact with specimen bu~ rather forms a capacitance type Oe pickup. A beam of light from source 103 is then 3~5~

directed througll fle~ible refe~ence electrode assembly 81 onto front surface 75 of specimen 15 to generate the SPV signals. As can be appreciated, the light beam from sou~ce 103 will pass through reference electrode 97 since i-t is transparent. Source 103, which may be a light emitting diode, is driven by light source driver 105 which modulates the light beam that is emitted. Light source driver 105 is driven by oscillator 107. A variable dc bias voltage of from about 0 to about 500 volts is applied to reference electrodes 97 and 99 from variable dc bias voltage source 109, the voltage being applied through separation resistors 103 and 105, respectively. The ac pllotovoltage si~nals developed across the surface 75 of specimen 15 upon illumination as the bias voltage from source iO9 is varied are capacitively picked up by reference electrode 97 and fed throu~h an isolation capacitor 108 into ampliPier 111, Signals picked up by guard electrode 99 are shunted to ground. As can be appreciated, guard electrode 99 serves to avoid fringing field problems in the applicatiol) of the bias field and also serves to limit the area on surface 75 of specimen lS that provides the SPV signal to reference electrode 97. The output of amplifier 111 is t'ed into deolodulator 113. The output of demodulator 113 is fed into A/D converter 115 whose output is fed into computer 117, Bias voltage source 109 may be, for example, in the form of a voltage ramp generator or an amplifier with the ramp supplied by computer 117.
If desired, an insulator of appropriate material and thickness (not shown) may be placed between specimen lS and back electrode 79 for protective purposes.

~3~195~
An electrical schematic showing how reference electrode 97 and guard electrode 99 are interconnected is shown in Fi~. 7. As can be seen, guard electrode 99 is electrically isolated from reference electrode 97 but is at the sarne dc potential as reference electrode 97.
After the SPV measurements have bcen made, assembly 81 is raised.
Instead of having a fixed pressure inside chamber 89, a pump for selectively increasing the pressure could be connected to chamber 89. In this case, assembly 81 would be brought close to specimen '5 and the pressure increased to push di.aphragm 87 against soecimen 15 to insure uniform compliance between electrode 97 and specimen 15 and then decreased after the testing has been completed.
The pickup arran~ement shown in Fig. 3 for sensin~ the SPV
signals has many advantages which are readily apparent.
For example, by making the reference electrode a flexible rather than a rigid type of structure, the reference electrode can be bent to conform to the shape of the specimell to achieve uniform compliance with the specimen rather than having to make the specimen conform to tlle shape of the re~erence electrode. Also, by making tlle reference electrode flexible, the pressure that must be applied to obtain close compliance between the reference electrode and the specimen is less than it would be if it were rigid. Also, if the pressure that must be applied to insure compliance between the reference electrode and specimen is reduced the likelyhood of damaging the specimen and/or the insulator on whicll the reference electrode is formed will be reduoed.

.

. . .: . . ~ i^, ~3~1119~;4 By making the ~eference electrode smaller than the specimen.
several benefits are also realized. First, and most important, since the SPV xi.gnal-to-noise (S/N) ratio depends mainly on the ratio of the mutual capacitallce between tlle illuminated part of t~le specimen and the reference electrode and the total mutual capacitance hetween the specimen and the reference electrode, if the reference electrode is made smaller relati.ve to the specimen and the size of the illuminating light beam is not changed, the SPV signal to noise ratio will be made larger. I~ amnlifier 111 is a voltage amplifier then the S/N ratio will increase since the signal will increase and if amplifier 111 is a current amplifier the S/N ratio will increase since the noise will decrease. Also, if the reference electrode is reduced in size, the pressure needed to achieve compliance of the specimen to the reference electrode surface may be reduced. Also. by making the reference electrode smaller than the semiconductor, the insulator can also be made smaller and will consequently be easier to keep clean and dus~ free. Also, if the reference electrode is small relative to the specimen, then the corresponding insulator area exposed to higtl voltage stress will be reduced, thus minimizing the likelyhood of hig~h voltage breakdown. Damage and contamination of the specimen will also be reduced. Also, if the SPV signal to noise ratio is increased, then the SPV signals can be measured for less time and the insulator will be less liable to high voltage breakdown. Furthermore. by making the reference electrode small, accommodation of the apparatus to specimens of various sizes is simpler. Also, by making the insulator smaller than the specimen, then only a portion of the specimen surface will actually be touched by the insulator at any one time.

.. . . . . .

-:

~l3~ 5~

In Figs. 8 and 9 there is shown another modification of construction of diaphragm 87. the modification heing identified by reference numeral ll9. Also shown in Pig. 8 is specimen ]5. As can be seen, diaphragm 119 comprises two substantiall~ disc shaped sheets 121 and 123 of transparent, fle~ible, dielectric material instead of a single sheet as in diaphragin 87. Sheets 121 and 123 are stacked one on top of the other and fixedly secured around at their outer edges by an adhesive 125, such as glue. Each sheet 121, 123 includes a peripheral tab area. 127 and 129, respectively. Sheets 121 and 123 may be made of mylar or other equivalent material.
Sheet 121, which is the upper sheet, has a transparent conductive coating 131 on its top surface. Coating 131 is shaped to define a small circular area 133 at the center of the sheet which serves a reference electrode, a rectangu1ar area 135 on tab 12q which serves as an electrlcal con~act and a narrow strip 137 for connecting area 133 to area 135. Lower sheet 123 has a nontransparent conductive coating 139 which aovers the entire top surface, including the tab area 129, e~cept for a small circular area 141 at the center which is slightly smaller in si~,e thall area 133. Sheets 121 and 123 are stacked so that area 73 is itl registration with area 141. Conductive coating 79 on sheet 63 serves as a guard electrode. Coatings 131 and 139 may both be aluminum. Coating 131 is of appropriate thickness so as to be transparent. As can be seen, reference electrode 133 is much smaller in si2e than specimen 15.
In Fig. 10 there is shown a modification 119-1 of diaphragm 119, the difference being that in diaphragm 119-1 the stripe 131 and the tab portion have been omitted.

~3~gi54 In Fig. 11 -there is shown another modification of diaphragm 119.
the modificatiotl bein~ identified by reference slumeral 141. Also shown in ~ig. 11 is specimen 15. DiapBragm 141 differs from diaphra~m 119 in that sheet`143, which is the bottom sileet, has a circular hole 1~5 at the center rather than a circularly shaped uncoated area at the center as with sheet 133. Except for hole 1~5.
bottom sheet 143 is identical to bottom sheet 123. When diaphragm 141 is pushed in the direction of specimen 15, the center of top sheet 121 will partly extend through bottom sheet at hole 145, as shown. Diaphragm 119-1 may be modified in a similar manller.
In Fig. 12 ~llere is shown a modification of reference electrode assembly 81 whereill pressure is transmitted to the diaphra~m to maintain the reference electrode in compliance witil the specimen through an elastomeric button rather than through a fluid, the modification being identifled by reference numeral 147. Also shown in Fig. 12 is specimen 16 and li~rht source 103.
Reference electrode assenlbly 147, parts of which are also sho~n in Fi~. 13, includes a ~lat ri~id transparent plate 149, such as glass, having a transparent conductive coating 151 on its bottom sureace. Reference electrode assembly 147 also includes a flexible d.iaphragm 87. Diaphragm 8q is disposed below plate 149. A flexible and deformable button 153 is disposed between plate 149 and diaphragm 87. Button 153 includes an inner section 155 which is circular in cross-section and made of a transparent elastomer, an intermediate section 157 which is annularly shaped in cross section, surrounds inner section 155 and is made of an elastomer that is conductive and opaque and an outer section 159 which is rin~ shaped in cross section, surrounds intermediate section 157 and is made of an ,, .

~3~95~

elastomer that is insulatillg and opaque. A conductor 151-1 is connected to coati3l~ 151. Tntermediate section 157 of button 153 is in registration with reference electrode 97 on diaphl~agm 87. A
conductor 160 is connected to coating g~ on diaphragm 87. An annular shaped spacer 161 made Oe nonconductive material, such as mylar, surrounds button 153. Spacer 161 is glued to the bottom of plate 89 and diapllragm 87 is glued to the bottom surface of spacer 161.
In use, reference electrode 97 is brought into uniform compliance with specimen 15 by applying pressure to plate 149, with the pressure being transmitted from plate 149 to diaphragm 87 through button 153.
As can be seen, button 153 serves simultaneously the four functions of a) transmitting uniform pressure to diaphragm 27 with minimal displacement b) allowin~ light to be passed to transparent reference electrode 97 c) confining the modulated light to a limited area of reference electrode 97 and d) providing an easy to asseml-le and low parasitic electrical connection to reference electrode 97 from coatin~ 151.
In Fig. 14 there is shown a modification of the reference electrode assembly shown in Fig, 12, tlle modification bein~
identified by reference numeral 162. Also shown is specimen 15 and light source 103. As can be seen, modification 162 includes a button 163 having an inner section 165 which is circular in cross-section and made of a conductive elastomer and an outer section 167 which is annular in cross-section. Outer section 167 surrounds inner section 165 and is made of a transparent, insulating elastomer. Button 163 is mounted in place between plate 149 and diaphragm 87. Inner 35~

section 165 is in contact with reference electrode 97 and serves to conduct the SPV signals from reeerence electrode to coating 1~!. As can b~ a?p~ a~ l, mocli~icatio~ 2 differs from modification 147 only in the construction of tlle button. In modification 1~2 button 163 is made from only two different materials while in modification 147 button 153 is made from three different materials. A perspective view of button 163 and diaphragm 152 are also shown in F'ig. 15.
Assembly 162 is used in the same way as assembly 14q.
In Fig. 16 there is shown another arrangement for holding the specimen and maintaining uniform compliance as the SP~ signals are being made, the arrangement being identified by refere!lr.e numeral 171. Also shown ln Fig. 16 is a specimen 1S. As can be seen.
specimen 15 is seated on a back electrode 79 which is located inside a chamber 173 filled wi.th a fluid 114 such as air. An outlet llS on chamber 173 is connected to a vacuum pUlDp 177 which i9 controlled by computel~ 117. Chamber 173 includes a diaphraglll 87 which is formed as a window on chanlber 113. In use, when SPV measuremellts are to be made, a negative pressure is created inside chanlber 173 by pump 177, the negative pressure pulling diaphragm in toward specimen 15 so that reference electrode 87 is brought into contact with specimen 15 and held in uniform compliance.
In Fig. 17 there is shown another arrangement for illuminating the specimen and for picking up the SPV signals and supporting the spec.imen during the measurement process. A light source 103 and a focusing lens 181 are fixedly mounted in a frame 183. Frame 183 is fixedly positioned (by means not shown) in the Z direct.ion at a dlstance so that light from source 103 is brougl~t to focus at .

~3~9S~

specimell lS and is movable in the x and y directions (by any suitable means, not shown) so that the light spot from source 103 can be made to scan across the portion of specimen 15 ad~iacent reference electrode 97 and thus illuminate a very small area at a time.
Specimen 15 is seated on a support 79.
In Fig. 18 there is shown another arrangement for illuminating the specimen and for ~ensiny the SPV signals and supportin~ the specimen during the measurement process, the arrangemellt being identified by reference numeral 191. Sys~em 191 include a reference electrode assembly 193 whicll is also shown separately in Fig. 19. As can be seen in Fig. 19, a ~ocusing lens 195, a diapllragm 87 and a window 197 are mounted on a common frame 199 which is shaped to define an air tight chamber 201 which is under pressure. Returning back to Fig. 18,.a beam of light from a source 203 is collimated by a lens L3, deflected off a scanning mirror 205 driven by any mechallical means (not shown), passed through a pair of lens L1 and L2 which are spaced as shown, then deflected off a semitransparent mirror 207 into referellce electrode assembly 193 where it is brought to focus as a small spot on specimen 15. S.ince mirror 205 is scanning, the spot of ligllt is not stationary but rather will scan over the surface of specimen 15. Source 208, lens L3 and mirror 205 are mounted in a housill~ 209. Lenses L1 and L2 are mounted in housin~ 251. Mlrror 207 is mounted in a housing 213. Housing sections 209, 211 and 213 are fixed relative to each other. A
housing 215 which is tubular in shape and includes assembly 193.
; which functions as a microscope objective, at one end and an eyepiece 216 at the other end is movable vertically relative to housing 211 to ~3~ 5~

permit assembly ~93 to be brought into contact with specimen 15.
I,ight source 203 may comprise an LED 217 and a beam e~panrler 219.
In ~ig. 20 there is shown anotller arrangemellt for illuminati.llg the specimen along with another arrangement for sensing the SPV
signals and supporting the specimen during the measurement process.
Al`so shown is specimen 15. There is a light source assembly 221 and a reference electrode assembly 223. ~ight source assembly 221 includes an electrically shielded housing 225 having a window 227, a light source 103. and a collimating lens 229 for collimating the beam of light from source lC3 are ~ixedly mounted (by means not shown) in housing 225. Reference electrode assembly 223 includes a ring 231 made of iron or other material which is conductive and magnetic. A
glass plate 233 is glued to ring 231. Coatings defining a reference electrode 235 and a guard electrode 236 are deposited on the bottom surface of plate 233. Ring 231 is fixedly attached to a frame 237 by a flexible annularly shaped suspension 239 made o a nonmagnetic material. A sheet of flexible transparent dieLectric material 241.
such as mylar, is attached to frame 237 below plate 238.
An electromagllet 243 is dlsposed underneath a support 245.
Support 245 is made of any rigid, conductive, nonmagnetic material, such as aluminum.
Specimen 15 is seated on support 245. Conductors 235-1 and 236-1 are attached to electrode 235 and 236, respectively. In use, reference electrode assembly 223 is brought close to specimen 15.
Electromagnet 243 is then energized, creating a magnetic field which pulls ring 231 downward by magnetic attraction, carrying with it plate 233 and sheet 241. As a result, plate 233 and sheet 241 are , ~3~

pressed evenly and controllably a~ainst specimell 15. sheet 24l electrically separating plate 238 from specimen 15. If desired, a guard electrode can also be formed on plate 233.
In Fig. 22 there is shown another arrangement for illuminating the specimen and for sensing the SPV signals and supporting the specimen during the measurement process, tlle arrangement being desig~ated by reference numeral 251.
A support plate 79 is mounted on an insulator 253 which is made of rigid material such as glass. Insulator 253 is seated on the floor 255 of a gas tight enclosure 257 which is flashed periodically with an inert gas. The specimen 15 being examined is seated on support plate 79. A reference electrode assembly 259 is also disposed in enclosure 257. Reference electrode assernbly 259 includes a rigid transparent plate 261 of nonconduct:ive material such as glass which is condllctively coated on its bottom surface. The conductive coating i9 shaped, as shown in Fi~. 22, t:o define a central pickup area 263 and three edge pickup areas 265, 2¢q and 269. The coating making up area 263 is transparent. Central p.iclcup area 263 serves as a reference electrode while ed~e areas 265, 267 and 269 serve as pickups to sense the distance from the bottorn of plate 261 to speclmen 15. A sheet 271 of transparent dielectric material, such as mylar, which serves as an insulator is attached to the bottom of plate 261. Alternately, the insulator could be a coating on plate 261. Plate 261 is attached to a support 273 by a set of three piezoelectric actuators 275, 277 and 279 which are used to maintain the desired separation and parallelism be~ween reference electrode 263 and specimen 15. Support 273 is movably mounted (by means not ~301~5~L

shown) inside enclosure 257 so that refe~ellce electrode 268 can be placed over any desired area on spec.imell 15. A light source 103 for illuminating specimen 15 is fi.Yedly moullted O~l support 273 above plate 261. Light source 103 is enclosed in an electrically shielded housing 281 having a window 283 at tlle bottom. When SPV measurements are being made, insulator 271 is not in contact with specimen 15 but, rather, is spaced above specimen lS a predetermined distance.
Pickups 265, 267 and 269 are used to sense the distance between plate 261 and specimen 15 and actuators 275, 277 and 279 are used to maintain the distance and keep reference electrode 263 (locally) parallel with specimen 15.
In Figs. 23 and 24 there are showll silnp].ified representations of an apparatus 361 for implementing this invelltion. Apparatus 361 includes a turntable 363 which is driven by a motor 365. Turntable 365 is made of conductive material. A l~robe assembly 367 is attached to an arm 369 which is movable irl the .x and ~ directions. The electronics (including a computer) is disposed in a base housing 371. Probe assembly 367 may comprise referellce electrode assembly 21 and light source 103.
Referring now to Fig. 25, there is sllown another modification of the reference electrode assembly showll in Fig. 12, the modification being identified by reference numeral 461.
Reference electrode assembly 461, parts of which are also shown in Figs. 26 and 27, includes a flat plate 463 made of bakelite or other rigid nonconductive material. A hole 465 is provided in plate 403 to allow light from source ~3 to pass through to specimen 15. If plate 463 is made of a rigid nonconduct~ve material which is ~301~
transparent rather -than opaque as is the case with bakelite, then hole 465 is no-t necessary and may be omitted. A flexible and deformable button 407 is fixedly secured by glue, not shown, in a recess 469 formed on -the bottom side 471 of plate 463. Button 467 is made of a transparent insulatin~ elastomer, such as silicone rubber and includes a top 473 which is substantially circular in cross-sectional shape, a main Llody portion 475 which is in the shape of a part of an inverted pyramid and a bottom 477 which is rectangular in shape. A flat rigid disc 479 of transparent material, such as Lexan~ is positioned between plate 463 and button 467 to prevent button 467 from extendin~ into hole 465 when plate 463 is pushed downward, (by external means, not: shown) toward specimen 11 as will hereinafter be described. A filnl ~8l, made of a flexible insulating material, such as Mylar, is fixedly attached to plate 463 and to button 467 by glue (tlOt ShOWII). First and second conductive coatings 483 and 485, respectively are forlned on the top side 487 of ellm 481. First coating 483 ls transl)arent, is shaped to include an end portlon 488 which is positioned underneath the bottom 477 of button 467 and serves as a reeerence electrode. Coating 485 is spaced from coatlng 483, is sllaped to surround at least end portion 488 of coating 483 and serves as a guard electrode. ~oth coatings may be made of gold. A pair of electrical contacts 487 and 489 are press eit into openings formed in plate 463, contact ~87 being electrically coupled to coating 483 and contact 489 being electrically coupled to coating 480.
In use, refèrence electrode 483 is brought into uniform compliance with specimen 15 over the area underneath bottom side 477 .

~3~\~9~;~
! apvlying pressure to vlate a63 fro.~ an exterllal source, the pressure being transmitted from plate 4G3 to film 48~ througil sheet 419 and button 467.
The results of the surface photovoltage (SPV) .measurements for a region of specimen actually examined are presented in the graph shown in Fig. 28as a dependance of tlle reciprocal of the space-charge capacitance. 1/Csc versus the charge induced in the semiconductor specimen Qind due to the application of the exterrlal DC bias voltage (Vg).
The space charge capacitance (Cs ) is obtained using the equation ~V = ~ ) qC c 1 noted above.
Qind can be determined by measuring the capacitance, (Ci), between the specimen and the reference electrode using a conventional capacitance meter and then multiplying the capacitance (Ci) by the externally applied dc bias voltage (V ), as shown in the equation below:

Qind=C~. x Tlle induced charge (Qind) can be also determined by measuring and integrating directly the current charging specimen/reference electrode capacitance. The results shown in the graph in Fig. 2 are for a wafer of p-type silicon (Si) coated by thermal oxidation with a 250A thick layer of SiO2.
The graph in Fig. 28shows that a plo~ of 1/Csc versus Qind saturates at high 1/C values. This saturation is known to sc correspond to a minimum space-charge capacitance related to a maximum depletion width. The limitation on the maximum value of the depletion-layer width is associated with occurance of the strong ~L30~5~
.. ..
inversion at the semicotlductor surface. This effect in relation to the conventional capacitance meas-lrements is described in chapter 7 section 7.2.2 of the book "Physics of Semiconductor Devices" by S,M.
Sze (John Wiley & Sons Inc., New York 1981, second edition) and for ac surface photovoltage in tile article by R.S. Nakhmanson entitled "~requency dependence of the photo-emf of strongly inverted Ge and Si MIS structures -I. Theory" published in Solid-State Electronics Vol 18, 1975, pages 617-626. Inversion discussed in this paper was actually induced due to the built-in charge (charge in the insulator).
The method of determining of the doping concentration according to this invention will now Le explalned.
The maximum value of 1/C c is proportional to the maximum depletion layer width, Wm, by the equation (1/CSc) max = Wm/
~s where ~s is the permit:tivity of -the semiconductor. The dependence of Wm and hence (1/Csc~lllax v concentration i9 dlscussed ~os~ dleferent materials including silicon in Chapter 7 section 7.2.2 of the book "Physics of Semiconductor Devices" by S.M. Sze, John Wiley and Sons Inc., New York 1981, second edition (eq. 28) This relationshlp may therefore be used to determine doping concentra~ion (ln the region ad~acent to the surface) of the senliconductor specimen even if the specimen is coated with a dielectric film.
The method of determining the surface (interface) state density according to this invention will now be explained.
Under d,epletion conditions. a change in the dc blas voltage and hence a change of the surface potential leads not only to a chan~e in the ~3~ S~
~emiconductor space charge O (used for determination of the doping concentration) but also to a change of the surface (interface) state charge. Therefore a change of the charge induced in the semiconductor is due to the challge of the semiconductor space char~e ~Qsc and change of the surface (interface) state charge~Qss.

Qind ~Qsc+~Qss This relation may be used to determine surface state density. ~ Q can be determined from the change in the value of l/CSc. using the equation~Qsc=~Nsc dW where NSc is the average doping concentration in the space charge region (which can be determined from (l/CsC)max), ~ ~is the change in the width of the depletion layer, ~ s x ~(1/Csc). and q is an elementary charge. The chall~e of charge in the surface states~ Qs = ~Qind - ~Qsc' The surfa(:e potential, Vs, can be determined from the measured value of l/CSc using the equation: Vs= -1/2 q ~ ~Sc (IIc ~ Hence the s~lrface state density in the ran~e of surface potentials from Vsl to Vs2 differing by ~Vs is, Dit ~Qss/q ~ s The energy levels of these states can be calculated knowing tlle surface potential and the Fermi level (see e.g. Sze book noted above).
It should be noted tllat determination of the surface state density from t}le SPV method according to this invention is more accurate than from the capacitance/conductance measurements because of the substantial simplification of the equivalent circuit as discussed by Emil Kamieniecki in the article dated Nov., 1983 noted above. The interface state density for the sample plotted in ~ig. 2a is shown in Fig.~9.

The method of determinillg the oxide/insulator charge (charge in the dielectric film such as an o~ide) according to this inventio will now be explained.
In the absence of the external bias voltage, the semiconductor/interface/oxide (insulator) system is neutral. Since the reference electrode is far away its disturbance can be neglected. Therefol~e, Qox Qsc( g ss g At some bias voltage V the charge induced in the semiconductor/insulator (oxide) system is given by the eauation:

Q = r~ - O (V =O)] + Q - Q (V = 0)1 ind - sc sc g ss ss g -' where Qind~ Qsc and Qss are thé values at the bias voltage, and it is assumed that the total charge in the oxide is not changillg due to external voltage (charge may move in the insulator/oxide- mobile charge, but may not be itl jected: if it is injected from the semiconductor than this change is attributed to charging of the surface states) FL~OIII thc above equations.

Qox Qitld Qsc Qss The bias voltage can be selected for instance in such a way that Fermi Level coincldes with the middle of the band gap or mininlum ir the dens.ity of the surface state (see e.g. Sze book). This can be realized by determining sur~ace potential corresponding to the va!ue q*abs(V ) = EG/2 - EF, where EG is the bandgap of the semiconductor ~ is the absolute value of the Fermi energy related to the appropriate band edge (conduction band for n-type, ~alence band for p-type ): from tnat we can determine space charge capacitance at this surface potential using equation (Vs =-1/2q ~s x Nsc(l/Cscj2. The induced charge corresponding to this space ~30195~
charge capacitance can be determined from the measurements shown in Fig~ 2a. ~OE we assume that the surface state char~e for tine i~ermi Level coinciding with the middle of the band gap (or minimum of the surface state density) is vanishing. thell O = Qi d ~

--~s c It should be noted that oxide charge measured this way re~resents total charge in t}le oxide. This is unlike conventional capacitance methods where the measured oxide charge represents the charge located in tlle oxide region adjacent to the semiconductor. This difference between the method of this invention and the conventional capacitance methods may be of importance for determination of the total contamination of the dielectric layer (e.g.SiO2 on Si).
The embodiments of the present invention are intended to be merely exemplary and those skilled in the art shall be able -to make numerous variations and modifications to it without departin~ from the spirit of the present invention. All such variations and modificatiolls are intended to be within the scope of the present invention a~ defined in the appended claims.

,,

Claims (32)

WHAT IS CLAIMED IS:

1. A method of characterizing a semiconductor comprising:
a. illuminating a region of the surface of the semiconductor with an intensity modulated beam of light of wavelengths shorter than that of the energy gap of the semiconductor, b. applying a variable bias voltage (Vg) to the semiconductor, c. measuring the resulting surface photovoltage (SPV) induced at the region of the semiconductor illuminated by the light beam, d. intensity of the light beam and frequency of modulation being selected so that the surface photovoltage (SPV) is directly proportional to the intensity and reciprocally proportional to the frequency of modulation, e. determining the space charge capacitance (Csc) from measured surface photovoltage (SPV), f. determining the charge induced in the semiconductor space-charge region (Qsc) from the space charge capacitance (Csc), g. determining the charge induced in the semiconductor (Qind) from the bias voltage (Vg), and then h. characterizing the semiconductor using the charge induced in the semiconductor space-charge region (Qsc) and/or the charge induced in the semiconductor (Qind).

2. The method of claim 1 and wherein characterizing the semiconductor comprises determining the surface state density in a range of surface potentials from Vs1 to Vs2 differing by Vs using the equation Dit = Qss/ Vs where Qss is equal to the change in charge of the surface states and Vs is equal to the change in surface potential.

3. The method of claim I and wherein characterizing the semicoductor comprises determining the oxide/insulator charge using the equation Qox = Qind - Qsc where Qind and Qsc are the induced charge in the semiconductor, the charge in the space charge region and the charge in the surface state, respectively at the bias voltage.

4. A method of determining the surface state density of a semiconductor in a range of surface potentials Vs1 to Vs2 differing by .delta.Vs comprising:
a. illuminating a region of the surface of the semiconductor with an intensity modulated beam of light of wavelengths shorter than that of the energy gap of the semiconductor, b. applying a variable bias voltage (Vg) to the semiconductor, c. measuring the resulting surface photovoltage (SPV) induced at the region of the semiconductor illuminated by the light beam, d. the intensity of the light beam and frequency of modulation being selected so that the surface photovoltage (SPV) is directly proportional to the intensity and reciprocally proportional to the frequency of modulation, e. determining the space charge capacitance (Csc) from measured surface photovoltage (SPV), f. determining the charge induced in the semiconductor space-charge region (Qsc) from the space charge capacitance (Csc), g. determining the charge induced in the semiconductor (Qind) from the bias voltage (Vg), and then h. determining the surface stage density using the equation Dit = .DELTA.Qss/?.DELTA.Vs, where .DELTA.Qss is equal to the change in the charge of the surface state and .DELTA.Vs is equal to the change in the surface potential.

5. A method of determining the oxide/insulator charge in a surface semiconductor semiconductor comprising:
a. illuminating a region of the surface of the semiconductor with an intensity modulated beam of light of a wavelength shorter than that of the energy gap of the semiconductor, b. applying a variable bias voltage (Vg) to the semiconductor, c. measuring the resulting surface photovoltage (SPV) induced at the region of the semiconductor illuminated by the light beam, d. the intensity of the light beam and frequency of modulation being selected so that the surface photovoltage (SPV) is directly proportional to the intensity and reciprocally proportional to the frequency of modulation.

e. determining the space charge capacitance (Csc) from measured surface photovoltage (SPV), f. determining the charge induced in the semiconductor space-charge region (Qsc) from the space charge capacitance (Csc), g. determining the charge induced in the semiconductor (Qsc) from the bias voltage (V.epsilon.), and then h. determining the oxide charge using the formula Qox =
Qind - Qsc where Qind and Qsc are the induced charge in the semiconductor and the charge in the space charge region, respectively at the bias voltage.

6. A method of determining the doping type of a semiconductor comprising:
a. illuminating a region of the surface of the semiconductor with an intensity modulated beam of light of wavelengths shorter than that of the energy gap of the semiconductor, b. applying a variable bias voltage (Vg) to the semiconductor, c. measuring the resulting surface photovoltage (SPV) induced at the region of the semiconductor illuminated by the light beam, d. the intensity of the light beam and frequency of modulation being selected so that the surface photovoltage (SPV) is directly proportional to the intensity and reciprocally proportional to the frequency of modulation, and e. determining the polarity of the SPV signal with the real component at a minimum, the doping type being related to the polarity so determined.

7. The method of claim 1 and wherein the bias voltage is a stepping voltage.

8. The apparatus for use in making ac surface photovoltage measurements of a specimen of semiconductor material under variable dc bias voltage conditions, said specimen having first and second major surfaces, said apparatus comprising:
a. means for illuminating at least a portion of said first major surface of said specimen with a beam of intensity modulated light, b. a back electrode adapted to receive said second major surface of said specimen, c. a reference electrode assembly positionable in the path of said light beam in close proximity to said first major surface of said specimen, said reference electrode assembly comprising:
i. a rigid transparent plate, and ii. a flexible diaphragm mounted on said rigid transparent plate, said flexible diaphragm comprising a first sheet of transparent flexible insulating material and a coating of transparent conductive material on at least a portion of a surface of said first sheet of transparent flexible insulating material, said coating constituting a reference electrode, said first sheet of transparent flexible insulating material insulating said reference electrode from said specimen, and d. means for transmitting pressure applied to said rigid transparent plate to said flexible diaphragm so as to push said flexible diaphragm against the first major surface of the specimen and hold said reference electrode in uniform compliance with said first major surface of said specimen, and e. means for applying a variable dc bias voltage between said back electrode and said reference electrode, f. the ac surface photovoltage signals generated by said light beam appearing between said reference electrode and said back electrode.

9. The apparatus of claim 8 and wherein said first sheet of flexible transparent insulating material is smaller in size than said specimen.

10. The apparatus of claim 8 and further including a guard electrode on said first sheet of flexible transparent insulating material surrounding said reference electrode for reducing fringing effects in the bias field produced by the applied variable dc bias voltage.

11. The apparatus of claim 8 and wherein said flexible diaphragm further includes a second sheet of flexible transparent insulating material fixed to first sheet and a guard electrode on said second sheet of flexible transparent insulating material, said guard electrode reducing fringing effects in the bias field produced by the applied variable dc bias voltage.

12. The apparatus of claim 8 and further including an annular spacer disposed between said rigid transparent plate and said flexible diaphragm.

13. The apparatus of claim 8 and wherein said pressure transmitting means comprises a fluid.

14. The apparatus of claim 8 and wherein said pressure transmitting means comprises a button which is flexible and deformable.

15. The apparatus of claim 14 and wherein said button comprises a transparent inner section of elastomeric material, a conductive opaque intermediate section of elastomeric material and an insulating opaque outer section of elastomeric material, said conductive opaque intermediate section being electrically coupled to said reference electrode for transmitting SPV signals picked up by said reference electrode.

16. The apparatus of claim 14 and wherein said button comprises a conductive inner section of elastomeric material and a nonconductive transparent outer section of elastomeric material.

17. The apparatus of claim 8 and wherein said illuminating means comprises a light source, focusing means for focusing said light source on said first surface of said sample and a frame for supporting said light source and said focusing means, said frame being movable relative to said reference electrode in the x-y directions for scanning purposes and fixed in the z direction.

18. The apparatus of claim 8 and wherein said light beam is collimated and said reference electrode assembly includes a focusing lens for bringing said light beam to focus as a spot on said first surface of said specimen.

19. The apparatus of claim 18 and wherein said illuminating means further includes scanning means for causing said illuminating beam to scan said first surface of said specimen.

20. Apparatus for use in making ac surface photovoltage measurements of a specimen of semiconductor material under variable dc bias voltage conditions, said specimen having first and second major surfaces, said apparatus comprising:
a. means for illuminating at least a portion of said first major surface of said specimen with a beam of intensity modulated light, b. a back electrode adapted to receive said second major surface of said specimen, c. a flexible reference electrode assembly positionable in the path of said light beam in close proximity to said first major surface of said specimen, said reference flexible electrode assembly comprising:
i. a reference electrode, and ii. a support for supporting said reference electrode, d. means for applying a variable dc bias voltage between said back electrode and said reference electrode, e. the ac surface photovoltage signals generated by said light beam appearing between said reference electrode and said back electrode.

21. The apparatus of claim 20 and wherein said reference electrode is smaller in size than said specimen.

22. The apparatus of claim 20 and wherein said reference electrode is flexible and said support is rigid.

23. The apparatus of claim 20 and wherein said reference electrode is rigid and said support is flexible.

24. A reference electrode assembly for use in apparatus for making ac surface voltage measurements of a specimen of semiconductor material, said reference electrode assembly comprising:
i. a frame, ii. a flexible diaphragm attached to said frame, said flexible diaphragm comprising a first sheet of transparent flexible insulating material having a coating of transparent conductive material on at least a portion of one side thereof, said coating constituting a reference electrode, said frame and said diaphragm defining a chamber, and iii. means in said chamber for transmitting pressure applied to said frame to said diaphragm.

25. The reference electrode assembly of claim 24 and wherein said first sheet of flexible transparent insulating material also includes a conductive coating serving as a guard electrode.

26. The reference electrode assembly of claim 24 and wherein said flexible diaphragm further includes a second sheet of flexible transparent insulating material, said second sheet being fixed to first sheet and having therein a guard electrode.

27. The reference electrode assembly of claim 24 and wherein said pressure transmitting means is a fluid.

28, The reference electrode assembly of claim 27 and wherein said pressure transmitting means is air.

29. The reference electrode assembly of claim 24 and wherein said pressure transmitting means is an elastomeric button.

30. The reference electrode assembly of claim 29 and wherein said elastomeric button includes a conductive elastomeric inner section and a transparent insulating elastomeric outer section.

31. The reference electrode assembly of claim 30 and wherein said button includes a transparent inner elastomeric section, a conductive, opaque, elastomeric intermediate section and an insulating, opaque, elastomeric outer section.

32. The reference electrode assembly of claim 30 and further including a light source.