Method and apparatus for measuring electric characteristics of semiconductor wafers

Abstract: A method of measuring C-V characteristics of a semiconductor wafer without forming an electrode on an oxide film thereof. An electrode 201 for C-V measurement is held above a semiconductor wafer 100 across a gap Ge of 1 micrometer or less, and a total capacity including that of the gap Ge is detected. The gap Ge is measured by utilizing the tunneling effect observed in total reflection of light wave. Parallelism of the electrode 201 to the wafer is adjusted by measuring the width of the gap or measuring the capacity of the gap at three different locations on the periphery of the electrode 201.

Description: The present invention relates to an apparatus and a method for examining a surface of a semiconductor wafer by C-V curve measurement, as defined in the preamble parts of patent claims 1 and 7. Manufacture of semiconductors, especially of LSIs includes many process steps. Each process step should be executed to attain the quality of the semiconductors. Manufacturers of semiconductors therefore carry out various tests to confirm that each process step is duly executed. C-V curve measurement is one method generally used for evaluating process of semiconductor having MIS (Metal Insulator Semiconductor) structure. Figs. 1(a) through 1(c) are explanatory views showing a conventional method for obtaining a C-V curve. A semiconductor of MIS structure shown in Fig. 1(a) includes a semiconductor substrate 101, an oxide film 102 formed on the substrate, and an electrode 201 formed further on the film 102. Another electrode 202 is also formed on the opposite surface of the substrate 101. A total electrostatic capacity Ct between the two electrodes 201 and 202 is expressed by a series connection of a capacity Cs of the substrate 101 and a capacity Ci of the oxide film 102 as seen in Fig. 1(b). A solid line and a broken line in Fig. 1(c) respectively represent an ideal or calculated C-V curve and an actual or measured C-V curve. In this C-V curve measurement, a flat band voltage Vfb is first obtained according to a shift between the ideal curve and the actual curve, and the quality of the semiconductor and manufacturing process thereof are then evaluated based on the flat band voltage Vfb. The conventional C-V curve measurement thus requires the electrode 201 on the oxide film 102. The step of forming the electrode 201 on the oxide film 102, however, sometimes causes contamination and damage in the oxide film 102 with impurity ions such as sodium. As generally known, the C-V characteristics depends on the concentration of the impurity ions in the oxide film 102. It has been difficult therefore to single out a process causing poor quality of the oxide film 102 from the oxide film formation process and the electrode formation process. The process of forming an oxide film includes plural steps, such as cleaning of a semiconductor substrate, thermal oxidation, and stabilizing heat treatment of an oxide film. The adequacy of each step may be determined by evaluating the C-V curve after each step. However, evaluation of each step requires to form a test electrode on the substrate after each step; for example, C-V curve measurement after cleanup treatment of the substrate requires to form a test electrode on the cleaned substrate. According to the conventional method described above, a test electrode is required in each evaluation of electric characteristics of the semiconductor wafer. Formation of a test electrode on the semiconductor wafer is, however, both time- and labor-consuming, and moreover affects the electric characteristics of the semiconductor wafer. Another method using a C-t curve, that is, variation of electrostatic capacity over time, is sometimes used for evaluating the characteristics at the surface portion of a semiconductor wafer. This method also requires a test electrode formed on the semiconductor as is the case with the C-V measurement; namely, it has the same problems as above. In a method for measuring the semiconductor surface as known from DE-A 38 32 298, which has been used to formulate the preamble parts of patent claims 1 and 7, to determine the electric characteristics of the surface of a semiconductor body, by means of an electrode having a plane surface opposite to the surface of the semiconductor substrate, the gap between said electrode and said surface of the semiconductor substrate is kept less than 0,5 µm. The semiconductor substrate is exposed to light, intermittently or not, the wavelength thereof being specified to have constant frequency. The known method uses the fact that in consequence of the surface structure of the semiconductor wafer the gap between electrode and said surface will vary. The apparatus disclosed in EP-A 0 235 696 uses a reflecting surface located substantially parallel to a surface to be examined and having a refractive index larger than the refractive index of the gap between the reflecting surface and the surface to be examined, thereby creating geometric optical total reflection conditions, the said apparatus also using a photosensor receiving the light reflected by said reflecting surface. However, a larger range of wavelengths of light is used in contrast to using monochromatic light of a known wavelength. Therefore, the gap between the test electrode and the semiconductor wafer will not be easily determined from the reflectance or transmittance of the light beam. Especially, this apparatus requires gray scale calibration before the measurement of the spacing. Therefore, it is an object of the present invention to examine a surface of a semiconductor wafer without forming a test electrode on the semiconductor wafer while precisely determining a gap between the test electrode and the semiconductor. This object is achieved by improving the known apparatus and/or method by the features of claims 1 and 7. A gap between the test electrode and the semiconductor wafer electrically acts as a condenser. The electrostatic capacity of the gap is determined based on the width of the gap measured by the electrode supporting means. The total electric characteristics of the condenser and the semiconductor wafer are determined based on the electric characteristics between the measuring electrode and the semiconductor substrate detected by the detection means. Measurement of a gap by the electrode supporting means can be accomplished by employing the theory of the tunneling effect, which is briefly explained here but will be given in detail later. When a gap between the reflecting surface of the reflector and the surface of the semiconductor wafer is substantially the same in size as the wavelength of monochromatic light or less than the wavelength while the laser beam is reflected by the reflector upon conditions of total reflection in geometrical optics, part of the monochromatic light is not reflected from the reflecting surface but is transmitted into the semiconductor wafer through the gap. Both the intensity of the transmitted light into the semiconductor wafer and the intensity of the reflected laser beam from the reflecting surface of the reflector vary with the width of the gap. The width of the gap is thus determined by measuring the intensity of the reflected light. According to another embodiment of the invention, the apparatus further comprises: plural parallelism-adjusting electrodes formed on the reflecting surface; and parallelism adjusting means for measuring an electrostatic capacity between the semiconductor wafer and each of the plural parallelism-adjusting electrodes, and adjusting the parallelism of the reflecting surface to the surface of the semiconductor wafer on the basis of comparison among the measured electrostatic capacities. The electrostatic capacity between each of the plural parallelism-adjusting electrodes and the semiconductor wafer is inversely proportional to the width of the gap between the semiconductor wafer and each of the plural electrodes. The parallelism of the reflecting surface to the surface of the semiconductor wafer is thus adjusted by comparing the electrostatic capacities measured by the plural electrodes to one another. Alternatively, as means for maintaining the parallelism, the apparatus comprises: light generation means for generating plural monochromatic light beams incident on the reflecting surface on the condition of total reflection in terms of geometrical optics; plural photosensors for measuring the intensities of the plural reflected beams of the plural monochromatic light beams reflected by the reflecting surface; and parallelism-adjusting means for adjusting the parallelism of the reflecting surface to the surface of the semiconductor wafer on the basis of comparison among the intensities of the plural reflected beams. The intensity of light reflected by the reflecting surface depends on the width of the gap between the electrode supporting face and the surface of the semiconductor wafer. Accordingly, the parallelism of the reflecting surface to the surface of the semiconductor wafer is adjusted by comparing the intensities of plural monochromatic light beams detected by the plural photosensors with one another. The light source generates monochromatic light and preferably comprises a laser beam, and the gap is kept to be less than 1 micrometer. According to another embodiment of the invention, the apparatus further comprises: a suction pump; and a conductive suction table on which the semiconductor wafer is sucked by the suction pump to be fixed; and wherein the measuring means comprises means for measuring an electric characteristic between the test electrode and the conductive suction table to obtain the electric characteristic between the test electrode and the semiconductor wafer. In the preferred embodiments, the electric characteristic is a dependance of an electrostatic capacity on a voltage between the test electrode and a substrate of the semiconductor wafer. These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 (a) to (c) are explanatory views showing a conventional method for measuring C-V curves. Figs. 2(a) to (c) are explanatory views showing a method of measuring electric characteristics of semiconductors according to the invention; Fig. 3 is a diagram showing transmittance of light in a double layered structure; Fig. 4 is a diagram showing transmittance of light in triple layered structure ; Figs. 5A through 5C are graphs showing the relationship between the width of the gap and the reflectance; Figs. 6A through 6C are graphs showing the relationship between the width of the gap and the transmittance; Figs. 7A, 7B, 8A, and 8B are graphs showing the relationship between the width of the gap and the transmittance under other conditions; Fig. 9 is a schematic block diagram showing the structure of an apparatus for measuring electric characteristics embodying the invention; Fig. 10 illustrates a sample table; Fig. 11(a) is an enlarged front view illustrating a prism; Fig. 11(b) is an enlarged bottom view illustrating the prism of Fig. 11(a); Fig. 12 is a graph showing an example of a C-V curve; Figs. 13A and 13B are graphs showing ideal C-V curves with varying width of the gap; Fig. 14 is a schematic block diagram showing the structure of an apparatus for measuring electric characteristics as a second embodiment of the invention; Fig. 15 is a view showing an arrangement of electrodes; Fig. 16 is a view showing another arrangement of electrodes; Figs. 17 (a) to (c) and 18 (a) and (b) schematically illustrate the relationship between the slope of the bottom surface of the prism and the capacity of each electrode for parallelism adjustment; Fig. 19 is a view showing a further arrangement of electrodes; and Fig. 20 is a bottom view illustrating a prism in a third embodiment. DESCRIPTION OF THE PREFERRED EMBODIMENT A. Outline of Measurement Fig. 2 is a schematic view illustrating the outline of the electric characteristics measurement according to the present invention. As shown in Fig. 2(a), an oxide film 102 is formed on one surface of a semiconductor substrate 101 and an electrode 202 is formed on the other surface of the substrate 101. A test electrode 201 is held above the oxide film 102 across a gap Ge by an electrode supporting unit 300. The gap Ge between the oxide film 102 and the test electrode 201 is controlled to be no more than about one micrometer by the electrode supporting unit 300 as described later in detail. An electrostatic capacity Ct between the two electrodes 201 and 202 is expressed, as shown in Fig. 2(b), by a series connection of a capacity Cs of the substrate 101, a capacity Ci of the oxide film 102, and a capacity Cg of the gap Ge as follows: 1/Ct = 1/Cs + 1/Ci + 1/Cg The total electrostatic capacity Ct is measured by a measuring unit 400. The C-V curve to be measured here is the voltage dependence of a resultant capacity Cta which consists of the capacities Cs and Ci. The resultant capacity Cta is expressed as follows: 1/Cta = 1/Cs + 1/Ci The resultant capacity Cta of Equation (2) is obtained by measuring the total capacity Ct of Equation (1) because the capacity Cg of the gap Ge can be calculated. Here an extremely small value for the capacity Cg of the gap Ge makes the result of the capacity Cta inaccurate as clearly shown by Equation (1). The capacity Cg of the gap Ge is generally expressed as follows: Cg = ε 0 *S/Ge where ε 0 denotes a dielectric constant of vacuum, S denotes an area of the lower face of the electrode 201, and the symbol * denotes multiplication. The capacity Cg is inversely proportional to the gap Ge; that is, a larger gap Ge makes the capacity Cg smaller. When the gap Ge is greater than about one micrometer, the capacity Cg becomes far smaller than the other capacities Cs and Ci, and this makes the total capacity Ct substantially equal to the capacity Cg of the gap Ge. Namely, when Cg << Cs and Ci, Equation (1) im rewritten as: C t =C g In the apparatus of the invention, control of the gap Ge by the electrode supporting unit 300 prevents the capacity Cg from being too small. This ensures measurement of a desired C-V curve shown by broken line in Fig. 2(c), and allows accurate evaluation of electric characteristics of semiconductors. B. Principle of gap meamurement The gap Ge is measured by employing the theory of the tunneling effect observed in total reflection of light at a boundary of media . The tunnel effect is generally observed in physical phenomena expressed by a wave function, including reflection of light. More specifically, when boundary conditions are provided on the boundary plane of a medium, the wave function exponentially damps outside the boundary plane. This means that the wave oozes or tunnels from the boundary plane as far as its wavelength. When light is incident on an optically thin medium, e.g., air, from an optically dense medium e.g., glass, at an incident angle greater than its critical angle, total reflection in terms of geometrical optics is observed. When the propagation of light (or electric field) is expressed by a wave function, the wave function representing the light (or the electric field) is transmitted into the optically thin medium (or the air) in spite of the total reflection condition in terms of geometrical optics. Fig.3 is a diagram showing the optical path of light transmitted from an optically dense medium M1 to an optically thin medium M2. Incident light Li is linearly polarized light having a polarization direction in the x-y plane. In other words, its electrostatic vector is parallel with the x-y plane. The incident light Li is incident on a boundary between the media M1 and M2 at an incident angle 1. Refractive indices of the medium M1 and the medium M2 are n1 and n2, respectively. Fig. 3 shows light Lt oozing into the medium M2 at an angle 2 as well as reflected light Lr although this figure satisfies the total reflection condition in terms of geometrical optics. The electric field Et of the through-light Lt is expressed by the following wave function: Et = E0 * exp(-kt*α*y) exp[i*{kt(n1/n2)sin1*x-ω*t}] where E0 is a constant; kt and α are constants expressed by the following equations (6) and (7), respectively; ω represents angular frequency of the light; and the operator "*" denotes multiplication. kt = n2 * (2 * π) /λ α = [(n1/n2) 2 * (sin1) 2 - 1] 0.5 where λ denotes a wavelength of the light. In equation (5), the term exp(-kt*α*y) shows that the electric field of the light wave is transmitted into the medium M2 and the amplitude of the electric field |Et| exponentially and monotonously decreases in a distance as far as the wavelength λ. Equations used here are equivalent to the equations shown in Fineman, Rayton, and Sanz, "Fineman Physics", Vol. IV, Electromagnetic Wave and Properties (Japanese version); Iwanami, March, 1986; page 196, line 20. If the medium M2 is air, n2 is equal to 1.0 and the equations (5) through (7) are rewritten as the following equations (5a) through (7a): Et = E0 * exp(-kt*α*y) exp[i*{kt*n1*sin1*x-ω*t}] kt = (2π) / λ α = [(n1*sin1) 2 -1] 0.5 Suppose that, from the state of Fig. 4, another optically dense medium M3 approaches from in the air M2 down to the medium M1, as shown in Fig. 4. When a gap G between the medium M1 and the third medium M3 decreases to be approximately equal to the wavelength λ, dipole moment of the third medium M3 is excited and the light wave begins to be transmitted into the third medium M3. The electric field of the light Lt transmitted into the third medium M3 is approximated by the equations (5a) through (7a) stated above. Here the intensity of the transmitted light Lt is known to be proportional to the square of the amplitude |Et| of the electric field. The gap G can be found from the relation between the intensity of the transmitted light Lt and the gap G according to the equations (5a) through (7a); namely, the gap G can be measured by calculating in advance the relation between the intensity of the transmitted light Lt and the gap G, and by substituting the observed intensity of the transmitted light Lt in the relation. For more precise calculation of the electric field Et of the transmitted light Lt, Maxwell's equations, instead of the equations (5a) through (7a), will be required to be solved on the boundary conditions for the three layer structure consisting of the three media M1, M2, and M3. The calculation can be executed with a computer; a computer program applicable to the calculation is, for example, shown in Tohru Kusakawa, "Lens Optics", Tokai university Press, 1988, pages 295-310. Practically, the intensity of the reflected light Lr is measured instead of the intensity of the light Lt transmitted into the optically dense medium M3 on account of the difficulty in measurement of the latter. The relation among the intensities Pi, Pr, and Pt of the incident light Li, the reflected light Lr, and the transmitted light Lt, respectively, is expressed as follows: Pt / Pi = 1 - Pr / Pi Table 1 shows the reflectance Rr (= Pr/Pi) of the reflected light Lr given by the precise calculation of the electric field of the light wave propagated through the three layer structure. Results in Table 1 are for s-polarized light, p-polarized light and linearly polarized light having a polarization direction at an angle of 45 degrees to each of s- and p-polarized light (hereinafter referred to as s+p polarized light). Gap G (µm) Reflectance Rr (=Pr/Pi) s-polarized p-polarized s+p polarized 0.0 0.285 0.081 0.183 0.1 0.508 0.152 0.330 0.2 0.758 0.325 0.542 0.3 0.881 0.524 0.702 0.4 0.939 0.691 0.815 0.5 0.967 0.810 0.889 0.6 0.982 0.887 0.935 0.7 0.990 0.934 0.962 0.8 0.994 0.962 0.978 0.9 0.997 0.979 0.988 1.0 0.998 0.988 0.993 Conditions for Table 1 are as follows: n1 = 1.5103, k1 = 0.0 (borosilicate glass, BK7) n2 = 1.0, k2 = 0.0 (air) n3 = 3.673, k3 = 0.005 (silicon) λ = 0.827 µm (GaAlAs laser) 1 = 45 degrees where k1, k2, and k3 denote extinction coefficients of the media M1 (BK7), M2 (air), and M3 (silicon), respectively. Any linearly polarized light can be separated into an s-polarized light component and a p-polarized light component. When the polarization direction of a linearly polarized light deviates at an angle of  to that of the s-polarized light, the reflectance Rra of the linearly polarized light is a function of the reflectance Rrs of the s-polarized light and the reflectance Rrp of the p-polarized light as given below: Rra = cos 2  * Rrs + sin 2  * Rrp Figs. 5A through 5C are graphs showing the relations in Table 1. In these graphs, the abscissa is the gap G and the ordinate is the reflectance Rr. When the gap G is far larger than the wavelength λ, the reflectance Rr is approximately 100%, which means total reflection. When the gap G is equal to zero, on the other hand, the reflectance Rr is close to zero, which means that almost all of light is transmitted. As equation (8) clearly shows, the transmittance Rt (= Pt/Pi) is equal to (1 - Rr). Figs. 6A through 6C are graphs showing the transmittance Rt plotted against the gap G. The ordinate scale for the transmittance Rt is logarithmic. The transmittance curve log(Rt) can be approximated by a straight line except for a range of the gap G near zero. Broken lines in Figs. 6A through 6C show approximations in the range of the gap G between 0.2 and 1.0 µm and are respectively expressed as follows: log Rt = -2.642G - 0.135 log Rt = -2.216G - 0.345 log Rt = -2.291G - 0.165 Figs. 7A and 7B show variation of the transmittance Rt of laser beam of another wavelength in the three layered structure of BK7/air/silicon. Figs. 8A and 8B show vaxiations of the transmittance Rt in a four layered structure of BK7/air/silicon dioxide film/silicon. The constituents are shown in the order of midiums M1/M2/M3(/M4). A thickness Ti of the silicon dioxide is equal to 100 nm (1,000 angstrom). The transmittance of (s+p)-polarized light is omitted in the figures because it can be calculated from those of s-polarized light and p-polarized light. The reflectance and transmittance for the four layered structure of Figs. 8A and 8B are obtained according to Maxwell's equation in the same manner as the three layered structure. The transmittance curve of "log Rt" plotted against the gap G is approximated by a straight line irrespective of the wavelength of laser beam and of difference in material of the third medium M3. As described above, the gap G is measured by firstly measuring the reflectance of laser beam for the multi-layer structure, and by secondly substituting the reflectance in the relation between the gap G and the reflectance or the transmittance calculated in advance. C. Structure of Apparatus Fig. 9 is a schematic block diagram showing the structure of an apparatus MD1 for measuring electric characteristics of a semiconductor wafer including C-V curve. The apparatus MD1 includes a device for measuring a gap between the test electrode 201 and the surface of the semiconductor wafer according to the above theory. The apparatus MD1 includes a base 1, a piezoelectric actuator 2 attached to the lower face of the base 1, and a trapezoid stand 3 further mounted on the lower end of the piezoelectric actuator 2. Two slant faces of the stand 3 are formed perpendicular to each other. A prism 4 is further mounted on the lower end of the stand 3. A laser source 5, for example, GaAlAs laser, is fixed to an end of one slant face of the stand 3, and a photosensor 6 such as a photodiode to an opposite end of the other slant face thereof. The prism 4 is made of borosilicate glass (BK7) and has a bottom face 4a located in parallel with the surface of a sample table 7 (or x-y plane), on which a semiconductor wafer 100 is placed. The prism 4 has an incident face 4b and an exit face 4c which are perpendicular to each other and arranged to have an angle of 45 degrees to the bottom face 4a. The semiconductor wafer 100 on the sample table 7 is held below the prism 4 across a gap G in such a manner that its surface 100a is substantially in parallel with the bottom face 4a of the prism 4. A position controller 11 is connected to the piezoelectric actuator 2. An intensity measuring device 12 is connected to the photosensor 6, and an LCR meter 13 is connected to the test electrode 201 and the metal table 7. The LCR meter 13 measures the total capacity Ct and conductance between the test electrode 201 and the table 7. The position controller 11, the intensity measuring device 12, and the LCR meter 13 are connected to a computer 14 such as a personal computer, which controls the whole apparatus and processes measured data. The electrode supporting unit 300 in Fig. 2 is implemented by the combination of the base 1, the piezoelectric actuator 2, the stand 3, the prism 4, the laser source 5, the photosensor 6, the position controller 11, the intensity measuring device 12, and the computer 14. The LCR meter 13 and the computer 14 constitute the measuring unit 400 in Fig. 2. The piezoelectric actuator 2 has a piezoelectric element extensible and contractible in z direction, and moves the stand 3 in z direction in response to the voltage applied by the position controller 11. The sample table 7 is made of a metal such as SUS. A semiconductor wafer 100 is sucked on the sample table 7 by a vacuum pump 8. Fig. 10(a) is a plan view of the sample table 7 and Fig. 10(b) is a cross sectional view thereof taken on the line A-A. A groove 7a formed on the surface of the table 7 includes two concentric parts and plural radial linear parts connecting the circles to each other. The center of the groove 7a is connected to a through hole 7b which runs through the sample table 7 and is connected with the vacuum pump 8. When the groove 7a is evacuated by the vacuum pump 8, the semiconductor wafer 100 shown by one-dot chain line in Fig. 10 is drawn onto the sample table 7 with suction. The contact between the semiconductor wafer 100 and the sample table 7 is ensured by mirror like finishing of the surface of the table 7 and plural O-rings (shown by broken line) attached to the side of the table 7. An electrostatic capacity Cb between the semiconductor wafer 100 and the sample table 7 is part of the total capacity Ct measured by the LCR meter 13. Accordingly, Equation (1) is rewritten as: 1/Ct =1/Cs + 1/Ci + 1/Cg + 1/Cb The capacity Cb is required to be sufficiently larger than the other three capacities Cs, Ci, and Cg in the left hand side of Equation (11) to obtain a C-V curve as shown in Fig. 2(c). Here the total capacity Ct is approximated by Equation (1). In the embodiment, the capacity Cb is maintained large by the contact of the semiconductor wafer 100 onto the sample table 7. Ohmic contact of the lower face of the semiconductor wafer 100 with the upper face of the table 7 is preferable in getting a C-V curve. The ohmic contact eliminates the need for the capacity Cb to be taken into account for the total capacity Ct. However, the ohmic contact requires an ohmic electrode formed on the lower face of the semiconductor wafer 100. According to the suction contact by the vacuum pump 8, on the other hand, the semiconductor wafer 100 is sucked to the sample table 7 and thus the capacity Cb therebetween becomes negligible in the total capacity Ct. The suction contact does not require the formation of an ohmic electrode and enables to readily measure electric characteristics of semiconductor wafers at higher speeds. Figs. 11(a) and 11(b) are an enlarged front view and an enlarged bottom view of the prism 4. A circular test electrode 201 is formed on the bottom face 4a of the prism 4. A reflecting area RA surrounded by the test electrode 201 is part of the bottom face 4a and reflects laser beams emitted from the laser source 5. The electrode 201 having a double layered structure of chromium (Cr) and gold (Au) is formed on the bottom face 4a by vacuum evaporation in the following steps. A photo resist is firstly applied on the bottom face 4a to form a resist pattern with an opening for the electrode 201. Chromium and gold are successively vacuum-evaporated to form a double layered structure, and the resist pattern is then peeled off. A preferable thickness Te of the electrode 201 ranges between 100 and 20 nm (1,000 and 2,000 angstrom). The thickness Te is measured with an interference microscope prior to attachment of the prism 4 to the stand 3. As seen in Fig. 9, a laser beam Li emitted from the laser source 5 enters the incident face 4b of the prism 4 at an incident angle of 90 degrees and then incident on the reflecting area RA of Fig. 11 on the bottom face 4a at an incident angle of 45 degrees. Since the critical angle of incidence is 41.5 degrees under the conditions of Table 1, all conditions required for total reflection in terms of geometrical optics are fulfilled. The reflected light Lr perpendicularly passing through the exit face 4c is received by the photosensor 6 and measured by the intensity measuring device 12. The computer 14 calculates the transmittance Rt, the gaps G and Ge, and the electrostatic capacity based on the quantity of the reflected light Lr measured by the intensity measuring device 12. D. Details of gap measurement and detection of electric characteristics of semiconductors The gap G between the bottom face 4a of the prism 4 and the surface of the semiconductor wafer 100 is measured in the following steps. First, the gap G is held to be several times as large as the wavelength λ of the laser beam. In this state, the laser beam Li is emitted to be reflected by the prism 4. The intensity measuring device 12 detects the intensity of the reflected light Lr received by the photosensor 6. The laser beam Li is not transmitted through the gap G of the width several times the wavelength λ, but is totally reflected by the bottom surface 4a of the prism 4. The intensity of the reflected light Lr thus detected by the photosensor 6 represents an intensity Pi of the incident light Li. The intensity Pi of the incident light Li is thus measured. Second, the position controller 11 drives the piezoelectric actuator 2 to move the stand 3 in (-z) direction to reduce the gap G to a value of about the wavelength λ. The laser beam Li is again emitted, and the intensity Pr of the reflected light Lr is detected by the photosensor 6. Here part of the laser beam Li is transmitted into the semiconductor wafer 100. The computer 14 calculates the reflectance Rr and the transmittance Rt (= 1-Rr). The reflectance Rr is calculated by dividing the intensity Pr of the reflected light Lr by the intensity Pi of the incident light Li. When the incident light Li is s-polarized light, the gap G is determined based on the relationship between the transmittance Rt and the gap G shown in Fig. 7A or Fig. 8A. The gap G can be determined by referring to a look-up table and by interpolating the data if necessary, which look-up table represents the relationship shown in Fig. 7A, 7A, 8A or 8B and is memorized in the computer 14 in advance. Alternatively, the gap G can be determined based on the relationships between the gap G and the transmittance Rt expressed by Equations (10a) through (10c) stored in the computer 14. Other functions representing relationships between the transmittance Rt or reflectance Rr and the gap G can be used for Equations (10a) through (10c); for example, higher order, logarithmic, and exponential functions. The computer 14 calculates the C-V curve in the following steps. The gap Ge between the lower surface of the electrode 201 and the upper surface of the semiconductor wafer 100 is calculated by: Ge = G - Te where Te is a thickness of the electrode 201 from the bottom surface 4a of the prism 4. In order to eliminate the calculation of Equation (12), the electrode 201 can be fully embedded in the prism 4 so that the bottom surface of the electrode 201 becomes even with that of the prism 4; this makes the value Te qual to zero. The electrostatic capacity Cg of the gap Ge is calculated by Equation (3). The total capacity Ct against the voltage is measured with the LCR meter 13. The voltage dependence of the resultant capacity Cta of the capacities Cs and Ci is obtained by subtracting the capacity Cg of the gap Ge from the total capacity Ct measured. Fig. 12 shows an example of such C-V curves obtained according to the high frequency measurement under the following conditions: the gap Ge is equal to 140 nm (1,400 angstrom), and the frequency of voltage applied is 100 kHz. Here, a sample wafer includes the oxide film 102 of 14 nm (140 angstrom) in thickness formed on a p-type semiconductor substrate 101. The quality of the oxide film 101 and Its manufacturing process are then evaluated based on the C-V, curve in the same manner as the conventional method. Figs. 13A and 13B are graphs showing the C-V curves in the cases that the gap Ge varies from 100 nm (1,000 angstrom) to 2 micrometer. Figs. 13A and 13B show C-V curves for the samples of a n-type semiconductor substrate without an oxide film 102 and with an oxide film 102 of 100 nm (1,000 angstrom) in thickness Ti, respectively. Plateau ranges are observed in both Figs. 13A and 13B, in which the capacity is kept constant while the voltage is changing; the capacity at the plateau range is known to be equal to the so-called inversion capacity. Since the C-V curves for the gap Ge of greater than 1 micrometer show very small changes, the flat band voltage is difficult to measure for those gaps. Accordingly, the preferable width of the gap Ge between the electrode 201 and the semiconductor wafer 100 is no more than 1 micrometer. E. Other Embodiments Fig. 14 is a block diagram showing the structure of an apparatus MD2 for measuring electric characteristics as a second embodiment of the invention. The apparatus MD2 includes a prism 41, the laser source 5, and the photosensor 6 fixed onto the stand 3. The sample table 7 is supported by three piezoelectric actuators 21 through 23 erecting on an x-y table 31. The x-y table 31 is supported by a vertical column 34 erecting on a base 33. The x-y table 31 is driven by an x-axis driving motor 32x and a y-axis driving motor 32y to move in the x-y plane. The x-y table 31 is further driven by a z-axis driving motor 32z to move in z-direction. The sample table 7 is brought close to the prism 41 in the following manner. First, the sample table 7 is lifted by the z-axis driving motor 32z to the height of an initial positioning sensor 35. The height of the table 7 is then finely adjusted with the piezoelectric actuators 21 through 23. Here the piezoelectric actuators 21 through 23 and the motors 32x, 32y, and 32z are all controlled by the position controller 11. Fig. 15 is a bottom view showing the prism 41. A test electrode 201, and three electrodes 111 through 113 for adjusting parallelism are formed on the bottom face of the prism 41. Wires 201a, 111a, 112a, and 113a are respectively connected to the electrodes 201, 111, 112, and 113. The piezoelectric actuators 21 through 23 are installed at the exterior of the electrodes 111 through 113 as shown by broken line in Fig. 15. The piezoelectric actuators 21 through 23 are separately driven by the position controller 11 to adjust the parallelism of the surface of a semiconductor wafer 100 to the surface of the electrode 201. The parallelism is maintained by the three piezoelectric actuators 21 through 23, the position controller 11, the LCR meter 13, and the computer 14. The electrodes 111 through 113 for adjusting parallelism are in the shape of identical arcs as seen in Fig. 15. Although circular electrodes 111 to 113 shown in Fig. 16 may also be used, the arc electrodes are preferable because of their area efficiency. Fig. 17 shows the relationship between the inclination of the bottom surface of the prism 41 to the wafer and the capacity Ce of the electrodes 111, 112, and 113. The dimensions of the electrodes 111 through 113 in Fig. 17(a) are as follows: Inner diameter r0 = 0.08cm; Outer diameter r1 = 0.12cm; and Space between electrodes Δg = 0.07cm. Fig. 17(b) shows the capacity Ce of the electrodes 111 through 113 plotted against the inclination of the bottom face of the prism 41 round an axis α of plane symmetry between the electrodes 112 and 113 shown in Figs. 16(a) and 16(c). When the prism 41 is inclined by an angle , the electrodes 111 through 113 are deviated from the position in parallel with the surface of the semiconductor wafer 100, as shown in Fig. 17(c). The deviation Δd of the end of the electrodes 111, 112, and 113 from the parallel position is plotted as abscissa in Fig. 17(b). The capacity Ce of the electrodes 111 through 113 are measured with the LCR meter 13. The precision of capacity measurement with the LCR meter 13 and the computer 14 can be easily maintained as high as 0.1pF. As shown in Fig. 17(b), when the precision of the measurement is 0.1pF, the parallelism is adjusted to have the deviation Δd of at most 0.01 micrometer. When the deviation Δd is 0.01 micrometer, the angle of the inclination  of the prism 41 is approximately 0.0005 degrees and negligibly small. Fig. 18 shows the relationship between the deviation Δd and the capacity Ce of the electrodes 111 through 113, where the prism 41 is inclined round another axis β perpendicular to the axis α. When the precision of the measurement is 0.1pF, the parallelism is also adjusted to have the deviation Δd of at most 0.01 micrometer as is the case with Fig. 17(b). As described above, parallelism of the bottom of the prism 41 (and hence the surface of the test electrode 201) to the semiconductor wafer 100 is precisely adjusted by measuring the capacities of the three electrodes 111 through 113 and by controlling the three piezoelectric actuators 21 through 23 to make the capacities equal to each other. For adjusting parallelism, at least three electrodes are generally required . When the test electrode is an identical shape as the parallelism-adjusting electrodes, two parallelism-adjusting electrodes are sufficient because the test electrode is also used for adjusting parallelism. The bottom of the prism 41 can be provided with a guard ring wire 120 as shown in Fig. 19. The guard ring wire 120 runs between the spaces of the test electrode 201 and each parallelism-adjusting electrode 111, 112, or 113, and is maintained at a ground level. With the guard ring wire 120, the capacity of each electrode 111, 112, or 113 is measured without influences from the others and the parallelism is adjusted with higher precision. Although the parallelism is adjusted based on the capacities of the three parallelism-adjusting electrodes in the second embodiment, the same can be accomplished by gap measurement employing tunneling effect of light wave. Fig. 20 is a bottom view of a prism 41 of such an apparatus shown as a third embodiment of the invention. The apparatus of the embodiment includes three laser sources 51 through 53 and three photosensors 61 through 63 for detecting parallelism besides the elements shown in Fig. 14. Laser beams L1 through L3 emitted from the three laser sources 51 through 53 are reflected at reflecting points P1 through P3 on the bottom of the prism 41 and received by the photosensors 61 through 63, respectively. The reflecting points P1 through P3 are positioned on straight lines respectively connecting the center of the circular test electrode 201 with the piezoelectric actuators 21 through 23. The gap G between each reflecting point P1, P2, or P3 on the bottom of the prism 41 and the surface of a semiconductor wafer is measured as described before. The parallelism is adjusted by controlling the piezoelectric actuators 21 through 23 in such a manner that the gaps G at the reflecting points P1 through P3 are equal to each other. Here the absolute value of the gap G at each reflecting point P1, P2, or P3 is not required, but the intensities of laser beams received by the photosensors 61 through 63 is equalized for parallelism adjustment. A beam splitter can be used to divide one laser beam emitted from the laser source 5 into three beams instead of the plural laser sources 51 through 53 for parallelism adjustment. Three laser beams are generally required for parallelism adjustment; but one beam can also be used for gap measurement. In the latter case, three photosensors are sufficient instead of the four as shown in Fig. 20. The invention is not limited to the above embodiments but there may be many other modifications and changes without departing from the scope of the invention. Some examples of such modifications are shown below. (1) The present invention is applicable to measurement of electric characteristics between a test electrode and a semiconductor substrate other than the C-V curve measurement. For example, characteristics of the surface area of a semiconductor can be evaluated based on the time dependence of the resultant capacity Cta.Interface charges and surface-state at the boundary between the semiconductor substrate 101 and the oxide film 102 can be evaluated by measuring the conductance with the LCR meter 13.Furthermore, cleaning process of semiconductor wafers, heat oxidation, and stabilizing heat treatment of oxidation films can be evaluated according to the present invention by measuring C-V curves of semiconductor wafers, even if an oxide film is not formed on the wafers. (2) Although laser beams are used for gap measurement and parallelism adjustment in the above embodiments, any monochromatic light can be used instead. Monochromatic light gives constant optical factors (refractive index and absorption factor) to a substance, and thereby transmittance and reflectance are easily calculated. Monochromatic light of long wavelength can be used instead of laser beams for precisely measuring gaps of greater than 1 micrometer. An example of such monochromatic light of long wavelength is obtained by passing far infrared ray, emitted from a globar light source (that is, silicon carbide (SiC) bar), through a band pass filter. Polarized light of long wavelength is also obtained by using a polarization plate.According to the present invention, electric characteristics are measured between a semiconductor substrate and a test electrode, which is held above the surface of a semiconductor wafer across a gap. Namely, electric characteristics of a semiconductor wafer are detected without forming a test electrode on the surface of the semiconductor wafer.The size of a gap maintained by the electrode supporting unit 300 is accurately and precisely measured by a method employing the tunneling effect. Accordingly, an apparatus embodying the present invention detects an electrostatic capacity of the gap and thereby electric characteristics of a semiconductor wafer at high precision.Using the parallelism adjustment by comparing electrostatic capacities between plural parallelism-adjusting electrodes and the semiconductor wafer further improves the precision in measuring an electrostatic capacity of the gap and thereby electric characteristics of a semiconductor wafer. Alternatively, the parallelism can be adjusted by comparing the intensities of monochromatic light detected by plural photosensors.

Claims: An apparatus (MD1; MD2) for examining a surface of a semiconductor wafer (100), comprising a test electrode (201), electrode supporting means (300) for supporting said test electrode above the surface (100a) of said semiconductor wafer to form a gap (Ge) therebetween and for measuring the size of said gap, and comprising a light source (5; 51-53) for monochromatic light of a known wavelength; and measuring means (400) for measuring an electric characteristic of a series connection of said test electrode and said semiconductor wafer, characterized in that said electrode supporting means (300) comprises a light transmission reflector (4; 41) having a reflecting surface (4a) located substantially parallel to said surface (100a) of said semiconductor wafer (100) on the opposite side of said gap (Ge) for reflecting said monochromatic light at said reflecting surface, and having a refractive index larger than the medium in the gap (Ge) such that geometric optical total reflection occurs upon incidence of the monochromatic light at an angle greater than the critical angle of the medium of the light transmission reflector (4, 41) said reflecting surface (4a) carrying said test electrode (201); a photosensor arrangement (6) for measuring the intensity of said monochromatic light reflected by said reflecting surface of said reflector; a stand (3) on which said light source, said reflector and said photosensor are mounted; a first driving mechanism (2, 11, 14; 21 - 23, 327, 11, 14) for relatively moving at least one of said stand and said semiconductor wafer in a first direction defining said gap; and gap determining means (12 - 14) for measuring the size of said gap on the basis of said intensity of said reflected monochromatic light measured by said photosensor, in that said gap determining means (12 - 14) comprises means (14) for comparing said intensity of said reflected monochromatic light with a calculated value of said intensity according to Maxwell's equations on the condition that total reflection occurs with respect to said reflecting surface (4a), thereby determining said gap (Ge), such that the electric characteristic measured by said measuring means (400) is the voltage dependency of the total electrostatic capacity (Ct) said capacity (Ct) consisting of the series connection of the capacity (Cg) of the substrate, the capacity (Ci) of the oxide film on the surface of the substrate facing the test electrode (201) and the capacity (Cg) of the gap (Ge) and in that a computer (14) is provided to calculate the capacity (Cg) of the gap to obtain the voltage dependence of a resultant capacity (Cta), the reciprocal of which is defined by the difference of reciprocals of the total electrostatic capacity (Ct) and the capacity (Cg) of the gap. The apparatus of claim 1, characterized by plural parallelism-adjusting electrodes (111 - 113) formed on said reflecting surface (4a); and parallelism-adjusting means (13, 14, 11, 21 - 23) for measuring an electrostatic capacity between said semiconductor wafer (100) and each of said plural parallelism-adjusting electrodes, and adjusting the parallelism of said reflecting surface to the surface of said semiconductor wafer on the basis of comparison among the measured electrostatic capacities. The apparatus of claim 1, characterized by a light source (51 - 53) for generating plural monochromatic light beams (L1 - L3) incident on said reflecting surface (4a) on the condition of total reflection in terms of geometrical optics; said photosensor arrangement (6) having plural photosensors (61 - 63) each single one measuring the intensity of a respective one of the plural reflected beams of said plural monochromatic light beams reflected by said reflecting surface; and parallelism-adjusting means (21 - 23, 11, 14) for adjusting the parallelism of said reflecting surface to the surface (100a) of said semiconductor wafer (100) on the basis of comparison among the intensities of said plural reflected beams. The apparatus as claimed in any one of claims 1 to 3, characterized in that said light source (5) generates monochromatic light in the form of a laser beam, and that said test electrode (201) is supported to maintain said gap (Ge) less than 1 micrometer. The apparatus as claimed in any one of claims 1 to 4, characterized by a suction pump (8) ; and a conductive suction table (7) for fixing said semiconductor wafer at said gap (Ge) to said test electrode (201), said semiconductor wafer (100) is sucked by said suction pump on said suction table (7) ; and by said measuring means (400) comprising means (13) for measuring an electric characteristic relating said test electrode (201) and said conductive suction table (7). The apparatus as claimed in claim 5, characterized in that the reciprocal of the resultant electric capacity (Cta) being additionally corrected by subtracting the reciprocal of the capacity (Cb) between said semiconductor wafer (100) and said suction table (7) therefrom. A method of examining a surface of a semiconductor wafer, comprising the steps of: (a) preparing a test electrode (201) and a semiconductor wafer (100); (b) supporting said test electrode above the surface (100a) of said semiconductor wafer (100) to form a gap (Ge) therebetween, and measuring the size of said gap, and using monochromatic light of a known wavelength for examining the surface of said semiconductor wafer; and (c) measuring an electric characteristic of a series connection of said test electrode and said semiconductor wafer; characterized by the step (b) comprising the steps of: (b-1) emitting monochromatic light of a known wavelength incident on a reflecting surface (4a) of a light transmission reflector (4 or 41), said reflecting surface being located substantially parallel to said surface (100a) of said semiconductor wafer (100) on the other side of said gap (Ge) and holding said test electrode (201); (b-2) reflecting said monochromatic light at said reflecting surface upon incidence of said monochromatic light at an angle greater than the critical angle of its medium, such that geometric optical total reflection occurs; (b-3) measuring the intensity of said monochromatic light reflected by said reflecting surface; (b-4) measuring the size of said gap on the basis of said intensity of said reflected monochromatic light by (b-5) comparing said intensity of said reflected monochromatic light with a calculated value of said intensity according to Maxwell's equations on the condition that total reflection occurs with respect to said reflecting surface (4a), thereby determining said gap (Ge) ; and by step (c) consisting of (c-1) measuring the electric characteristic as the voltage dependence of the total electrostatic capacity (Ct) and calculating the capacity (Cg) of the gap to achieve the voltage dependence of a resultant capacity (Cta) a reciprocal of which is defined by the difference of reciprocals of total electrostatic capacity (Ct) and capacity (Cg) of the gap. The method as claimed in claim 7, characterized by, before said step (b), the steps of: (d) preparing plural parallelism-adjusting electrodes (111 - 113) formed on said reflecting surface (4a); and (e) measuring an electrostatic capacity between said semiconductor wafer (100) and each of said plural parallelism-adjusting electrodes, and adjusting the parallelism of said reflecting surface to the surface (100a) of said semiconductor wafer on the basis of comparison among the measured electrostatic capacities. The method as claimed in any one of claims 7 and 8, characterized in that said step of emitting monochromatic light comprises emitting a laser beam, and said gap is adjusted to be less than 1 micrometer.

Inventors: KOUNO MOTOHIRO, NAKATANI IKUYOSHI, SAKAI TAKAMASA, KOUNO, MOTOHIRO, NAKATANI, IKUYOSHI, SAKAI, TAKAMASA, Kouno, Motohiro, c/o Dainippon Screen Mfg. Co. Ltd, Nakatani, Ikuyoshi, c/o Dainippon Screen Mfg. Co., Sakai, Takamasa, c/o Dainippon Screen Mfg. Co. Ltd
Applicants: DAINIPPON SCREEN MFG, DAINIPPON SCREEN MFG. CO., LTD.
Application year: 1991
Main classification: G01R31
IPC codes: H01L21, G01N27, G01R31, G01B11