RU2121732C1 - Device and technique for measuring surface resistance of semiconductor plates - Google Patents

Abstract

FIELD: measurement technology; quality control of semiconductor materials. SUBSTANCE: device has inductance sensor, holding frame with one or more reference specimens, and measuring unit. Measurement procedure includes measurement of signal background value as well as information signals coming from reference and analyzed specimens. Information signals coming from reference specimens are used to calculate correction function, including background signal, for calibration function; surface resistance of analyzed specimen is determined by using these functions and information signal coming from analyzed specimen. EFFECT: extended measurement range enabling taking measurements without re-calibration at high accuracy and proximity. 5 cl, 2 dwg

Description

 The invention relates to the field of measuring equipment, in particular, to the field of measuring the electrophysical parameters of materials and can be used to control the quality of semiconductor materials, in particular, semiconductor wafers.

 To control the electrophysical parameters of semiconductor materials, both contact and non-contact measurement methods are used. Contact control methods are essentially probe methods. In this regard, they lead to contamination of the surface of the analyzed sample, and in the vast majority of cases, they are destructive control methods.

Non-contact methods for measuring the electrical conductivity of semiconductor wafers, or the reciprocal of the surface resistance of the wafers, are formed by measuring the loss of high-frequency (HF) electromagnetic field at the eddy currents induced in the investigated semiconductor wafer. In order to measure these losses, an LC high-quality oscillatory circuit is created. Structurally, the inductance is performed so that its core has a gap gap. The studied semiconductor wafer is placed in the gap of the inductance core, as a result of which the primary rf induction field induces eddy currents in the semiconductor sample, the value of which is proportional to σh, where σ is the specific conductivity of the semiconductor, h is the thickness of the sample, and the surface resistance R _s = (σh) ^{is 1} . The eddy currents induced in this way create their own RF field, which, in accordance with the Lenz law, reduces the primary RF field of the oscillatory circuit. From a radio technical point of view, the loss of the RF field of the oscillatory circuit at the eddy currents of the plate under study looks like the appearance of an additional active component of the impedance of the oscillatory circuit, as a result of which the quality factor of the circuit decreases. Further, depending on the device, in one way or another, the additional active component of the impedance of the circuit is measured, or a change in its Q factor, or a decrease in the voltage drop across it due to a decrease in its complex resistance.

 It is known that for low-resistance samples it is preferable to place the test sample in the RF field of the inductor, and for high-resistance samples, i.e. with low electrical conductivity, use capacitively coupled circuits.

 Known technical solution (SU, copyright certificate 164068 G 01 R 31/26, 1964), characterizing the method and device for measuring the surface resistance of semiconductor materials. The device contains an X-shaped sensor core with windings. The windings are connected to an RF generator whose voltage is modulated by a modulator. At the ends of the X-shaped ferrite core, two measuring and two compensation windings are symmetrically placed, each pair of which is connected through a detector to a comparison circuit. The signal value is measured without making the test plate and making the test plate in the RF field of the measuring windings. The measured values of the signals judge the surface resistance of the test plate. The disadvantages of this technical solution include insufficient measurement accuracy and the relative complexity of the design.

 Also known is a method and apparatus for non-contact resistance measurement of semiconductor materials (DE, application 3625819, G 01 R 31/26, 1987). The device contains an induction sensor and a recording unit, including an RF generator connected to the sensor excitation coil, and an information signal meter, which is used as a voltmeter. The studied semiconductor wafer is introduced into the induction sensor, in which the primary RF electromagnetic field is preliminarily made.

 Closest to the proposed is a method and apparatus for non-contact measurement of the surface resistance of semiconductor wafers (Miller GL et al. Contactless measurement of semiconductor conductivity by radio frequency-free carries power absorption. Rev. Sci. Instrum. 1976, vol. 47, N 7, pp. 799-805). The device comprises an induction sensor and an information signal recording unit. This unit includes: an RF generator, to which an induction sensor coil is connected, an RF signal amplitude meter, an automatic amplitude control circuit, and a DC amplifier. The semiconductor wafer under investigation is introduced into the gap of the ferrite core of the sensor, in which an electromagnetic field is created by the RF current of the coil. The variable magnetic component of this field causes the appearance of Foucault eddy currents in the semiconductor wafer under study. These eddy currents generate their electromagnetic field, which reduces the primary field, which generated eddy currents. As a result, the impedance of the LC circuit, in which the sensor coil plays the role of inductance, changes and the amplitude of the RF signal decreases by a value proportional to the electrical conductivity σh of the test sample. The difference between the amplitudes of the initial RF signal (the sensor without the plate) and the reduced RF signal (the sensor with the inserted plate) is an information signal ΔI. The automatic amplitude control circuit restores the reduced RF signal to the previous value, which adds ΔI to the sensor coil.

When measuring reference samples with a known value of σh, a calibration curve ΔI (σh) is built, due to which the electric conductivity of the measured plate and, therefore, its surface resistance R _s = (σh) ^{-1 are} determined from the measured value of ΔI.
One of the main components of the measurement error is associated with the irreproducibility of the ΔI measurement. Because of this error, an error arises in measuring the surface resistance of the measured semiconductor, as a result of which it is necessary to recalibrate the device practically before each new plate measurement with an unknown surface resistance value.

 This technical solution can be taken as the closest analogue of the present invention.

 The technical problem to which the present invention is directed is to develop a device and method having higher accuracy and expressivity in measuring the surface resistance of semiconductor materials.

 The technical result obtained by the implementation of the invention consists in the development of a device and method that allows, without recalibrating the device during repeated measurements of various samples with an unknown previously, even approximately the value of surface resistance, to make measurements in a wide range of the parameter with high accuracy and expressivity.

 The device comprises an inductive sensor, an information signal registration unit, and a holder frame for the test plate. The regeneration unit includes an RF generator, to which an inductive sensor coil is connected, an RF signal amplitude meter, an automatic amplitude control circuit, and a DC amplifier (Fig. 1). The holder frame of the test plate includes at least one control sample with a known value of electrical conductivity (Fig. 2). These control samples on the holder’s frame are fixed in the plane of the test plate and are thus positioned along the center line of the frame so that a space with linear dimensions of at least linear dimensions of the sensor, free from objects, is left in front of the first control sample and the test plate electrical conductivity. In addition, the sensor coil is connected to the RF generator according to a partial switching circuit, i.e. according to the scheme with autotransformer communication.

с помощью которой по калибровочной функции определяют поверхностное сопротивление исследуемой пластины

В этой формуле вычитание из каждого сигнала фонового значения, измеренного непосредственно перед измерением информационного сигнала, позволяет уменьшить ошибку, связанную с возможным "дрейфом нуля" прибора, т.е. по сути фоновый сигнал дает истинный ноль отсчета для тут же измеряемого информационного сигнала. Функция F(β) = β^α позволяет вести поправку на "уход" калибровочной функции со временем, т.е., по существу, на нестабильность измерения информационных сигналов, связанную со многими случайными процессами, изменяющими параметры как радиотехнической системы, так и самих датчиков. В этой поправочной функции ΔI $\genfrac{}{}{0ex}{}{\text{o}}{\text{k}}$ -ΔI $\genfrac{}{}{0ex}{}{\text{o}}{\text{k,o}}$ сигнал из калибровочной функции для контрольного образца. Если контрольных образцов несколько, то в формуле используют данные измерения для тех из них, для которых значение R_{s контр} наиболее близко к поверхностному сопротивлению исследуемой пластины R_{s иск}. Другими словами, в расчет берут измерения контрольных образцов, значение поверхностного сопротивления которых оказалось наиболее близким к поверхностному сопротивлению исследуемой пластины.The holder frame, when moving relative to the sensor along its center line, brings the free space in front of the first control sample under the sensor and the background signal ΔI _{k is} measured _{, o} then the holder frame sequentially brings each of the control samples under the sensor and the information signal ΔI _{k is} sequentially measured for each of the control samples. Next, measure the background signal ΔI _o in the second free space located in front of the investigated plate. And finally, measure the information signal ΔI from the test plate in one or more areas of this plate. As a result of the following mathematical operations with the measured signals, the final value of

with the help of which the surface resistance of the test plate is determined by the calibration function

In this formula, subtracting from each signal the background value measured immediately before the measurement of the information signal allows one to reduce the error associated with the possible “zero drift” of the device, i.e. in fact, the background signal gives a true zero reference for the immediately measured information signal. The function F (β) = β ^α allows correction for the "departure" of the calibration function with time, i.e., essentially, for the instability of the measurement of information signals associated with many random processes that change the parameters of both the radio system and the sensors themselves . In this correction function, ΔI $\genfrac{}{}{0ex}{}{\text{o}}{\text{k}}$ -ΔI $\genfrac{}{}{0ex}{}{\text{o}}{\text{k, o}}$ signal from the calibration function for the control sample. If there are several control samples, then the measurement data are used in the formula for those for which the value of R _{s counter} is closest to the surface resistance of the test plate R _{s suit} . In other words, the measurements of control samples are taken into account, the surface resistance of which turned out to be the closest to the surface resistance of the plate under study.

The value of α lies in the range from 0.5 to 1.5 and depends on how far apart are the values of R _{s suit} and R _{s counter} and some characteristics of the device.

Thus, before starting to measure each new test plate, in essence, the calibration function is checked against one or several reference points corresponding to control samples and, using the function F = β ^α , without recalibration of the device, a correction for “leaving” is introduced coordinates of points of the calibration function for the time from the moment of calibration to the moment of the current measurement cycle.

от нескольких измерений контрольных образцов в предыдущих циклах измерений.It should also be noted that all signal values included in the indicated formula are the averaged result of a large number of measurements (up to several thousand) of a given signal performed automatically at short intervals (for example, after 100 μs) in one region of the measured sample. This minimizes the high-frequency noise of the measured signal and improves the accuracy of the measurement. In addition, in order to avoid the imposition of a random error in the measurement ΔI _k from the control samples on a random error in the measurement ΔI from the test plate, the average value is used in the correction function F = β ^α

from several measurements of control samples in previous measurement cycles.

 The applicant notes that the combination of features introduced by him in the independent claims is necessary and sufficient to achieve the above technical result. The features introduced by the applicant in the dependent claims develop and supplement the features introduced in the independent claims.

 The invention is illustrated in graphic material, where in FIG. 1 is a block diagram of a registration unit with a sensor connection circuit; FIG. 2 - design of the holder frame with two (as an example) control samples. The following notation is introduced on the graphic material: RF generator - 1, RF signal amplitude meter - 2, RF signal amplitude automatic control unit - 3, DC amplifier - 4, induction sensor - 5, holder frame - 6, control samples - 7, the investigated semiconductor wafer - 8, areas not containing electrical conductive objects for measuring the background signal - 9.

 The invention can be implemented as follows.

и по калибровочной функции находят соответствующее значение поверхностного сопротивления исследуемой пластины. Далее, с целью устранить погрешность измерения, связанную с влиянием температуры измерения на поверхностное сопротивление измеряемого объекта, возможно введение температурной поправки на найденное из калибровочной функции значение поверхностного сопротивления.In accordance with the procedure described above, in this measurement cycle, the background signal is sequentially measured in the region free of electrically conductive objects before the first control image, then the information signals in the area of each control sample, then the background signal values are read again (i.e., the value of "zero ") in the area free of electrically conductive objects located in front of the plate under study and finally, the measurement of the information signal in one or more areas of the studied p Astina. The information signal in each of these areas, as already mentioned, is associated with the electrical conductivity of objects in the sensor gap, unless the background signal is measured due to factors of external influence on the radio circuits and sensors (for example, thermal radiation, drafts, vibrations, etc.). This information signal is equal to the signal ΔI, which is supplied by the RF amplitude generator 1 from the automatic amplitude control unit 3 to restore the RF signal in it, which has decreased due to the appearance of Foucault eddy currents in the measured conductive objects. In accordance with the above expression for F, calculate the value

and by the calibration function find the corresponding value of the surface resistance of the investigated plate. Further, in order to eliminate the measurement error associated with the influence of the measurement temperature on the surface resistance of the measured object, it is possible to introduce a temperature correction to the surface resistance value found from the calibration function.

Claims (5)

1. A method of measuring the surface resistance of semiconductor wafers, including the construction of a calibration function, a measurement procedure consisting of mutual plane-parallel movement of the holder frame with the test plate and an induction sensor, measurement of the information signal from the test plate, which is used to change the high-frequency signal in the sensor coil, and determination by information signal using the calibration function of the desired surface resistance of the investigated plates, characterized in that the measurement procedure additionally introduces the measurement of the information signal from the control samples and the measurement of background signals in spaces free of objects with electrical conductivity and located directly in front of the control samples and in front of the plate under study, and enter into the measured information signal from the plate under investigation correction by multiplying it by the value of the correction function given by the relation

where ΔI _to - information signal for the control sample,
_{ΔI KO} - background signal from the space in front of the control sample,

- signal from the calibration function for the control sample,
α is an exponent equal to 0.5 - 1.5.  2. The method according to p. 1, characterized in that when constructing the correction function, the average value of the signals from the control sample for the current and immediately preceding measurement procedures is used, and when constructing the correction function, data are used for one or more control samples closest to the measured parameter to the test plate.  3. The method according to claim 2, characterized in that all information signals are calculated taking into account the background signal, i.e. calculated as the algebraic difference between the measured information and the corresponding background signals.  4. The method according to claim 3, characterized in that the information and background signals within the measurement cycle are averaged values.  5. A device for measuring the surface resistance of semiconductor wafers, comprising an induction sensor, a holder frame for the plate under study, a registration unit, which includes a high-frequency generator, characterized in that the holder frame is made with at least one control sample and spaces free from objects with electrical conductivity, and the sensor coil is connected to a high-frequency generator according to the scheme with a partial inductance.

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