WO2003102557A1

WO2003102557A1 - Method of measuring electric characteristics of flat substrate using terahertz light

Info

Publication number: WO2003102557A1
Application number: PCT/JP2003/006887
Authority: WO
Inventors: Ryoichi Fukasawa; Toshiyuki Iwamoto
Original assignee: Nikon Corporation; Tochigi Nikon Corporation
Priority date: 2002-06-03
Filing date: 2003-05-30
Publication date: 2003-12-11
Also published as: AU2003241695A8; AU2003241695A1; JP2004003902A

Abstract

A terahertz light L3 is repeatedly applied to a semiconductor substrate (5) to receive a reflection light L4 from the substrate (5). The reflection light L4 includes a multiple-reflection light occurred in the substrate (5). When the reflection light L4 is received, a delay time length required to lead a probe pulse light L2 to a terahertz light detector (4) is set so as not detect at the terahertz light detector (4) the reflection light L4 reaching via surfaces other than the illuminated surface of the semiconductor substrate (5). A time region spectrometry method is used to measure the time-series waveform of the field intensity of the detected reflection light L4, and the electric characteristic parameters of the substrate (5) are calculated from a spectral reflectance determined based on the time-series waveform.

Description

Specification

Method for Measuring Electrical Characteristics of Flat Substrate Using Terahertz Light This application is based on the following application, the contents of which are incorporated herein by reference. Japanese patent application 2002 No. 161085 (filed on June 3, 2002)

The present invention relates to a method for measuring electric characteristics of a flat substrate using terahertz time-domain spectroscopy. Background art

In the semiconductor device industry, physical properties related to the electrical properties of semiconductor materials, such as carrier concentration, mobility, resistivity, and electrical conductivity, are important factors influencing the performance of semiconductor devices. Physical properties such as carrier concentration and mobility of an epitaxial film formed on a semiconductor substrate are also important factors that determine device characteristics. In the case of a semiconductor substrate having an optical thickness, a method of evaluating the electrical characteristic parameters by analyzing the spectral characteristics in the far-infrared region is known. It is a technique that is implemented without contact. Further, Japanese Unexamined Patent Application Publication No. 2000-282497 discloses a method of measuring a time-series waveform using terahertz time-domain spectroscopy, performing a Fourier transform on the time-series waveform, and performing spectral reflectance or spectral transmission of a semiconductor material. The electrical characteristics parameters are calculated based on this value. Disclosure of the invention

When evaluating the electrical characteristics of a semiconductor substrate in a non-destructive and non-contact manner, if multiple reflections occur inside the semiconductor substrate, they appear as a thousand fringes on the reflectance spectrum. In this case, the reflectance spectrum becomes complicated, and it becomes difficult to analyze the spectral characteristics. Similarly, when measuring the spectral reflectance characteristics of an epitaxial film formed on a semiconductor substrate, the reflection on the back surface of the substrate is superimposed, so that correct spectral reflectance characteristics cannot be obtained. Furthermore, the Chihatsu fringe caused by multiple reflection inside the semiconductor substrate This makes it extremely difficult to evaluate physical properties from the spectral shape in order to distort the shape of the reflectance spectrum.

Conventionally, in order to eliminate the influence of reflection from the backside of the board that causes the strongest interference fringe, the cross-sectional shape in the thickness direction of the board is made to be wedge-shaped, or the backside of the board is made rough and diffusely reflected. I was working to eliminate the returning light. However, processing a semiconductor sample to be measured in a semiconductor manufacturing process is not desirable because it cannot be used as a product and processing is troublesome. Therefore, there is a demand for the development of a simple measuring method for measuring these electrical characteristics without processing a semiconductor sample in the semiconductor device manufacturing process.

An object of the present invention is to provide a method for simply measuring the electrical characteristics of a semiconductor sample without being affected by a thousand fringes caused by multiple reflection inside a semiconductor substrate.

The method for measuring the electrical characteristics of a flat substrate using terahertz light according to the first aspect of the present invention is a method for repeatedly irradiating a terahertz pulsed light to a flat substrate and receiving reflected light or transmitted light from the flat substrate. In order to prevent the detection of light arriving via a surface other than the irradiated surface of the flat substrate, a time domain for detecting terahertz pulse light from the flat substrate is set, and time domain spectroscopy is used. The time series waveform of the electric field intensity of the detected light is measured, and the electrical characteristic parameters of the flat substrate are calculated from the spectral reflectance or the spectral transmittance obtained based on the time series waveform.

The method for measuring electrical characteristics of a flat substrate using terahertz light according to the second aspect of the present invention includes: irradiating a terahertz pulse light repeatedly to a flat substrate; receiving reflected light or transmitted light from the flat substrate; Using time-domain spectroscopy, measure the time-series waveform of the electric field intensity of the received light, and Fourier transform only a predetermined time range on the time-series waveform to obtain the spectral reflectance or spectral transmittance. This is to calculate the electrical characteristics and parameters of the flat substrate from the spectral reflectance or spectral transmittance.

According to a third aspect of the present invention, there is provided an apparatus for measuring electrical properties of a flat substrate using terahertz light, comprising: a laser light source that emits light pulses; A terahertz light source for irradiating a plate, a terahertz photodetector for receiving reflected light or transmitted light from a flat substrate, a light pulse, and one is guided to the terahertz light source, and the other is guided to the terahertz light source. Detects terahertz pulse light from a flat substrate so that the optical system leading to the Hertz photodetector and the terahertz photodetector do not detect light arriving via a surface other than the irradiated surface of the flat substrate And a control unit for limiting a time region to be used. An apparatus for measuring electrical characteristics of a planar substrate using terahertz light according to a fourth aspect of the present invention includes a laser light source that emits a light pulse, and a terahertz light source that repeatedly irradiates a planar substrate with terahertz pulsed light. A terahertz photodetector that receives reflected light or transmitted light from a planar substrate; and an optical system that splits an optical pulse and guides one to the terahertz light source and the other to the terahertz light detector. And an arithmetic unit that measures the time-series waveform of the terahertz pulse light from the flat substrate received by the terahertz photodetector and performs a Fourier transform on only a predetermined time range on the time-series waveform. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic configuration diagram of an electric characteristic measuring device used in the electric characteristic measuring method according to the embodiment of the present invention.

Figure 2 shows a time-series waveform.

FIG. 3 is a schematic diagram showing a state of multiple reflection of light inside the semiconductor substrate.

FIG. 4 is a reflectance spectrum obtained from the time-series waveform of FIG.

FIG. 5 is a time-series waveform according to the embodiment of the present invention.

FIG. 6 is a reflectance spectrum obtained from the time-series waveform of FIG.

FIG. 7 is a measured value of the reflectance spectrum excluding the influence of interference fringes according to the embodiment of the present invention.

FIG. 8 is a calculated value of the reflectance spectrum excluding the influence of Chikko Fringe.

FIG. 9 is a graph showing the relationship between the carrier concentration obtained by the electrical characteristic measuring method according to the embodiment of the present invention and the carrier concentration obtained from the film forming conditions.

FIG. 10 shows the relationship between the carrier concentration and the mobility obtained by the electrical property measurement method according to the embodiment of the present invention, and the relationship between the carrier concentration and the mobility obtained from the electrical measurement. It is a graph which shows a comparison.

FIG. 11 is a flowchart showing the procedure of the electrical characteristic measuring method according to the embodiment of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

First, terahertz time-domain spectroscopy, which is currently established as a technique for measuring terahertz pulsed light, is described. Terahertz time-domain spectroscopy measures the time-series waveform E (t) of the electric field intensity of the terahertz pulse light, and performs Fourier transform on the time-series waveform to obtain a reflectance spectrum or a transmittance spectrum. This is the spectroscopy that gives Based on the spectral reflectance or the spectral transmittance, the electrical characteristics of the planar substrate, that is, physical properties such as carrier concentration, mobility, resistivity, and electrical conductivity can be obtained.

FIG. 1 is a schematic configuration diagram of an electric characteristic measuring apparatus using terahertz light of the present invention, and is also a diagram for explaining time-domain spectroscopy.

The light pulse emitted from the femtosecond pulse laser 1 passes through the beam splitter 2 and is divided into a pump pulse L1 and a probe pulse L2. The pump pulse L1 is guided to the terahertz light source 3, and irradiates the terahertz light source 3 to generate the terahertz pulse light L3. On the other hand, the probe pulse L 2 is guided to the terahertz photodetector 4 to receive (detect) the terahertz pulse light that has passed through the semiconductor sample (semiconductor substrate) 5. A movable mirror 6 is provided on the optical path for guiding the probe pulse L2, and moving the movable mirror 6 in the direction indicated by the arrow changes the time for the probe pulse L2 to reach the terahertz photodetector 4. Can be done. The movable mirror 6 and the drive mechanism 7 for displacing the movable mirror 6 in the direction of the arrow are called a time delay device.

The pulse width of the light pulses emitted from Fuemuto second pulse laser 1 is about lOOfsec ⁽¹ X 1 0- 1 ³ sec), the repetition period is several tens MH z. Therefore, the emitted terahertz pulse light L 3 is also emitted at a repetition of several tens of MHz. With the current terahertz photodetector, the waveform of the terahertz pulse light cannot be measured instantaneously as it is. Therefore, this measurement method utilizes the fact that the terahertz pulse light L4 of the same waveform arrives at the terahertz photodetector 4 at a repetition rate of several tens of MHz. The pump-probe method is used to measure the terahertz pulse light waveform with a time delay between the two. That is, by delaying the timing of the probe pulse L 2 for operating the terahertz light detector 4 by Δt seconds with respect to the pump pulse L 1 for operating the terahertz light source 3, the time delayed by Δt seconds The electric field intensity of the terahertz pulse light L4 can be measured. In other words, the probe pulse L 2 gates the terahertz photodetector 4. Also, moving the movable mirror 6 gradually is nothing more than gradually changing the time Δt.

In this measurement method, the terahertz pulse light L4 that repeatedly arrives is detected while shifting the timing of applying a gate by a time delay device, and these terahertz pulse lights L4 are spliced to reproduce one waveform. is there. Thus, the time-series waveform E (t) of the electric field of the terahertz light can be measured.

The terahertz photodetector 4 generates a carrier according to the incidence of the probe pulse L2. If a terahertz pulse light L4 is incident at the same time as the probe pulse L2 is incident and an electric field is generated, a photoconductive current proportional to the electric field flows. The current J (t) measured at this time is expressed by the convolution of the electric field strength E (t) of the terahertz pulse light and the optical conductivity g (te1 t) of the photoexcited carrier as shown in equation (1). Can be expressed as

J (t) oc S E) g — t) d

If the photoconductivity g is close to the delta function, the measured current will be proportional to the amplitude of the electric field I E (t) I of the incoming terahertz pulsed light. FIG. 2 is a time-series waveform of the electric field E (t) of the terahertz pulse light obtained in this manner. This time-series waveform is a result of actually measuring the semiconductor sample 5 using the terahertz time-domain spectroscopy with the electric characteristic measuring apparatus of FIG. In this measurement, 1024 points were measured at a sampling interval Δt of time-series waveform data of 0.06667 ps.

FIG. 3 is a schematic diagram showing a state of multiple reflection inside the semiconductor substrate 5. Semiconduct An epitaxy film 8 is formed on the substrate 5, and a part of the incident light incident from the epitaxy film 8 side is reflected on the substrate surface (that is, the epitaxy film 8), and a part of the light is reflected inside the substrate. Through. A part of the light transmitted through the inside of the substrate is reflected on the back surface of the substrate, and the light is transmitted through the inside of the substrate again, emitted from the surface of the substrate, and returned to the inside of the substrate again. Such repetitive reflection is called multiple reflection.

Referring again to FIG. 2, several peaks appear in the time-series waveform. The first peak P 1 is caused by the reflection of the terahertz pulse light on the surface of the semiconductor substrate 5, and the second peak P 2 is caused by the reflection of the terahertz pulse light on the back surface of the substrate 5. are doing. Similarly, peak P 3 is caused by two reflections on the back surface of substrate 5, and peak P 4 is caused by reflection three times on the back surface of substrate 5. The numbers of the multiple reflections in FIG. 3 correspond to the peak numbers of the time-series waveform in FIG. 2, respectively.

In conventional spectroscopy, all the reflected light reflected by multiple reflections is observed at the same time.In terahertz time-domain spectroscopy, the time when the reflected light arrives at the photodetector is represented by a time-series waveform. The point is that the observation is performed in a time-resolved manner. Therefore, as the number of reflections inside the substrate increases, the peak due to the reflection appears with a time delay.

When obtaining a reflectance spectrum using terahertz time-domain spectroscopy, a Fourier transform is performed on a time-series waveform. Peaks caused by multiple reflections of light inside the substrate complicate the shape of the entire reflectivity spectrum.

FIG. 4 is a graph of a reflectance spectrum obtained by Fourier-transforming the time-series waveform of FIG. 2, in which the vertical axis represents the reflectance and the horizontal axis represents the frequency. At a glance, it can be seen that repetition of the magnitude of the reflectance, that is, intense chikko fringe is observed. Such a fringe affects the shape of the entire reflectance spectrum, complicating the analysis of the spectrum and complicating the accurate physical properties of the thin film such as an epitaxial film formed on the substrate. It is extremely difficult to determine the value.

In the present embodiment, the above problem is solved by stopping the measurement before the reflected light (see FIG. 3) from the back surface of the substrate returns when measuring the time-series waveform. That is, as shown in FIG. 1, the control mechanism 8 controls the drive mechanism 7 Then, the stroke range for displacing the movable mirror 6 in the direction of the arrow is changed from the long setting indicated by S1 to the short setting indicated by S2. As a result, the measurement is stopped before the reflected light from the back surface of the substrate returns. In this way, the reflectance spectrum can be measured without being affected by a thousand fringes due to multiple reflections inside the substrate, and accurate physical properties of the film on the substrate can be obtained from the analysis. .

The present invention is a measurement technique utilizing the features of terahertz time-domain spectroscopy, and such measurement could not be performed by conventional spectroscopy. This measurement method is extremely effective when measuring the optical constant of a film on a substrate, because the influence of interference fringes can be eliminated.

Fig. 5 shows the time-series waveform of the electric field E (t) of the terahertz pulsed light obtained in this manner. The semiconductor sample, which is the sample, is the same as that obtained when the time-series waveform of FIG. 2 was obtained. However, in this measurement, the sampling interval Δt of the time-series waveform data was measured at 256 points at 0.06667 ps, which is 1 to 4 times the measurement time compared to the 1024 points measurement in FIG. The peak appearing on the time-series waveform in FIG. 5 is only P1 reflected on the surface of the semiconductor substrate 5, and the peak P2 and below are removed. In this way, by selecting the time within a predetermined range of the time-series waveform, the reflectance spectrum is measured without being affected by interference fringes due to multiple reflections inside the substrate, and from the analysis, the reflectance spectrum of the film on the substrate is determined. Accurate physical property values can be obtained.

FIG. 6 is a reflectance spectrum obtained by Fourier-transforming the time-series waveform of FIG. 5, and is a graph corresponding to FIG. Comparing the reflectance spectrum of Fig. 6 with that of Fig. 4, it is clear that the influence of interference fringes due to multiple reflection inside the substrate has been removed.

The advantages of the measurement method of the present invention are that (1) a spectrum that is not affected by the fringe is obtained, and (2) the measurement time can be reduced because the number of data points is reduced.

In addition, in the time series waveform of FIG. 2 including multiple reflections, even if the time range in which the first peak P1 exists is selected and Fourier-transformed, as in (1) above, a pulse that is not affected by a thousand fringes is obtained. Vector is obtained. The control shown in Fig. 1 includes Fourier transform and waveform analysis. The calculation is performed by the calculation unit 8.

Next, the points of the procedure of the measurement method of the present invention will be described with reference to FIG. In the first embodiment of the present invention, in step 5, detection data is acquired until the stroke amount of the movable mirror 6 becomes S2, and in step 6, a time-series waveform is measured. On the other hand, in the second embodiment of the present invention, in step 5, detection data is acquired until the stroke amount of the movable mirror 6 becomes S1, and in step 9, a time-series waveform is measured. In step 10, a time range for performing Fourier transform on the time-series waveform is set, and in step 7, only this time range is subjected to Fourier transform.

Subsequently, quantitative measurement of physical property values of an epitaxial film formed on a semiconductor substrate using the measurement method of the present invention will be specifically described. Here, the carrier concentration and mobility, which are important parameters of the electrical characteristics of the epitaxial film, were calculated. The n-type GaAs films with four different carrier concentrations used for the measurement were formed by molecular beam epitaxy on a semi-insulating GaAs substrate (thickness: 625 μηι). is there. The thickness of the film is 2. Kiyaria concentration of guaranteed value of n-type G a A s layer that is determined from the film formation conditions, respectively ^{3 X 1 0 1 5, 1} X 1 0 1 6, 4 X 1 0 1 6, 1 X 1 0 1 ⁷ is a cm _ ^3.

FIG. 7 is a reflectance spectrum that does not include the influence of interference fringes due to multiple reflection, and is obtained by the terahertz time-domain spectrometry of the present invention.

FIG. 8 is a reflectance spectrum calculated by a theory that does not consider the reflection from the back surface of the substrate. Looking at the reflectance spectra in Figs. 7 and 8, it can be seen that the measured and theoretical values agree very well.

FIG. 9 is a graph illustrating the reliability of the terahertz time domain measurement method of the present invention with respect to the carrier concentration that best reproduces the shape of the observed reflectance spectrum. Figure 9 shows the results of plotting the relationship between the carrier concentration determined by the non-linear optimization method from the reflectance spectrum obtained by the spectroscopic measurement of the present invention and the carrier concentration determined from the epitaxial film formation conditions. Is shown. Both are roughly on the proportional straight line, and are in good agreement.

FIG. 10 shows the reflectance spectrum obtained by the terahertz time-domain spectrometry of the present invention. This is the result of plotting the carrier concentration and mobility determined from the torque on a graph showing the relationship between the carrier concentration and mobility determined from electrical measurements. The point indicating the relationship between carrier concentration and mobility obtained from terahertz time-domain spectroscopy (indicated by a triangle in the figure) is the point indicating the relationship between carrier concentration and mobility determined from electrical measurements. (Indicated by a square mark in the figure). Therefore, it can be seen that the values obtained from terahertz time-domain spectroscopy almost match the values obtained from electrical measurements.

The method of measuring physical properties according to the present embodiment is based on HI-V compound semiconductors such as p-type GaAs, n-type AlGaAs, and p-type A1GaAs on a semiconductor substrate. — It is also applicable to thin films of Group VI compound semiconductors and IV—VI compound semiconductors.

In this embodiment, the method for measuring the parameter of the electrical characteristics of the semiconductor thin film on the semiconductor substrate has been described. However, the measurement of the reflectance spectrum of the ion-implanted layer formed on the semiconductor substrate is also described in this embodiment. The invention is applicable. Further, the present invention is also applicable to reflectance spectrum measurement of thin films other than semiconductor thin films such as dielectrics and superconductors. The embodiments of the present invention have been described above, but the present invention is not limited to these embodiments. Other modes that can be considered within the technical idea of the present invention are also included in the scope of the present invention.

Claims

The scope of the claims

1. The method of measuring the electrical characteristics of a flat substrate using terahertz light

Terahertz pulse light is repeatedly irradiated on the flat substrate,

When receiving reflected light or transmitted light from the planar substrate, detect terahertz pulse light from the planar substrate so as not to detect light arriving via a surface other than the irradiated surface of the planar substrate. Set the time domain to

Using a time-domain spectroscopy, a time-series waveform of the electric field intensity of the detected light is measured, and an electrical characteristic parameter of the flat substrate is obtained from a spectral reflectance or a spectral transmittance obtained based on the time-series waveform. Calculate overnight.

2. In the method for measuring electrical characteristics of claim 1,

The time region is set by an optical path length range that guides an optical pulse necessary for detecting the terahertz pulse light from the planar substrate to the terahertz photodetector.

3. The method for measuring the electrical characteristics of a flat substrate using terahertz light

Irradiate the planar substrate repeatedly with terahertz pulse light,

Receiving reflected light or transmitted light from the flat substrate,

Using time domain spectroscopy, a time series waveform of the electric field intensity of the received light is measured, and only a predetermined time range on the time series waveform is subjected to Fourier transform to obtain a spectral reflectance or a spectroscopic transmittance. ,

An electrical characteristic parameter of the flat substrate is calculated from the spectral reflectance or the spectral transmittance.

4. In the method for measuring electrical characteristics of claim 3,

The predetermined time range is set so as to correspond to an optical path length range for guiding a light pulse necessary for detecting terahertz pulse light from the flat substrate to the terahertz photodetector.

5. In the method for measuring electrical characteristics of claims 1 to 4,

The measurement target is a thin film on a flat substrate instead of the flat substrate.

6. In the method of measuring electrical properties of claim 5, The thin film is a semiconductor, a dielectric, or a superconductor.

7. The device for measuring electrical properties of flat substrates using terahertz light

A laser light source that emits light pulses;

A terahertz light source that repeatedly irradiates a terahertz pulse light to a planar substrate; a terahertz light detector that receives reflected light or transmitted light from the planar substrate; An optical system for guiding to a Hertz light source and for guiding the other to the terahertz photodetector;

A time region for detecting terahertz pulse light from the flat substrate is limited so that the terahertz light detector does not detect light arriving via a surface other than the irradiated surface of the flat substrate. A control unit.

8. In the electrical characteristic measuring device of claim 7,

The optical system includes a movable mirror for providing an optical path length difference by a distance corresponding to the time domain between the light pulse guided to the terahertz light source and the light pulse guided to the terahertz light detector. Have.

9. An apparatus for measuring electrical characteristics of flat substrates using terahertz light

A laser light source that emits light pulses;

An arithmetic unit that measures a time-series waveform of the terahertz pulse light from the flat substrate received by the terahertz light detector and performs a Fourier transform only on a predetermined time range on the time-series waveform.

10. In the electrical characteristic measuring device of claims 7 to 9,

1 1. In the method for measuring electrical characteristics of claim 10,

The thin film is a semiconductor, a dielectric, or a superconductor.