JP5925634B2 - Semiconductor defect evaluation method - Google Patents

Description

The present invention relates to a semiconductor defect evaluation method using a CPM measuring device in a wide gap semiconductor.

Conventionally, a transistor using amorphous silicon has been used for a display device typified by a liquid crystal television. However, an oxide semiconductor has been attracting attention as a material replacing a silicon semiconductor. For example, an amorphous oxide containing In, Ga, and Zn is used as a channel region of a transistor in an active matrix display device, and the electron carrier concentration of the amorphous oxide is less than 1 × 10 ¹⁸ cm ⁻³ . Have been disclosed (see Patent Document 1).

However, a transistor including an oxide semiconductor has several problems. One of them is characteristic instability, and it has been pointed out that the threshold voltage changes when irradiated with visible light and ultraviolet light (see Non-Patent Document 1). In terms of transistor reliability, a problem has been pointed out that characteristics are changed by a bias-thermal stress test (see Non-Patent Document 2).

JP 2006-165528 A

P. Barquinha, A.D. Pimentel, A.M. Marques, L.M. Pereira, R.A. Martins, E.M. Fortunato, “Effect of UV and visible light radiation on the electrical perforations of trans-ant TFTs based on the two of the six-in-one-in-the-lens” Kwang-Hee Lee, Ji Sim Jung, Kyoung Seok Son, Joon Seok Park, Tae Sang Kim, Rino Choi, Jae Kyeong Jeong, Jang-Yeon Kwon, Bonwon Koo, and Sangyun Lee, "The effect of moisture on the photon-enhanced negative bias thermal instability in Ga-In-Zn-O thin film transistors ", APPLIED PHYSICS LETTERS 95, (2009) 232106

As one of semiconductor defect evaluation methods, a constant photocurrent measurement method (CPM) is known.

In CPM measurement, the amount of light applied to the sample surface between the terminals is adjusted so that the photocurrent value is constant while a voltage is applied between the first electrode and the second electrode provided on the sample, The extinction coefficient is derived from each wavelength at each wavelength. In the CPM measurement, when a sample has a defect, an extinction coefficient at an energy (converted from a wavelength) corresponding to the level where the defect exists is increased. By multiplying the increase in the extinction coefficient by a constant, the defect density of the sample can be derived.

An oxide semiconductor is one of wide gap semiconductors with a wide band gap. Until now, it has been difficult to accurately evaluate the defect density of wide gap semiconductors by CPM measurement.

Therefore, even in a wide gap semiconductor, it is possible to evaluate the defect density with high accuracy by CPM measurement.

Below the band gap wavelength (λ _Eg ) of the wide gap semiconductor and above the predetermined wavelength range, the absorption coefficient derived from the irradiation light quantity obtained by CPM measurement, and below the λ _{Eg of the} wide gap semiconductor measured separately and the predetermined wavelength G (hν) is changed so that the fitting value F (x) with the absorption coefficient above the range is 0.0001 or more and 1 or less, preferably 0.0001 or more and 0.1 or less.

The fitting value F (x) is defined by Equation (1).

In the formula (1), f (hν) is an absorption coefficient of λ _Eg or less of the separately measured wide gap semiconductor and a predetermined wavelength range or more. In addition, g (hν) is obtained by multiplying a standardized irradiation light amount obtained by standardizing the irradiation light amount in a predetermined wavelength range or less, which is equal to or less than λ _{Eg of the} wide gap semiconductor derived by CPM measurement, by an arbitrary number. It is a thing.

Specifically, the shape of the sample corresponds to an area that is effectively irradiated with light, and the optical characteristic corresponds to the transmittance of the sample.

The predetermined wavelength is a value obtained by subtracting 30 nm to 100 nm from λ _Eg . For example, λ _Eg −50 nm.

Further, the fitting value F (x) is calculated using λ _Eg or less, a wavelength of 3 points or more and 10 points or less, for example, 5 points at a predetermined wavelength or more.

g (hν) is obtained by multiplying the normalized irradiation light quantity obtained by CPM measurement by an arbitrary number. The normalized irradiation light quantity may be read as the number of photons derived from the normalized irradiation light quantity. At this time, the normalized irradiation is performed so that the fitting value F (x) with the separately measured absorption coefficient is 0.0001 or more and 1 or less, preferably 0.0001 or more and 0.1 or less at λ _Eg or less and a predetermined wavelength or more. Multiply the light quantity by an arbitrary number.

As described above, by setting the fitting value F (x) within the above range, the defect density of the wide gap semiconductor can be evaluated with high accuracy. Specifically, a defect density of 1 × 10 ¹⁵ pieces / cm ³ or less, preferably 1 × 10 ¹⁴ pieces / cm ³ or less can be evaluated.

Here, the wide gap semiconductor refers to a semiconductor having a band gap of 2.2 eV or more and 6.0 eV or less, 2.5 eV or more and 5.0 eV or less, or 2.8 eV or more and 4.0 eV or less.

Examples of the semiconductor include an oxide semiconductor, an organic semiconductor, or a compound semiconductor (excluding an oxide semiconductor).

Even in a wide gap semiconductor, the defect density can be evaluated with high accuracy.

FIG. 6 is a flowchart illustrating a measurement method which is one embodiment of the present invention. The schematic diagram of the CPM measuring apparatus which is 1 aspect of this invention. FIG. 10 shows an absorption coefficient derived from CPM measurement of an oxide semiconductor film.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the following description, and it will be easily understood by those skilled in the art that modes and details can be variously changed. In addition, the present invention is not construed as being limited to the description of the embodiments below. Note that in describing the structure of the present invention with reference to drawings, the same portions are denoted by the same reference numerals in different drawings. In addition, when referring to the same thing, a hatch pattern is made the same and there is a case where it does not attach a code in particular.

In many cases, the voltage indicates a potential difference between a certain potential and a reference potential (for example, a ground potential). Thus, voltage, potential, and potential difference can be referred to as potential, voltage, and voltage difference, respectively.

In this specification, even when expressed as “connected”, in an actual circuit, there may be only a case where there is no physical connection portion and a wiring is extended.

The ordinal numbers attached as the first and second are used for convenience and do not indicate the order of steps or the order of lamination. In addition, a specific name is not shown as a matter for specifying the invention in this specification.

(Embodiment 1)

First, a CPM measuring apparatus will be described with reference to FIG.

FIG. 2 is a schematic diagram of a CPM measuring apparatus. For simplicity, the light path is indicated by an arrow, and the wiring is indicated by a solid line. Light emitted from the lamp 201 enters the sample 205 via the monochromator 202, the filter 203 and the beam splitter 204. In some cases, the filter 203 is not provided or a plurality of filters 203 are provided. The beam splitter 204 transmits and reflects light, and transmits the transmitted light to the sample 205 and the reflected light to the photodiode 210. However, the transmitted light is not necessarily incident on the sample 205 and the reflected light is not incident on the photodiode 210, and may be reversed.

As the lamp 201, for example, a xenon lamp, a mercury lamp, a halogen lamp, or the like may be used. Note that the lamp 201 may be used in combination with the lamps described above. A xenon lamp is preferably used.

Any monochromator 202 may be used as long as it can extract only a narrow range of wavelengths from a wide range of wavelengths.

The filter 203 may be one or more selected from a neutral density (ND) filter, a wedge filter, and a cut filter. The cut filter refers to an optical filter having a function of passing a specific range of wavelengths and attenuating other wavelengths. Moreover, you may use multiple types of each filter. By doing so, the controllability of the irradiation light quantity and / or the irradiation wavelength can be enhanced.

After the light irradiated by the photodiode 210 is changed to an electric current, the electric current can be measured by the lock-in amplifier 209, and the amount of irradiation light can be estimated by the computer 208.

The sample 205 is provided with a wide gap semiconductor, a first electrode, and a second electrode. The first electrode and the second electrode are connected to the DC power source 206 via a resistor, and the photocurrent value can be measured by the lock-in amplifier 207 in parallel with the resistor. Note that the obtained photocurrent value is fed back to the filter 203 via the calculator 208. If the photocurrent value is too high, the transmittance of the filter 203 is lowered to reduce the amount of irradiation light. On the other hand, when the photocurrent value is too low, the transmittance of the filter 203 is increased to increase the amount of irradiation light.

Note that the lock-in amplifier 207 and the lock-in amplifier 209 have a function of amplifying, detecting, and outputting a signal having a specific frequency among the input signals. For this reason, the influence of noise and the like is reduced, and a signal can be detected with high sensitivity.

The first electrode and the second electrode provided in the sample 205 are Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ag, Ta, W, Pt and Au, and nitrides thereof, One or more oxides and alloys may be selected and used in a single layer or a stacked layer. Alternatively, a transparent conductive film including a plurality of types of materials selected from Si, Ti, Ni, Cu, Zn, Ga, In, and Sn may be used. Preferably, a material that does not form an insulating film at the interface with the wide gap semiconductor is selected.

It is preferable that the first electrode and the second electrode have the same layer and the same material because the sample can be easily manufactured.

Note that the first electrode and the second electrode may have a rectangular shape or a comb-like shape. In addition, the shapes of the first electrode and the second electrode can be appropriately modified.

If the wide gap semiconductor provided in the sample 205 is an oxide semiconductor film (such as an In—Ga—Zn—O-based material), an organic compound, a silicon-based compound (such as silicon carbide), a carbon-based compound (such as diamond), or the like. Good.

Next, the CPM measurement method will be described with reference to FIG.

FIG. 1 is a flowchart of a CPM measurement method. Measurement is started in step S101, and measurement conditions (wavelength range, chopper frequency, constant current value) are arbitrarily determined in step S102. The wavelength range and the constant current value must be appropriately selected depending on the type of the wide gap semiconductor that is the target of CPM measurement. The wavelength range is determined from the band gap of a wide gap semiconductor. For example, in the case of a wide gap semiconductor having a band gap of 2.8 eV to 4.0 eV, the wavelength range may be 200 nm to 1200 nm. The chopper frequency corresponds to the switching speed between light irradiation and light shielding. The chopper frequency may be selected from the photoresponsiveness of the wide gap semiconductor. In CPM measurement, charge-up due to light irradiation may be a problem. In that case, the influence of charge-up by light irradiation can be reduced by lowering the chopper frequency. The constant current value is determined in advance by preliminary measurement.

The constant current value is determined by selecting several arbitrary wavelengths from the wavelength range scheduled for measurement. It is preferable to select the wavelength without deviation in the wavelength range in which measurement is planned. Next, a voltage is applied between the first electrode and the second electrode of the sample 205, and the photocurrent value at each wavelength selected in the vicinity of the maximum irradiation light amount of the apparatus is measured. Among the obtained photocurrent values, a current value that is 5% to 30% smaller than the smallest value is determined as a constant current value.

Next, in step S103, a voltage is applied between the first electrode and the second electrode of the sample 205. The voltage between the first electrode and the second electrode is, for example, 0.1 V or more and 50 V or less.

Next, an initial irradiation light amount is determined in step S104. The initial irradiation light amount may be, for example, the maximum irradiation light amount of the apparatus.

Next, in step S105, the sample 205 is irradiated with light blinked by the chopper. The blinking cycle can be selected according to the chopper frequency, and by using the lock-in amplifier 207, it is possible to detect a weak photocurrent value having a frequency similar to the chopper frequency.

In step S106, it is determined whether the photocurrent value detected in step S105 is within a certain range (for example, within plus or minus 10%) of the arbitrary photocurrent value determined in step S102. When the detected photocurrent value is outside a predetermined range of the photocurrent value determined arbitrarily (for example, outside of plus or minus 10%), the irradiation light amount is changed in step S107. Note that the accuracy of CPM measurement increases when the range of the photocurrent value determined in step S106 is narrowed, and the accuracy of CPM measurement decreases when it is widened. However, the narrower the range of the photocurrent value determined in step S106, the longer it takes to control the photocurrent value, and the longer the time for CPM measurement.

An ND filter or / and a wedge filter may be used for changing the irradiation light quantity.

When the irradiation light quantity is changed in step S107, the process returns to step S105 again to detect the photocurrent value. In step S106, it is determined whether the detected photocurrent value is within ± 10% of the arbitrary photocurrent value determined in step S102. When the detected photocurrent value is out of plus or minus 10% of the arbitrarily determined photocurrent value, the amount of irradiation light is changed in step S107. This is repeated until the photocurrent value determined in step S102 is within ± 10%.

If it is determined in step S106 that the detected photocurrent value is within ± 10% of the arbitrary photocurrent value determined in step S102, the process proceeds to step S108, and all measurements in an arbitrary wavelength range are performed. Judgment is made. If it is determined that all the measurements in the arbitrary wavelength range have not been performed, the wavelength of the irradiation light is changed in step S109, and the process returns to step S105.

In addition, it is preferable to provide a cut filter according to the wavelength of irradiation light. As the wavelength of irradiation light is changed in step S109, a step for changing to a cut filter having an optimum wavelength range may be added. By changing the cut filter to a cut filter having an optimal wavelength range, the influence of stray light can be reduced, and CPM measurement can be performed with high accuracy.

Steps S105 to S109 are repeated, and if it is determined in step S108 that all wavelength ranges have been measured, the process proceeds to step S110 and the CPM measurement is terminated.

The above is an example of the CPM measurement method.

The amount of irradiation light thus obtained for each wavelength is set as a normalized amount of irradiation light normalized by the shape and optical characteristics of the sample. By multiplying the normalized irradiation light quantity by an arbitrary number and fitting with an absorption coefficient for each wavelength measured separately within a range of approximately λ _Eg or less of the wide gap semiconductor, it can be handled as an absorption coefficient.

In addition, what is necessary is just to derive | lead-out the absorption coefficient for every wavelength from measurements, such as a transmittance | permeability and a reflectance by a spectroscopic ellipsometry or a spectrophotometer.

Since the absorption coefficient obtained from the CPM measurement is derived from the measurement of the current value, it is possible to evaluate the absorption of a trace amount of light that makes it difficult to measure the number of photons. Therefore, the CPM measurement can derive the absorption coefficient with high accuracy in the range of λ _Eg or less of the wide gap semiconductor, as compared with spectroscopic ellipsometry, spectrophotometer, and the like.

Therefore, a low-density defect level within the band gap of the wide gap semiconductor can be evaluated with high accuracy.

In addition, the value obtained by CPM measurement is recorded as the irradiation light quantity when it becomes a constant current value. Therefore, the absorption coefficient is derived from the normalized irradiation light amount. Specifically, it may be calculated using the irradiation area of the sample and the transmittance of the sample.

In general, the absorption coefficient is derived by fitting the normalized irradiation light quantity obtained by CPM measurement by an arbitrary number and fitting the absorption coefficient measured separately and in an appropriate range. At that time, the absorption coefficient derived from the CPM measurement and the absorption coefficient measured separately are in the range of λ _Eg or less of the wide-gap semiconductor, or only one point or two points coincide with each other, or very slightly In the case of matching only in a wide energy range, it is difficult to say that the absorption coefficient derived from the CPM measurement represents the actual absorption coefficient, and naturally, the defect density and accuracy to be evaluated are low. .

Therefore, λ _Eg or less of the wide gap semiconductor, the normalized irradiation light quantity obtained by CPM measurement at a predetermined wavelength or more (g (hν)), and λ _Eg or less of the wide gap semiconductor measured separately. The fitting value F (x) in Equation (1) with the absorption coefficient f (hν) at a predetermined wavelength or more is 0.0001 or more and 1 or less, preferably 0.0001 or more and 0.1 or less. hν) is changed. For the formula (1), the above description is taken into consideration.

The above-mentioned predetermined wavelength or more is a value obtained by subtracting 30 nm to 100 nm from λ _Eg . For example, λ _Eg −50 nm.

Also, the fitting value is calculated using λ _Eg or less, a wavelength of 3 points or more and 10 points or less, for example, 5 points at a predetermined wavelength or more.

g (hν) is obtained by multiplying the normalized irradiation light quantity obtained by CPM measurement by an arbitrary number. At this time, the normalized irradiation is performed so that the fitting value F (x) with the separately measured absorption coefficient is 0.0001 or more and 1 or less, preferably 0.0001 or more and 0.1 or less at λ _Eg or less and a predetermined wavelength or more. Multiply the light quantity by an arbitrary number.

As described above, by setting the fitting value within the above range, the defect density of the wide gap semiconductor can be evaluated with high accuracy. Specifically, a defect density of 1 × 10 ¹⁵ pieces / cm ³ or less, preferably 1 × 10 ¹⁴ pieces / cm ³ or less can be evaluated.

This embodiment can be used in combination with any of the other examples as appropriate.

In this example, a method for deriving the absorption coefficient of an oxide semiconductor that is a wide gap semiconductor from CPM measurement will be described.

As a sample to be measured, a silicon oxide film with a thickness of 300 nm is formed over a glass substrate, an oxide semiconductor film with a thickness of 200 nm is formed over the silicon oxide film, and a pair of tungsten is formed over the oxide semiconductor film. An electrode is formed to a thickness of 100 nm, an aluminum oxide film is formed to a thickness of 100 nm so as to cover the oxide semiconductor film and the pair of tungsten electrodes, and then a part of the aluminum oxide film is etched to form a pair of tungsten electrodes It was produced by exposing

Here, an In—Ga—Zn—O film was used as the oxide semiconductor film.

CPM measurement was performed according to the flow shown in FIG. The measurement range is 200 nm to 1200 nm.

Next, normalization is performed so that the absorption coefficient derived from the spectrophotometer and the normalized irradiation light amount for each wavelength obtained by the CPM measurement substantially coincide with each other at λ _Eg or less and a predetermined wavelength or more of the oxide semiconductor film. The irradiation light quantity was multiplied by an arbitrary number.

Next, using Equation (1), the absorption coefficient f (hν) derived from the spectrophotometer and the normalized irradiation light amount obtained by the CPM measurement at λ _Eg or less and a predetermined wavelength or more of the oxide semiconductor film The arbitrary number was changed until g (hν) multiplied by an arbitrary number and the fitting value F (x) became 0.0001 or more and 1 or less, preferably 0.0001 or more and 0.1 or less.

Thus, when F (x) reached 0.088, the absorption coefficient derived by CPM measurement was determined. The results are shown in FIG. In FIG. 3, the horizontal axis indicates a value obtained by converting the wavelength into energy.

As shown in FIG. 3, an absorption coefficient derived from a spectrophotometer (broken line) and an absorption coefficient derived from CPM measurement (solid line) at λ _Eg or less, a predetermined wavelength or more (energy range of band gap or more), and It turns out that it is fitting well.

Therefore, it can be seen from the absorption coefficient derived from the CPM measurement in this example that the defect density of the oxide semiconductor film can be evaluated with high accuracy.

S101 Step S102 Step S103 Step S104 Step S105 Step S106 Step S107 Step S108 Step S109 Step S110 Step 201 Lamp 202 Monochromator 203 Filter 204 Beam splitter 205 Sample 206 DC power supply 207 Lock-in amplifier 208 Computer 209 Lock-in amplifier 210 Photodiode

Claims (4)

Applying a voltage between the pair of electrodes to a sample having a pair of electrodes on a semiconductor having a band gap of 2.2 eV or more and 6.0 eV or less;
Irradiating light of an arbitrary wavelength to the semiconductor periodically to detect a photocurrent value flowing between the pair of electrodes;
A step of the photocurrent values Ru changing irradiation light amount to be constant current value,
Recording the arbitrary wavelength and the irradiation light amount;
The irradiation light intensity in the wavelength range below wavelength than the band gap of the prior SL 50nm than the wavelength of the bandgap wavelength shorter than the upper front Symbol semiconductor, and normalized to the sample, the steps of the normalized light quantity,
The absorption coefficient of the semiconductor before Symbol wavelength range, spectroscopic ellipsometry Ma et deriving from the measurement of transmittance and reflectance by a spectrophotometer, a,
Below shown in equation (1), wherein as the fitting value F between the normalized light quantity and the absorption coefficient (x) satisfies 0.0001 or more and 1 or less, Jill can multiply any number to the normalized irradiation dose A defect evaluation method for a semiconductor characterized.

(However, the lambda _Eg represents the wavelength of the band gap of the semiconductor, f (hv) shows a front Ki吸 yield coefficients, g (hv) shows are multiplied by the arbitrary number of prior Symbol normalization irradiation dose .) In claim 1,
When the photocurrent value is within a certain range from the constant current value, record the arbitrary wavelength and the irradiation light amount, and change the arbitrary wavelength,
When the photocurrent value is out of a certain range from the constant current value, the amount of irradiation light of the arbitrary wavelength is changed,
A semiconductor defect evaluation method, wherein the wavelength of light having an arbitrary wavelength and the amount of irradiation light are recorded in a predetermined wavelength range. In claim 1 or claim 2,
A semiconductor defect evaluation method, wherein the semiconductor is an oxide semiconductor. In claim 1 or claim 2,
A semiconductor defect evaluation method, wherein the semiconductor is an organic semiconductor or a compound semiconductor.