JP2004071867A - How to measure the amount of mask attached - Google Patents

Abstract

An object of the present invention is to provide a method for quantitatively and easily measuring the amount of a contaminant attached to a mask for irradiating a charged particle beam to an object through a mask.
In the method, after a wafer is irradiated with an electron beam a predetermined number of times using a stencil mask, a mask stage is moved, and the stencil mask is shifted from the optical axis center of the electron optical system to change the surface potential. It is moved below the meter probe 32. Next, the surface potential of the stencil mask is measured. The surface electrons of the stencil mask are measured before and after the electron beam irradiation, and the difference in surface potential is compared to detect the amount of contamination attached to the stencil mask. In advance, the limit thickness Tth of the adhesion layer of the contamination substance and the correlation between the limit thickness Tth of the adhesion layer of the contamination substance and the limit surface potential Vth are previously established by experiments or the like, and then the measured surface potential is limited. If the surface potential is lower than Vth, it can be seen that the amount of the contaminant attached to the stencil mask 26 is equal to or less than the allowable amount.
[Selection diagram] Fig. 1

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for measuring the amount of attached mask, and more particularly, to quantitatively and easily determine the amount of attached contaminant attached to a mask for irradiating a charged particle beam to an irradiation target via the mask. It relates to a method of measuring.
[0002]
[Prior art]
As an exposure apparatus for forming a next-generation pattern having a fine line width, an exposure method for forming a pattern by, for example, equal-size proximity exposure of a low-energy electron beam is being developed.
In the equal-size proximity exposure, a mask having a pattern of the same size as the pattern formed on the wafer is used. The mask is arranged at a position very close to the wafer about 50 μm above the wafer covered with the resist film. For example, in order to form an extremely fine pattern of 100 nm or less, the mask is also 100 nm or less. It is necessary to form an extremely fine pattern.
[0003]
As an example of the 1: 1 proximity exposure apparatus, for example, LEEPL (Low Energy E-beam Proximity Projection Lithography) can be used. Here, with reference to FIG. 4, the configuration of a 1: 1 proximity electron beam exposure apparatus will be described using LEEPL as an example. FIG. 4 is a schematic diagram showing the configuration of the LEEPL system.
The LEEPL system 10 is one of the same-size proximity electron beam exposure apparatuses, and includes an electron gun 12 for emitting an electron beam B, an aperture 14 for limiting the spread of the electron beam B, and an electron beam B as shown in FIG. Has a condenser lens 16 for making the beam parallel, a set of main deflectors 18 and 20 configured as a pair, and a pair of sub deflectors 22 and 24 for fine adjustment of the electron beam.
Further, the LEEPL system 10 has a mask stage 28 for holding a stencil mask 26 below the sub deflector 24, and further holds the wafer W close to below the stencil mask 26, and moves the wafer W in the X and Y directions. It has a stage 30.
[0004]
The sets of main deflectors 18 and 20 have the function of deflecting the electron beam B so that it remains parallel and in either raster or vector scanning mode and is incident perpendicular to the stencil mask 26.
The wafer W is mounted on the wafer stage 30 below the stencil mask 26 at a distance D (D = about 50 μm).
[0005]
The electron beam B emitted from the electron gun 12 is subjected to predetermined deflection control by a set of main deflectors 18 and 20 and sub deflectors 22 and 24, passes through a stencil mask 26, and is applied onto a wafer W. Reaches the resist film thus formed.
Since the electron beam cannot pass through an area other than the opening pattern which is the same size as the circuit pattern provided on the stencil mask 26, only the electron beam passing through the opening pattern changes the shape of the circuit pattern into a latent image on the resist film. Transcribe as
[0006]
Compared to the so-called photolithography using a krypton fluoride (KrF) laser with a wavelength of 248 nm or an argon fluoride (ArF) laser with a wavelength of 193 nm, the wavelength of an electron beam is as short as several nm. Therefore, in electron beam exposure, the pattern size can be reduced.
In addition, by performing close proximity exposure using a stencil mask as an exposure mask, higher throughput can be realized as compared with a conventional direct writing method using an electron beam.
[0007]
By the way, in the 1: 1 proximity exposure method represented by LEEPL, as described above, since the mask is arranged close to the position of about 50 μm from the wafer, particles generated from the resist film on the wafer at the time of pattern drawing are formed. Is more likely to adhere to the mask as compared to a mask arranged at a distance from the wafer.
Then, when particles adhere to these masks serving as a master for manufacturing a semiconductor device to form a mask defect, the mask defect is transferred as a defect to all semiconductor devices patterned using this mask. If the defect transferred to the semiconductor device is a defect that affects the performance of the semiconductor device, the semiconductor device is defective.
For this reason, it is usually necessary to inspect all the masks used as the original masters for foreign substances such as carbon-based contaminants as mask defects.
[0008]
Further, as the miniaturization of the semiconductor device progresses, the size of the problematic defect also becomes finer. In particular, in the electron beam equal-magnification exposure and X-ray equal-magnification exposure typified by the above-described LEEPL and the like, the pattern size of the mask is the same as the pattern size of the semiconductor device. It becomes a problem.
[0009]
When the mask is continuously irradiated with the electron beam, as described above, the contamination substance adheres to the electron beam irradiation area on the mask. The contamination substance is mainly a hydrocarbon-based organic compound, and the adhesion of the contamination substance causes a dimensional change of the exposed pattern at the time of pattern exposure.
Variations in the dimensions of the exposed pattern are caused by the contaminant adhering, which reduces the size of the pattern opening on the mask, reducing the number of electrons transmitted through the opening, and charging the mask with the contaminant. , There is a cause.
Therefore, it is necessary to remove the contaminant before the pattern dimension variation after exposure occurs, and for that purpose, a method for quantitatively and easily measuring the amount of the contaminant adhering is required.
[0010]
2. Description of the Related Art As a method of measuring the amount of a mask attached to a contamination substance, a method of measuring the thickness of an adhesion layer of the contamination substance using a stylus type step meter such as an AFM has been conventionally known.
There is also a method of calculating a contamination substance attached to a stencil mask used for exposure from a variation amount of a pattern dimension after exposure.
[0011]
[Problems to be solved by the invention]
However, the conventional method of measuring the amount of the contaminant attached has the following problems.
The first problem with the method using the stylus-type step gauge is that it is difficult to put the probe into practical use because there is a risk of pattern collapse due to the probe needle coming into contact with the adhesion layer.
Second, a dedicated device must be used to measure the thickness of the contaminant-adhered layer, which increases the cost and makes it difficult to improve the throughput.
Thirdly, since the mask to be measured must be removed from the vacuum chamber once when measuring the amount of the contaminant attached, fine particles in the air adhere to the removed mask, and the contaminant adheres. The problem is that it is difficult to measure the amount accurately.
[0012]
In addition, in the method of measuring the amount of the contaminant attached from the variation in the pattern dimension after exposure, it is not only difficult to quantify the amount of the contaminant adhered, but also the exposed wafer having a desired size is required. Since the pattern is not formed, there is a problem that the exposed wafer is wasted.
[0013]
As described above, no satisfactory method has yet been found for the conventional methods for measuring the amount of deposition of the contamination substance.
Therefore, an object of the present invention is to provide a method for quantitatively and easily measuring the amount of a contaminant attached to a mask for irradiating an irradiation target with a charged particle beam through the mask. .
[0014]
[Means for Solving the Problems]
The present inventor paid attention to the chargeability of the contamination substance when developing a method for measuring the amount of the contamination substance adhered to the stencil mask.
That is, the contamination substance attached to the stencil mask surface is generally positively or negatively charged. The surface potential of the region to which the contamination substance is attached is higher than the surface potential of the region to which the contamination substance is not attached so that it can be identified.
[0015]
Assuming that the contamination material is a dielectric and the potential of the electron beam irradiation part of the stencil mask is a ground potential, the potential V induced on the surface of the contamination material by charging can be expressed as follows.
V = (dQ) / (χε ₀ S)
Here, d is the thickness of the contaminant adhering layer, Q is the charge on the surface of the contaminant, χ is the relative permittivity of the contaminant, ε ₀ is the permittivity in a vacuum, and S is the adhering area of the contaminant. It is.
[0016]
For example, when d = 30 nm, Q = 4.8 × 10 ⁻⁵ C, χ = 3.0, and S = 1.6 × 10 ⁻³ m ² , the surface potential V becomes 33.9 V. This value is a value that can be sufficiently measured by a general surface electrometer.
In other words, the thickness of the contamination adhesion layer can be measured by measuring the surface potential of the contamination adhesion layer with a surface voltmeter.
[0017]
Therefore, the critical thickness Tth of the adhesion layer of the contamination substance and the surface potential when the thickness of the contamination layer of the contamination substance is the critical thickness Tth are defined as the critical surface potential Vth. If the measured surface potential is lower than the limit surface potential Vth, it can be understood that the amount of the contaminant attached to the stencil mask 26 is not more than the allowable amount.
Here, the critical thickness Tth of the contaminant adhesion layer is defined as: When the thickness of the contaminant adhesion layer is equal to or greater than the critical thickness Tth, close-to-one-size exposure of an electron beam through the stencil mask 26 is performed. When the pattern size variation exceeds the allowable value. Further, the thickness of the adhesion layer that is thinner than the thickness where the pattern dimension variation exceeds the allowable value may be used as the limit thickness Tth.
[0018]
In order to achieve the above object, a method for measuring a mask adhesion amount according to the present invention includes a method for measuring an adhesion amount of a contamination substance attached to a mask for irradiating a charged particle beam to an irradiation target through the mask. And
A first step of establishing a correlation between the surface potential of the mask and the amount of the contamination substance attached to the mask;
Stopping the irradiation of the charged particle beam, measuring the surface potential of the mask with a surface voltmeter, and calculating the amount of adhesion of the contamination substance on the mask according to the correlation based on the measured surface potential. It is characterized by:
[0019]
In the method of the present invention, according to the correlation between the surface potential of the mask and the amount of the masking substance adhering to the mask, which has been established in advance through experiments or the like, the amount of the contaminating substance adhered to the mask is quantitatively determined based on the measured surface potential of the mask. Measurement can be performed easily and easily.
In addition, since the amount of the contaminant attached can be measured in a non-contact manner by a surface voltmeter, the pattern of the mask to be measured is not destroyed by the measurement.
Further, since the probe of the surface electrometer is usually large enough to be accommodated in the vacuum container of the exposure apparatus, it is possible to measure the amount of the contaminant attached to the mask in the vacuum container. Therefore, unlike the related art, since it is not necessary to expose the mask to the atmosphere, there is no possibility that the contamination substance in the atmosphere adheres to the mask and an error occurs in the measurement of the amount of adhesion.
[0020]
In the second step of the preferred embodiment of the method of the present invention, the surface potential of the mask is measured while moving the mask with respect to the surface voltmeter.
Accordingly, the surface potential can be measured over the entire surface of the mask, that is, without distinction between the charged particle beam irradiation surface and the non-irradiation surface of the mask, and the distribution of the amount of the contaminant attached to the entire mask can be quantitatively measured.
[0021]
In the first step of the preferred embodiment of the method of the present invention, the critical layer thickness Tth, which is the allowable layer thickness of the contamination layer, and the surface potential Vth when the thickness of the contamination layer is Tth. (Hereinafter referred to as the critical surface potential),
In the second step, if the measured surface potential is lower than the threshold surface potential Vth, it is assumed that the amount of the contaminant attached to the mask is equal to or less than the allowable amount.
The critical layer thickness Tth refers to the thickness of the adherent layer that causes a problem in the irradiation result of the charged particle beam when the thickness of the contaminant adherent layer is equal to or greater than the critical layer thickness Tth. For example, when a pattern is formed by exposure to a charged particle beam, when the thickness of the adhesion layer is equal to or greater than the limit layer thickness Tth, the thickness of the adhesion layer is such that the shape accuracy of the formed pattern is equal to or less than an allowable value. Say However, it is free to set a thin layer thickness equal to or smaller than the allowable layer thickness as the limit layer thickness Tth.
[0022]
In a further preferred embodiment of the method of the present invention, when exposing a predetermined number of wafers by a charged particle beam exposure apparatus,
In the first step, in addition to the limit surface potential Vth, a charge amount Vc per wafer charged on a mask when one wafer is exposed at the time of exposure is obtained.
In the second step, it is checked whether or not the number of exposed wafers has reached a predetermined number. If the number has not yet reached the predetermined number, the exposed wafer is unloaded from the charged particle beam exposure apparatus, and then the mask surface is exposed. Measuring the potential V,
The magnitudes of the surface potential V and Vth−Vc are compared. If Vth−Vc> V, the exposure of the wafer is repeated. If Vth−Vc ≦ V, the mask is washed, and the surface potential V of the mask is reduced again. Return to the step of measuring.
[0023]
When applying the method of the present invention, the charged particle beam irradiation device is not limited as long as it is a device for irradiating a charged particle beam to an irradiation target, and may be an exposure device, a scanning electron microscope, a defect inspection device, a defect correction device, and the like. It can be suitably applied to
[0024]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described specifically and in detail with reference to the accompanying drawings by way of example embodiments.
Embodiment 1
This embodiment is an example of an embodiment in which the method for measuring the amount of attached mask according to the present invention is applied to a stencil mask used in LEEPL. FIG. 1 measures the amount of attached mask using the method of this embodiment. FIG. 4 is a schematic diagram illustrating a method for performing the method.
In the present embodiment, a predetermined number of wafers are irradiated with an electron beam using a stencil mask, and then, as shown in FIG. 1, the mask stage 28 is moved to move the stencil mask 26 to the optical axis of the electron optical system. And moved below the surface electrometer probe 32.
Next, the surface potential on the membrane side of the stencil mask 26 is measured by the surface voltmeter 34 via the surface voltmeter probe 32.
The surface electrons of the stencil mask 26 are measured before and after the electron beam irradiation, and the difference in the surface potential is compared to detect the amount of contamination adhering to the stencil mask 26.
[0025]
In the present embodiment, the limit thickness Tth of the adhesion layer of the contamination material, and the correlation between the limit thickness Tth of the adhesion layer of the contamination material and the limit surface potential Vth are previously established by experiments and the like, If the measured surface potential is lower than the threshold surface potential Vth, it can be understood that the amount of the contaminant attached to the stencil mask 26 is not more than the allowable amount.
Here, the critical thickness Tth of the contaminant adhesion layer is defined as: When the thickness of the contaminant adhesion layer is equal to or greater than the critical thickness Tth, close-to-one-size exposure of an electron beam through the stencil mask 26 is performed. When the pattern size variation exceeds the allowable value.
[0026]
Embodiment 2
The present embodiment is another example of an embodiment in which the method for measuring the amount of attached mask according to the present invention is applied to a stencil mask used in LEEPL. FIG. 2 shows the amount of attached mask using the method of this embodiment. It is a schematic diagram explaining the method of measuring.
In the present embodiment, a predetermined number of wafers are irradiated with an electron beam using a stencil mask, and then, as shown in FIG. 2, the mask stage 28 is moved to move the stencil mask 26 to the optical axis of the electron optical system. It is moved off the center and below the surface electrometer probe 32.
In the present embodiment, the surface potential on the membrane side of the stencil mask 26 is sequentially measured by the surface voltmeter 34 via the surface voltmeter probe 32 while moving the mask stage 28.
[0027]
The surface electrons of the stencil mask 26 are measured before and after the electron beam irradiation, and the difference in the surface potential is compared to detect the amount of contamination adhering to the stencil mask 26.
In the present embodiment, as shown in FIG. 2, since the surface potential of the stencil mask 26 is measured while moving the mask stage 28, the surface potential of the stencil mask 26 on the electron beam irradiated portion and the unirradiated portion is measured. Is easy. By comparing the surface potential value of the surface potential of the electron beam irradiated part and the surface potential of the unirradiated part, it is possible to detect the presence or absence of the contamination substance and the amount of the contamination.
[0028]
Embodiment 3
This embodiment is another example of an embodiment in which the method for measuring the amount of mask attached according to the present invention is applied to a stencil mask used in LEEPL. FIG. 3 shows the amount of mask attached by the method of this embodiment. 6 is a flowchart for explaining a procedure when measuring is measured.
First, in step S _1, in advance, by exposing the evaluation wafer through a stencil mask with an electron beam exposure apparatus, the contaminants adhering amount, i.e. between the surface potential of the layer thickness and the stencil mask of the deposited layer The correlation, or the correlation between the surface potential and the change in the resist dimensions after exposure, is clarified by experiments.
After repeated exposure evaluation wafer in step S _1, in step S _2, the limit surface potential Vth which affect the size of the resist pattern after exposure, and the stencil mask 26 upon exposure to one wafer during exposure The charge amount Vc to be charged is obtained.
[0029]
In step S _3, loads the exposure target wafer to an electron beam exposure apparatus.
In Step S _4, pattern exposure on the wafer through a stencil mask.
In step S _5, the exposure the number of wafers to determine whether reaches a predetermined number. Upon reaching a predetermined number terminates the pattern exposure proceeds to step S _6. Yet, when it does not reach the predetermined number, the process proceeds to step S _7, unloading the exposed wafer from the electron beam exposure apparatus.
[0030]
In the present embodiment, in the course of using the stencil mask has implemented an exposure process, for example, every the wafer unloading, to measure the surface potential V of the stencil mask in step S _8.
Further, in step S _1, by previously establishing the relationship between the surface potential of the thickness and the stencil mask of the deposit of contaminants in the stencil mask, on the basis of the surface potential V measured in step S _8, contamination The thickness of the deposited layer of the substance can be detected.
[0031]
In step _{S 9,} it compares the magnitude of the surface potential V and Vth-Vc. If the Vth-Vc> V, returns to the step _{S 3,} repeating the exposure of the wafer. Further, if Vth-Vc ≦ V, washing the stencil mask proceeds to step _{S 10,} then the process returns to step _{S 8,} to measure the surface potential V of the stencil mask.
If step S ₈ the surface potential V measured at that was Vth-Vc or more, in the course of exposing the wafer to the next exposure, the surface potential exceeds the Vth, occurs a bad effect on the transfer of the pattern. Therefore, when the surface potential is equal to or higher than Vth-Vc, exposure of the wafer is interrupted, and contamination removal such as cleaning of the mask is performed.
[0032]
In the present embodiment, the use of the mask to which the contamination substance has adhered can be prevented before the state in which the exposure is adversely affected, so that a defect of the exposure pattern due to the adhesion of the contamination substance can be prevented.
In addition, since the amount of the contaminant attached can be measured in the exposure apparatus, the work of loading and unloading the mask is not required. Therefore, an improvement in throughput in exposure can be expected.
[0033]
【The invention's effect】
According to the method of the present invention, unlike a conventional stylus-type measuring device such as an AFM, the surface potential of the mask is measured by a surface voltmeter, and the contamination substance on the mask is adhered according to the correlation based on the measured surface potential. The so-called non-contact measurement of the amount of the contaminant attached can be performed to calculate the amount.
Therefore, the pattern of the mask to be measured is not destroyed by the measurement.
In addition, since the probe of the surface electrometer used in the method of the present invention is large enough to be accommodated in the vacuum vessel of the exposure apparatus, it is possible to measure the amount of contamination of the mask in the vacuum vessel. . Therefore, since it is not necessary to expose the mask to the atmosphere, there is no possibility that the contamination substance in the atmosphere adheres to the mask and an error occurs in the measurement of the amount of adhesion as in the related art. Further, since it is not necessary to take out the mask to be measured from the vacuum container for each measurement or to perform a work such as evacuation associated therewith, the exposure throughput can be improved.
[Brief description of the drawings]
FIG. 1 is a schematic diagram for explaining a method for measuring the amount of mask attached by the method of Embodiment 1;
FIG. 2 is a schematic diagram for explaining a method of measuring the amount of mask attached by the method of Embodiment 2;
FIG. 3 is a flowchart showing a procedure for measuring the amount of mask attached by the method of Embodiment 3;
FIG. 4 is a schematic diagram showing a configuration of a LEEPL system.
[Explanation of symbols]
10 LEEPL system, 12 Electron gun, 14 Aperture 14, 16 Condenser lens, 18, 20 Main deflector, 22, 24 Sub deflector, 26 Stencil mask, 28 ... Mask stage, 30 ... Wafer stage, 32 ... Surface voltmeter probe, 34 ... Surface voltmeter.

Claims (4)

A method for measuring the amount of a contaminant attached to a mask for irradiating an irradiation target with a charged particle beam through a mask,
A first step of establishing a correlation between the surface potential of the mask and the amount of the mask adhering to the contamination substance;
A second step of stopping the irradiation of the charged particle beam, measuring the surface potential of the mask with a surface potentiometer, and calculating the adhesion amount of the contamination substance on the mask according to the correlation based on the measured surface potential; and A method for measuring a mask adhesion amount, comprising: 2. The method according to claim 1, wherein in the second step, a surface potential of the mask is measured while moving the mask with respect to the surface voltmeter. 3. In the first step, the critical layer thickness Tth, which is the allowable layer thickness of the contamination layer, and the surface potential Vth when the layer thickness of the contamination layer is Tth (hereinafter, referred to as the critical surface potential). )
3. The method according to claim 1, wherein in the second step, if the measured surface potential is less than the critical surface potential Vth, the amount of the contaminant attached to the mask is equal to or less than an allowable amount. 4. 3. The method for measuring the amount of mask attached described in 1. When exposing a predetermined number of wafers by a charged particle beam exposure apparatus,
In the first step, in addition to the limit surface potential Vth, a charge amount Vc per wafer charged on a mask when one wafer is exposed at the time of exposure is obtained.
In the second step, it is checked whether the number of exposed wafers has reached a predetermined number, and if the number has not yet reached the predetermined number, after unloading the exposed wafer from the charged particle beam exposure apparatus, Measuring the surface potential V of the mask,
The magnitudes of the surface potential V and Vth−Vc are compared. If Vth−Vc> V, the exposure of the wafer is repeated. If Vth−Vc ≦ V, the mask is washed, and the surface potential V 4. The method according to claim 3, wherein the method returns to the step of measuring the amount of mask attached.

Images