JP2005191166A - Projection aligner and projection exposure method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a projection aligner capable of preventing variations in the amount of light exposure caused by contaminations, or the like adhering to a reticle; and to provide a projection exposure method. <P>SOLUTION: Before exposure is started, a reticle supporting rack is filled with purge gas made of inert gas containing prescribed active gas, and fixed light beams are applied, thus recovering transmittance in the reticle deteriorated by the contaminations, and then obtaining real exposure time for executing exposure. As a result, variations in line width caused by a change in transmittance during the exposure of the reticle is suppressed, and the controllability of the line width can be improved. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a projection exposure apparatus and a projection exposure method, and more particularly, to improve line width control by suppressing "variation" of line width due to change in transmittance of a reticle in manufacturing a semiconductor integrated circuit or the like. The present invention relates to a projection exposure apparatus and a projection exposure method.

In manufacturing a semiconductor integrated circuit, a projection exposure apparatus plays an important role as an apparatus for transferring a predetermined pattern onto a semiconductor wafer.
FIG. 7 is a schematic diagram showing the main configuration of the projection exposure apparatus. That is, the projection exposure apparatus 200 includes an illumination optical system 11, a reticle support 13, a projection optical system 14, and a wafer stage 15. A reticle, that is, a photomask 12 is placed on the reticle support 13. A semiconductor wafer 16 is placed on the wafer stage 15.

  Light emitted from an exposure light source (not shown) passes through the illumination optical system 11, and the mask pattern on the reticle 12 is projected and exposed onto the wafer 16 via the projection optical system 14. An illuminance monitor 17 is provided on or near the wafer stage 15, and the detected value is output to the illuminance control unit 18 to control the illuminance, and the actual exposure time (shutter open time) is determined.

  FIG. 8 is a flowchart showing an algorithm for determining the actual exposure time. That is, in step S111, the illuminance of the light irradiated onto the wafer 16 is measured by the illuminance monitor 17. Next, in step S112, based on this monitor value, a time for opening the shutter (shutter open time) is determined. That is, the exposure time is determined so that proper exposure can be obtained. Thereafter, in step S113, the shutter is opened and exposure is started. In this manner, appropriate exposure according to the illuminance of the light irradiated on the wafer 16 can be realized.

  By the way, with the recent miniaturization of design rules for patterns of semiconductor integrated circuits, the exposure wavelength has been shortened. The currently mainstream light source is a KrF (krypton / fluorine) excimer laser with a wavelength of 248 nm, followed by an ArF (argon / fluorine) excimer laser with a wavelength of 193 nm. Next to the ArF excimer laser, an F2 (fluorine) laser (wavelength 157 nm) is listed as a candidate.

As a projection exposure apparatus using an F2 (fluorine) laser (wavelength 157 nm), there is disclosed a technique for removing contamination of an objective lens due to outgas from a resist applied to a wafer 16 by oxygen and ultraviolet light ( Non-patent document 1).
Proc. SPIE Vol. 4691 (2002) pp. 1665-1674

  However, since the F2 laser light is absorbed by oxygen and moisture in the atmosphere, it is necessary to purge the irradiation space with an inert gas such as nitrogen or helium. In other words, it is necessary to purge the spaces such as the illumination optical system 11, the reticle support base 13, and the projection optical system 14 of the projection exposure apparatus with an inert gas such as nitrogen or helium.

  However, as a result of the study by the present inventor, it has been found that there is a problem that “variation” occurs in an appropriate exposure amount for the wafer 16 even if purging with an inert gas is performed in this way. As a result of investigating the cause, the surface of the reticle (photomask) 12 is penetrated because contaminants (contaminants) such as organic substances and inorganic substances from the outside of the apparatus and contaminants from the structure inside the apparatus adhere to the surface. It has been found that this is because the transmittance decreases and the transmittance changes during exposure. Such contaminants may adhere, for example, when a reticle is loaded into a special case, or during transportation or storage.

  The present invention has been made on the basis of recognition of such a problem, and an object thereof is to provide a projection exposure apparatus and a projection exposure method capable of preventing “variation” of an exposure amount due to a contaminant attached to a reticle. There is.

  In order to achieve the above object, according to the present invention, before the start of exposure, the reticle support table is filled with a purge gas composed of an inert gas containing a predetermined active gas, and a constant light beam irradiation is performed, thereby causing contamination by contaminants. After recovering the lowered transmittance of the reticle, the exposure is performed for the actual exposure time. In this way, the “variation” of the line width due to the change in transmittance during the exposure of the reticle can be suppressed, and the controllability of the line width can be improved.

That is, the projection exposure apparatus of the present invention includes a light source, an optical system, a reticle stage capable of holding a reticle, a gas supply means capable of maintaining the periphery of the reticle in an atmosphere containing an active gas, and the reticle A transmittance measuring means capable of measuring a change in transmittance, and a wafer stage capable of mounting a wafer,
Light cleaning can be performed to remove contaminants attached to the reticle by irradiating light onto the reticle from the light source while maintaining the atmosphere around the reticle by the gas supply means in an atmosphere containing the active gas. It is characterized by that.

  Here, an illuminance sensor that measures the illuminance of light irradiated on the wafer, and a control mechanism that determines an exposure time for the wafer from the illuminance measured by the illuminance sensor, the control mechanism comprises: The exposure time may be determined after the light cleaning.

  A concentration measuring mechanism for measuring the concentration of the active gas in the atmosphere around the reticle; and a concentration control mechanism for controlling the concentration of the active gas according to the output of the concentration measuring mechanism; can do.

The active gas may be at least one selected from the group consisting of O ₂ , O ₃ , CO ₂ , CO, nitric oxides (NO _X ), sulfur oxides (SO _X ), and oxygen. There can be.

The atmosphere containing the active gas includes the active gas and an inert gas, and the inert gas may be any selected from the group consisting of N ₂ , Ar, and He. .

  Further, when the change in the transmittance measured by the transmittance measuring means falls below a predetermined value, the optical cleaning can be stopped.

  The transmittance measuring means may measure the transmittance at a portion of a transmittance measuring window provided on the reticle.

  The light source may emit F2 laser light.

  On the other hand, in the projection exposure method of the present invention, the reticle is subjected to optical cleaning for removing contaminants attached to the reticle by irradiating the reticle with light while maintaining the atmosphere around the reticle in an atmosphere containing an active gas. And a second step of performing exposure by irradiating the wafer with light through the reticle, and a second step of performing exposure by irradiating the wafer with light through the reticle. To do.

  Here, the concentration of the active gas in the atmosphere containing the active gas may be 500 ppm or less.

  In addition, purging with an inert gas is performed to remove the active gas contained in the atmosphere around the reticle after the light cleaning and before the exposure of the wafer. be able to.

  According to the present invention, the reticle is caused by organic contaminants or inorganic contaminants adhering to the reticle when it is loaded into the special case, transporting or storing, or contaminants adhering to the structure inside the projection exposure apparatus. It is possible to prevent the problem that the transmittance of the reticle surface of the support table 200 is lowered, the transmittance of the reticle is changed during exposure, and the line width is changed.

  As a result, it is possible to stably carry out microfabrication using short-wavelength light such as F2 laser light, and to stably manufacture highly integrated and high-performance semiconductor devices, There are significant industrial benefits.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a schematic diagram showing a main configuration of a projection exposure apparatus according to an embodiment of the present invention.
That is, the projection exposure apparatus 100 of this embodiment includes an exposure light source 1, an illumination optical system 11, a reticle stage 30, a projection optical system 14, and a wafer stage 15. The reticle stage 30 is provided with a reticle support 13 on which a reticle (photomask) 12 is placed. A semiconductor wafer 16 is placed on the wafer stage 15.

  The light emitted from the exposure light source 1 passes through the illumination optical system 11 and projects and exposes the mask pattern on the reticle 12 onto the wafer 16 via the projection optical system 14. An illuminance monitor 17 is provided on or near the wafer stage 15, and the detected value is output to the illuminance control unit 18 to control the illuminance, and the actual exposure time (shutter open time) is determined.

  Furthermore, in the projection exposure apparatus of the present embodiment, a space filled with a purge gas made of an inert gas containing a predetermined amount of active gas can be formed in the reticle stage 30, and in a state where the purge gas is filled. A certain amount of exposure light can be irradiated.

  For this purpose, the reticle stage 30 is provided with a flow rate adjusting mechanism 19, a purge gas supply port 20, a purge gas recovery port 21, a transmittance measuring device 22, and an oxygen concentration measuring mechanism 23. That is, an inert gas containing an active gas can be supplied to the reticle stage 30 through the purge gas supply port 20.

Here, a gas containing oxygen (O) can be used as the active gas. Specifically, at least one of O ₂ , O ₃ , CO ₂ , CO, nitric oxides (NO _X ), sulfur oxides (SO _X ), and other various organic gases containing oxygen is used as the active gas. Can be used. On the other hand, as the inert gas, any of noble gases of N ₂ , Ar, and He can be used. The concentration of the active gas is preferably 500 ppm or less, as will be described in detail later.

For example, the mechanism of decomposition and removal when the organic substance C _x H _y O _z adheres to the surface of the reticle 12 can be described as follows.
That is, first, when the organic substance C _x H _y O _z is irradiated with F2 (fluorine) laser light (wavelength 157 nm), the following reaction occurs, and an organic radical is formed.

_{C x H y O z + Hν} → C x H y O z *

On the other hand, when oxygen is supplied as the active gas, the following decomposition occurs by irradiation with an F2 (fluorine) laser (wavelength 157 nm).

O ₂ + Hν → O + O

The atomic oxygen thus formed is combined with O 2 gas to form ozone (O ₃ ).

O + O ₂ → O ₃

This ozone forms oxygen radicals (O ^* ) by irradiation with an F2 (fluorine) laser (wavelength 157 nm).

_{O 3 + Hν → O 2 +} O *

The oxygen radicals thus formed react with organic radicals and decompose into gas components having a high equilibrium vapor pressure.

^{_{C x H y O z * +}} O * → CO, CO 2, H 2 O

The gas component decomposed in this manner is desorbed from the surface of the reticle 12 and removed. That is, by irradiating light in a gas atmosphere containing oxygen, organic substances can be decomposed and removed by so-called “ashing”.

  As described above, before the exposure is started, the space enclosing the reticle 12 is filled with a purge gas made of an inert gas containing such an active gas, and is adhered to the surface of the reticle 12 by performing a constant light beam irradiation. The pollutants can be decomposed and removed to restore their transmittance.

  At that time, the transmittance of the reticle 12 can be measured by the transmittance measuring device 22 and the result can be fed back. The transmittance measuring device 22 includes, for example, a primary light measuring device 22a that measures the intensity of light incident on the reticle 12, and a secondary light measuring device 22b that measures the intensity of light transmitted through the reticle 12. By comparing the measurement results of the primary light measuring device 22a and the secondary light measuring device 22b, the transmittance of the reticle 12 can be determined.

  Further, the concentration of the active gas in the purge gas filled in the reticle stage 30 is monitored by the oxygen concentration measurement mechanism 23, and the flow rate adjustment mechanism 19 such as a mass flow controller is controlled according to the output of the oxygen concentration measurement mechanism 23. Therefore, the oxygen concentration can be controlled.

  After recovering the transmittance of the reticle 12 in this way, the exposure can be performed by obtaining the actual exposure time (shutter open time) by the illuminance monitor 17 provided on the wafer stage 15.

FIG. 2 is a flowchart showing an algorithm for determining the exposure time in the projection exposure apparatus of this embodiment.
First, in step S11, the surface of the reticle 12 is optically cleaned. That is, the reticle stage 30 is purged with a gas containing an active gas, and a predetermined amount of light is irradiated to decompose and remove contaminants such as organic substances and inorganic substances attached to the surface of the reticle 12.

  Next, in step S12, the transmittance of the reticle 12 is measured. That is, the light transmittance of the reticle 12 is measured using the transmittance measuring device 22 (22a, 22b).

  Next, in step S13, it is determined whether or not the change in transmittance of the reticle 12 exceeds a predetermined amount. That is, when the transmittance of the reticle 12 is improved to some extent by light cleaning, there is a high possibility that a removable contaminant remains on the surface of the reticle 12. For this reason, it is determined based on the amount of change in the transmittance of the reticle 12 whether further optical cleaning is necessary. In this case, the criterion for determining the amount of change in transmittance can be, for example, 0.1 percent.

When the change amount of the transmittance exceeds a predetermined amount (for example, 0.1%), the process returns to step S11 to perform light cleaning.
On the other hand, when the amount of change in transmittance is equal to or less than a predetermined amount (for example, 0.1%), it is determined that re-light cleaning is not necessary, and the process proceeds to step S14 to measure illuminance. That is, the illuminance of light irradiated on the wafer 16 is measured by the illuminance monitor 17 provided on the wafer stage 15.
Next, in step S15, based on this measurement value, a time for opening the shutter (shutter open time) is determined. That is, the exposure time is determined so that proper exposure can be obtained. Thereafter, in step S16, the shutter is opened and exposure is started. In this manner, appropriate exposure according to the illuminance of the light irradiated on the wafer 16 can be realized.

  Further, at the time of starting exposure in this way, the organic and inorganic contaminants adhering to the reticle 12 are sufficiently washed, so that the contaminants on the surface of the reticle 12 are further removed during the exposure of the wafer 16. Thus, the problem that the exposure amount deviates from an appropriate value can also be prevented.

FIGS. 3A and 3B are graphs illustrating the effect of light cleaning in the present invention. That is, the horizontal axis in FIG. 3A represents the time when the reticle 12 is irradiated with light, and the vertical axis represents the transmittance of the reticle 12. Here, the wavelength of light is 157 nanometers, the irradiation energy is 120 milliwatts (1000 hertz), the irradiation energy density is 0.1 mJ / cm ² , the atmosphere is a mixed gas of oxygen (O ₂ ) and nitrogen (N ₂ ), The oxygen concentration was 10 ppm or less.

  As can be seen from FIG. 3A, a rapid increase in transmittance is observed at the initial stage of light irradiation, and when light is further irradiated, the transmittance gradually increases. In the case of the specific example shown in the figure, the transmittance of the reticle is increased by about 2.5% by light cleaning for 600 seconds. According to the results of experiments by the inventors, it was possible to obtain an increase in transmittance of about 5%.

  In addition, according to the experiment by the present inventors, it is known that when a mixed atmosphere of oxygen and nitrogen is used, a higher cleaning effect can be obtained by increasing the oxygen concentration in the range of about 10 to 50 ppm. .

  FIG. 3B is a graph showing the change in the line width dimension of the pattern obtained corresponding to FIG. The line width shown here is the line width of a resist pattern formed using a positive resist. That is, it corresponds to the width of the part where the resist is not irradiated with light. The target value of the line width is 500 nanometers.

  Referring to FIG. 3B, it can be seen that the line width is about 850 nanometers before the optical cleaning, which is significantly larger than the target value of 500 nanometers. This is because the exposure amount to the resist is less than the appropriate value due to the low transmittance of the reticle.

On the other hand, when light cleaning is performed, it can be seen that the line width sharply decreases and approaches the target value in response to the rapid increase in the transmittance of the reticle seen in the initial stage. Then, by light cleaning for about 100 seconds, the line width is almost close to the target value of 500 nanometers. That is, it can be seen that the line width is stabilized corresponding to the recovery of the transmittance on the reticle surface.
As described above, according to the present invention, the contaminants attached to the reticle can be effectively removed and the transmittance can be recovered by performing optical cleaning in an atmosphere containing an active gas.

  In the case of a so-called “semi-transmissive” or “half-tone” reticle, the margin of variation in transmittance is several percent. That is, if the transmittance of the reticle varies by several percent or more, the “shift” of the line width of the pattern to be formed may exceed the allowable range. On the other hand, the effect of the light cleaning in the present invention can meet such a severe requirement, and the effect is great.

Next, the concentration of the active gas at the time of light cleaning in the present invention will be described.
That is, as a result of experiments by the present inventor, it has been found that the effect of decomposing contaminants attached to the reticle 12 is insufficient even if the concentration of the active gas supplied to the reticle stage 30 is too low or too high.

The results of examining the effect of photocleaning when oxygen (O ₂ ) is used as the active gas and nitrogen (N ₂ ) is used as the inert gas are as follows.

Active gas concentration Light cleaning effect
2ppm ×
5ppm △
10ppm ○
100ppm ○
200ppm ○
500ppm ○
700ppm △
2000ppm ×

Here, as the light cleaning effect, a value represented by “◯” represents a case where an increase of 5% or more was obtained in the transmittance of the reticle, and a value represented by “Δ” represents 1 in the transmittance of the reticle. A case where an increase of not less than 5 percent and less than 5 percent is obtained, and an “x” indicates that the transmittance of the reticle was obtained by less than 1 percent.

  If the oxygen concentration is 5 ppm or less, it is considered that the effect of photocleaning is insufficient because the supply amount of oxygen radicals is insufficient. On the other hand, when the concentration of oxygen is 700 ppm or more, the effect of photocleaning is insufficient because F2 (fluorine) laser (wavelength 157 nm) light is absorbed by the purge gas and does not reach the surface of the reticle 12 sufficiently. Conceivable.

  From the above results, it can be said that the concentration of the active gas in the purge gas in the present invention is preferably 5 ppm or more and 700 ppm or less, more preferably 10 ppm or more and 500 ppm or less.

  FIG. 4 is a schematic view showing a projection exposure apparatus according to a modification of the present invention. In this figure, the same elements as those described above with reference to FIGS. 1 to 3 are denoted by the same reference numerals, and detailed description thereof is omitted.

  In this modification, a light output monitor 25 is provided at the output of the exposure light source 1, and a transmittance monitor 22 c is provided on the wafer stage 15 or in the vicinity of the wafer 16.

  In order to evaluate the effect of light cleaning, it is necessary to measure the change in transmittance of the reticle 12. On the other hand, in this modification, the amount corresponding to the intensity of the primary light incident on the reticle 12 is measured by the light output monitor 25, while the reticle is measured by the transmittance monitor 22 c provided in the vicinity of the wafer 16. The amount corresponding to the intensity of the light transmitted through 12 is measured. The transmittance of the reticle 12 can be relatively evaluated by comparing these measured values.

  FIG. 5 is a schematic diagram showing a specific example of the reticle 12 that can be used in the present invention. That is, in the pattern formation region 12a provided on the reticle 12, a predetermined opening is formed in the light shielding film, and exposure according to the opening shape is possible. However, depending on the type of pattern to be formed (element isolation pattern, trench formation pattern, etc.) and the type of resist used therefor (positive resist, negative resist), most of the pattern formation region 12a is covered with a light shielding film and transmits light. In some cases, only a few openings are formed. In such a case, it is not easy to measure the transmittance of the reticle 12.

  Therefore, in order to reliably and easily measure the transmittance, a transmittance measuring window region 12b from which the light shielding film is removed may be provided in a part of the reticle 12. In this way, even when a low-sensitivity sensor is used or when the transmittance monitor 22c is provided in the vicinity of the wafer 16 as described above with reference to FIG. And it can be easily detected.

  FIG. 6 is a flowchart showing a modified example of the operation of the projection exposure apparatus of the present invention. In this figure, the same elements as those described above with reference to FIGS. 1 to 5 are denoted by the same reference numerals and detailed description thereof is omitted.

  In this modified example, an inert gas purge is performed as step S20 before the illuminance measurement in step S14. That is, when cleaning the light, the reticle stage 30 is purged with a mixed gas of an active gas and an inert gas. If exposure is carried out as it is, a loss due to absorption of exposure light occurs depending on the concentration of the active gas.

  Therefore, in this modification, when the optical cleaning is completed, the supply of the active gas is stopped, and the reticle stage 30 is purged only with the inert gas. In this way, when the wafer 16 is exposed, loss of F2 (fluorine) laser (wavelength 157 nm) light absorbed by the purge gas can be prevented, and exposure can be performed in a shorter time.

  The embodiments of the present invention have been described above with reference to specific examples. However, the present invention is not limited to these specific examples.

  For example, in addition to those described above, those skilled in the art will appropriately determine the specific structure and size of each element such as the exposure light source, illumination optical system, reticle stage, projection optical system, and wafer stage that constitute the projection exposure apparatus. Designed ones are also included in the scope of the present invention as long as they include the gist of the present invention. Furthermore, the types of active gas and inert gas, the flow rate, the flow velocity distribution, or the supply means and discharge means thereof, which are appropriately selected and used by those skilled in the art, are included in the scope of the present invention.

  In addition, all projection exposure apparatuses and projection exposure methods that include elements of the present invention and that can be appropriately modified by those skilled in the art are included in the scope of the present invention.

It is a schematic diagram showing the principal part structure of the projection exposure apparatus concerning embodiment of this invention. It is a flowchart showing the algorithm which determines exposure time in the projection exposure apparatus of embodiment of this invention. (A) And (b) is a graph which illustrates the effect of light washing in the present invention. It is a schematic diagram showing the projection exposure apparatus of the modification of this invention. It is a schematic diagram showing the specific example of the reticle 12 which can be used in this invention. It is a flowchart figure showing the modification of the operation | movement of the projection exposure apparatus of this invention. It is a schematic diagram showing the principal part structure of a projection exposure apparatus. It is a flowchart showing the algorithm for determining real exposure time.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light source for exposure 11 Illumination optical system 12 Photomask (reticle)
12a Pattern formation region 12b Window region 13 Reticle support base 14 Projection optical system 15 Wafer stage 16 Wafer 17 Illuminance monitor 18 Illuminance control unit 19 Flow rate adjusting mechanism 20 Purge gas supply port 21 Purge gas recovery port 22 Transmittance measuring device 22a Primary light measurement device 22b Secondary light measuring device 22c Transmittance monitor 23 Oxygen concentration measuring mechanism 25 Optical output monitor 30 Reticle stage 100 Projection exposure apparatus 200 Projection exposure apparatus

Claims (11)

A light source;
Optical system,
A reticle stage that can hold the reticle;
Gas supply means capable of maintaining the periphery of the reticle in an atmosphere containing an active gas;
A transmittance measuring means capable of measuring a change in transmittance of the reticle;
A wafer stage on which a wafer can be placed;
With
Light cleaning that removes contaminants attached to the reticle by irradiating light onto the reticle from the light source while maintaining the atmosphere around the reticle by the gas supply means in an atmosphere containing the active gas is enabled. A projection exposure apparatus. An illuminance sensor for measuring the illuminance of light irradiated on the wafer;
A control mechanism for determining an exposure time for the wafer from the illuminance measured by the illuminance sensor;
The projection exposure apparatus according to claim 1, further comprising: determining the exposure time after the light cleaning. A concentration measuring mechanism for measuring the concentration of the active gas in the atmosphere around the reticle;
A concentration control mechanism for controlling the concentration of the active gas according to the output of the concentration measurement mechanism;
The projection exposure apparatus according to claim 1, further comprising: The active gas is at least one selected from the group consisting of O ₂ , O ₃ , CO ₂ , CO, nitric oxides (NO _X ), sulfur oxides (SO _X ), and oxygen. The projection exposure apparatus according to any one of claims 1 to 3. The atmosphere containing the active gas includes the active gas and an inert gas,
The projection exposure apparatus according to claim 1, wherein the inert gas is one selected from the group consisting of N ₂ , Ar, and He.   6. The projection exposure apparatus according to claim 1, wherein the optical cleaning is stopped when a change in the transmittance measured by the transmittance measuring unit falls below a predetermined value.   The projection exposure apparatus according to claim 1, wherein the transmittance measuring unit measures the transmittance at a portion of a transmittance measuring window provided in the reticle.   The projection exposure apparatus according to claim 1, wherein the light source emits F2 laser light. Light cleaning that removes contaminants attached to the reticle by irradiating the reticle with light while maintaining the atmosphere around the reticle in an atmosphere containing an active gas causes the change in the transmittance of the reticle to fall below a predetermined value. A first step to be carried out until
A second step of performing exposure by irradiating the wafer with light through the reticle;
A projection exposure method comprising:   The projection exposure method according to claim 9, wherein the concentration of the active gas in the atmosphere containing the active gas is 500 ppm or less. Purging with an inert gas is performed to remove the active gas contained in the atmosphere around the reticle after the light cleaning and before the exposure of the wafer. The projection exposure method according to claim 9 or 10.

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