JP3428966B2 - Exposure control device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exposure amount control device when a pulse laser such as an excimer is used as a light source, and particularly to those suitable for application to a semiconductor exposure device that requires highly accurate exposure amount control. It is about.

[0002]

2. Description of the Related Art In a semiconductor exposure apparatus, a pulse laser such as an excimer laser has been used as a light source in recent years as semiconductors are highly integrated and miniaturized. In the exposure amount control of the exposure device using the conventional pulse laser,
The average output energy E ₀ and the total exposure pulse number N are determined in advance from the target set exposure amount E _dose before exposure, and the exposure target energy amount of each pulse is set so that the exposure energy of the irradiation target is constant for each pulse. Was set to control the exposure amount.

However, in the actual exposure sequence, there are various disturbance energies based on optical characteristics, laser characteristics, and the like that prevent exposure of the object to be exposed with the exposure target energy amount. However, it is necessary to control the exposure amount while correcting the laser output energy. Therefore, for example, the exposure algorithm shown in FIG. 8 has been used. In FIG.
In step 121, the set exposure amount E _dose is set. Also,
The initial pulse value i = 1. In step 122, E
The total number N of exposure pulses satisfying _dose = N × E ₀ and the laser average output energy E ₀ per pulse are obtained. In step 123, the exposure error up to the i- _1th pulse E _err =
_Find E _obj −E _meas (i ≧ 2). Here, E _meas is the total actual exposure amount up to the (i-1) th pulse, E _obj is the total exposure target amount up to the (i-1) th pulse, and each pulse is adjusted so that the exposure energy of the irradiation target is constant for each pulse. In the conventional technique of setting the exposure target energy amount of E _obj = (i
-1) It can be expressed as E ₀ . However, when i = 1, E _err = 0. In step 124, the laser output energy of the i-th pulse

[0004]

[Equation 1] Ask for. Here, the second term on the right side is the correction amount for removing the exposure error caused by the disturbance energy. In step 125, laser output is performed with energy y [i]. In step 126, the exposure amount of the i-th pulse is measured. Also, i
Obtain the total exposure amount E _meas up to the pulse. In step 127, the pulse number i is incremented by 1. Step 123-
The exposure is completed by repeating 127 times N times.

Energy y [i] as in step 125
In the case of outputting the laser by, the output energy control of the pulse laser is generally performed by setting the laser charging voltage to an appropriate value corresponding to the output energy. In exposure amount control of an exposure apparatus using a conventional pulsed laser, as shown in FIG. 9, the relationship between the charging voltage and the output energy is calculated in advance before the exposure, and the charging voltage corresponding to the target output energy is pulse-wise determined. And laser output was performed.

[0006]

By the way, in the exposure amount control for each pulse in the above-mentioned conventional example, the characteristic relationship between the laser charging voltage and the output energy is obtained in advance before the exposure, and the laser charging voltage is determined using this relationship. Because it was due to the open loop, When the characteristic relationship between the laser charging voltage and the output energy obtained in advance is different from the actual laser characteristic relationship,
It causes an exposure error. 2. If there is no compensator for the disturbance and the disturbance is applied, an exposure error will occur. There was a problem.

The above problems 1 and 2 will be described in more detail. The characteristic relationship between the laser charging voltage and the output energy shown in FIG. 9 is not always constant, but changes with time as the laser gas deteriorates. For this change over time,
In order to perform exposure with the exposure target energy amount for each pulse without an exposure error, it is necessary to constantly measure and update the characteristic relationship between the laser charging voltage and the output energy.

Further, the laser is characterized in that the output energy of several tens to several hundred pulses immediately after the start of oscillation becomes higher than the output energy in the steady state (hereinafter referred to as spiking phenomenon). In order to perform the exposure with the exposure target energy amount without the exposure error against the spiking phenomenon, it is necessary to obtain the characteristic relationship between the laser charging voltage and the output energy for each pulse.

It is difficult to accurately obtain the characteristic relationship between the laser charging voltage and the output energy which is different over time or for each pulse in the entire charging voltage range without deteriorating the throughput of the device. In the conventional exposure amount control by the open loop, there is a problem that the characteristic relationship between the laser charging voltage and the output energy obtained in advance is different from the actual characteristic relationship of the laser, and an exposure error occurs.

Next, an example of occurrence of disturbance in the exposure amount control of the exposure apparatus using the pulse laser will be described. A semiconductor exposure apparatus is required to have a uniform illuminance on a surface to be irradiated. However, when a laser is used as the light source, unevenness in illuminance occurs in the beam cross section of the laser, which causes unevenness in illuminance on the irradiated surface.
In order to remove this illuminance unevenness, as shown in FIG.
For example, a means has been devised for uniformizing the illuminance by temporally allocating a laser beam on a surface conjugate with an irradiation surface by rotating a plane-parallel plate. However, although the illuminance unevenness can be eliminated by using this means, periodical energy fluctuations occur on the entire surface to be illuminated in conjunction with the rotation of the plane-parallel plate. This is an example of a disturbance that becomes a factor that hinders exposure of the surface to be irradiated with the exposure target energy amount. Other examples of the illuminance nonuniformity remover include an example in which a wedge prism or a mirror is periodically moved.

In general, the relationship between the laser charging voltage and the actual exposure energy changes from pulse to pulse due to the influence of disturbance. In the conventional exposure amount control by the open loop, there is no compensation means for such a disturbance, and there is a problem that an exposure error occurs when the disturbance is applied.

The present invention has been made in view of the above-mentioned problems in the prior art. Even if the characteristics of the laser charging voltage and the output energy fluctuate, the exposure is performed to the exposure target energy for each pulse without an exposure error. , And even if a disturbance occurs, its effect on the exposure amount is eliminated.

[0013]

In order to achieve the above object, an exposure control apparatus of the present invention uses a pulse laser as a light source, and exposes an irradiated object by causing the pulse laser to emit a plurality of pulses. In the exposure amount control device, the exposure energy amount setting means for setting the exposure target energy amount for each pulse of the pulse laser, and the output energy control means for controlling the output energy for each pulse of the pulse laser based on the exposure target energy amount. When,
Detects the amount of exposure energy for each pulse of the pulsed laser
Exposure energy amount detecting means and the exposure energy amount setting means
Pre-compensator for determining the setting value of the adjusting means and the exposure energy
The amount of exposure energy detected by the
Serial characterized by comprising a feedback <br/> click system and means for feeding back to the predistorter.

Here, the predistorter is an integral compensator.
Good to have . The pre- compensator gain of the feedback system can be determined from the characteristic relationship between the laser charging voltage and the output energy. Also, before the feedback system
It is possible to determine the local compensator gain in multiple stages based on the characteristic relationship between the laser charging voltage and the output energy, and always switch to the optimum gain.

Furthermore, the pulse laser light quantity attenuation rate due to a change of lighting conditions measured or predicted, which can be switched the predistorter gain of the feedback system based on. Here, the illumination conditions of the pulse laser can be changed by inserting an ND filter, a prism, a diaphragm, or a lens in the laser optical path.

Generally, as a characteristic of the feedback system,
(1) Good modeling error removal characteristics, and (2) Good disturbance removal characteristics centered on the low frequency region.

In an exposure apparatus using a pulse laser,
When the feedback system is configured and the exposure amount is controlled,
It has the following effects. 1. Even if the characteristics of the laser charging voltage and the output energy change, the exposure target energy can be exposed without exposure error. 2. Disturbance can be removed mainly in the low frequency region.

[0018]

Embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) FIG. 1 shows a schematic structure of an exposure apparatus using an exposure amount control system according to the first embodiment of the present invention. In the figure, 1 is a laser light source such as an excimer, 2 is a reticle on which an integrated circuit pattern is formed, 3 is a projection optical system, 4 is a wafer, and the integrated circuit pattern formed on the reticle 2 is a projection optical system 3 Projection exposure is performed on the wafer 4 via. A half mirror 5 is arranged on the optical path from the laser light source 1, and a photosensor 6 is arranged on the optical path reflected by the half mirror 5. The output of the photo sensor 6 is converted into an exposure amount for each pulse of laser through a sensor amplifier 7. CPU8 is photo sensor 6
Calculate the next laser output value based on the exposure amount measured in
An output energy command value is given to the laser light source 1. An optical system 9 includes an illuminance unevenness remover described later.

When a pulsed laser is used as the light source, there is a problem that unevenness in illuminance occurs on the surface to be illuminated because unevenness in illuminance occurs in the beam cross section of the laser. In order to prevent this, a illuminance nonuniformity remover such as a wedge prism or a plane-parallel plate as shown in FIG. 10 for oscillating a laser beam on the irradiated surface or a surface conjugate with the irradiated surface is inserted and A method of periodically reducing the unevenness of illuminance by rotating the illuminance is used.

However, although the illuminance nonuniformity remover can be removed by rotating the illuminance nonuniformity remover periodically, the exposure energy reaching the wafer surface periodically varies depending on the rotational position of the illuminance nonuniformity remover. .

FIG. 2 shows an exposure amount control system of the exposure apparatus according to the first embodiment of the present invention. The hardware configuration of this exposure apparatus is as shown in FIG. The exposure amount control system of FIG. 2 is configured on the CPU 8 of FIG. In FIG. 2, the transfer function of the laser is G _c and the compensator gain is K _c . In addition, the target energy amount is r [i] and the actual exposure energy amount is x [i].
, Laser charging voltage v [i], disturbance energy amount d [i]
, And the error amount is e [i]. However, * [i] indicates that it is the i-th pulse. By configuring this control system, the laser charging voltage is

[0022]

[Equation 2] Determined by That is, the laser charging voltage of the i-th pulse is determined based on the error amount between the target energy amount of the i-th pulse and the actual exposure energy amount of the (i-1) th pulse. Also,

[0023]

[Equation 3] Have a relationship. From the expressions (1) and (2), the relationship between the exposure target energy amount r [i], the disturbance energy amount d [i], and the actual exposure energy amount x [i] is

[0024]

[Equation 4] Can be expressed as

As described above, the characteristic relationship between the laser charging voltage and the output energy changes with time and from pulse to pulse due to the influence of laser gas deterioration and spiking. Therefore, a laser modeling error occurs in the exposure amount control system. Here, the effect on the modeling error of the exposure control system will be described.

As a hypothesis, it is assumed that no disturbance occurs and the laser output energy becomes the exposure energy as it is.
Substituting d [i] = 0 into the equation (3),

[0027]

[Equation 5] The relationship is established. From this relationship, when a constant target energy amount r [i] = r is input to the exposure amount control system, the output energy of the i-th pulse is the output energy x [1] of the first pulse,

[0028]

[Equation 6] Can be expressed as From the above results, if the established _{1.1-K c G c = 0} , the output energy of the second pulse and subsequent y [i] = r, and the exposure error does not occur. 2. _{| 1-K c G c |} < If 1 is satisfied, the stability of the exposure control system is guaranteed, the laser output energy converges to the target energy x [i] → r.

The above will be described in detail. As shown in FIG. 3, it is assumed that the characteristic relationship between the laser charging voltage and the output energy is linear. At this time, the gain K _{c of the} compensator
Is the reciprocal of the laser gain G _c

[0030]

[Equation 7] To decide. Here, the maximum charging voltage of the laser is V _max ,
The minimum charging voltage was V _min , the maximum output energy of the laser was E _max , and the minimum output energy was E _min . In this case, the laser modeling error does not occur, and the relationship of K _c G _c = 1 is established, so that the exposure error does not occur.

Next, a case will be described in which the gain G _c of the laser does not change and characteristic fluctuations of the laser charging voltage and output energy parallel to the energy axis occur. This characteristic variation is also shown in FIG. Also in this case, the relationship of K _c G _c = 1 is established, and therefore, the actual exposure energy having no error from the target energy can be obtained from the equation (5) after one pulse.

Next, a case will be described in which the laser gain G _c fluctuates to G _c ′ as shown in FIG. in this case,

[0033]

[Equation 8] If the laser characteristic variation is within the range that holds, the stability of the exposure amount control system is guaranteed, and the laser output energy converges to the target energy x [i] → r.

The case where the characteristic relationship between the laser charging voltage and the output energy is linear has been described above. However, in general, the characteristic relationship between the laser charging voltage and the output energy is not always linear, but often nonlinear as shown in FIG. In this case, when performing a linear modeling of laser characteristics modeling error occurs, from the results described above, the gain of the full charge voltage _{| 1-K c G c '} | if satisfied <1 The stability of the exposure amount control system is guaranteed, and the actual exposure energy converges on the target energy y [i] → r.

Next, the influence of the disturbance of the exposure amount control system will be described.
To describe. Here, the ideal state without modeling error
State, K_c = 1 / G_c Is established. (3)
K in the formula _c = 1 / G_c By substituting

[0036]

[Equation 9] The relationship is established. From this result, the disturbance energy d
It can be seen that only the difference between [i] and the amount of disturbance energy in the previous pulse is transmitted to the exposure energy.

For example, as described above, the periodic disturbance energy having the amplitude α and the initial phase θ ₀ is accompanied by the rotation of the illuminance unevenness remover.

[0038]

[Equation 10] Occurs. Here, f _L : laser oscillation frequency, f
_N : The rotation frequency of the illuminance unevenness remover. Even if this periodic disturbance energy is added to the exposure amount control system, the error amount transmitted to the exposure energy y [i] from the result of the equation (8) is

[0039]

[Equation 11] Is reduced to.

This result can be generally applied to the case where the disturbance of a certain frequency is input to the exposure amount control system, and the larger the value of n, that is, the disturbance in the low frequency region, is the disturbance removal characteristic. It turns out that is excellent.

Further, in recent years, as a measure for widening the angle of view of semiconductors, a scan exposure method is used in which exposure is performed while synchronously driving the reticle 2 and the wafer 4 in FIG. 1 in opposite directions. When performing exposure by outputting a laser while moving the wafer at a constant speed, unevenness in the illuminance in the scanning direction occurs due to variations in the laser output of each pulse. 3 in FIG.
An example of scan exposure in which one area is exposed with a pulse is shown.

FIG. 6 shows an example in which the output energy at the (m + 3) th pulse is large and the illuminance becomes uneven.

By using the exposure amount control system shown in this embodiment, it becomes possible to eliminate the influence of laser characteristic variations and disturbances and to expose the wafer with constant exposure energy for each pulse. It is possible to realize exposure amount control with a small amount of

(Second Embodiment) As shown in FIG. 5, the characteristic relationship between the charging voltage of the laser and the output energy is generally non-linear. In the present embodiment, a means for performing exposure amount control by approximating the characteristic relationship between the laser charging voltage and the output energy, determining the gain K _c of the laser compensator at L stages, and constantly switching to the optimum gain is used. FIG. 7 shows an example in which the characteristic relationship is approximated when L = 3. By using this means,
The modeling error can be reduced, and the convergence to the target value can be realized with a small number of pulses.

(Third Embodiment) By inserting an ND filter, a prism, a diaphragm, a lens and the like on the laser optical path, the amount of laser light reaching the irradiated body is attenuated and the gain G _c of the laser is changed. In this embodiment, when the laser light amount is attenuated and the laser gain G _c is changed by changing these laser illumination conditions, a means for measuring or predicting this change and changing the value of G _c is used.

For example, when an ND filter having transmittance β (0 <β <1) is inserted in the laser optical path, the gain G shown in FIG.
_An exposure amount control system can be realized by replacing _c with βG _c .

[0047]

As described above, according to the present invention,
In an exposure apparatus using a pulsed laser, the target exposure energy amount is compared with the past actual exposure energy amount, and a feedback system is used to determine the laser charging voltage using the result, and the exposure amount control is performed to perform laser charging. Even if the characteristics of the voltage and output energy fluctuate, the exposure target energy can be exposed without exposure error.
In particular, the disturbance can be removed mainly in the low frequency region.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an exposure apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram showing an exposure amount control system using feedback according to the first embodiment of the present invention.

FIG. 3 is a diagram showing a linear characteristic relationship between a laser charging voltage and output energy, and a characteristic relationship when an output fluctuation parallel to the energy axis occurs.

FIG. 4 is a diagram showing a characteristic relationship when a gain variation of laser output energy occurs.

FIG. 5 is a diagram showing a non-linear characteristic relationship between laser charging voltage and output energy.

FIG. 6 is a diagram showing an example of occurrence of illuminance unevenness in scan exposure.

FIG. 7 is a diagram showing an L stage compensator gain according to a second embodiment of the present invention.

FIG. 8 is a diagram showing a conventional exposure algorithm.

FIG. 9 is a diagram showing a conventional method for determining a laser charging voltage.

FIG. 10 is a diagram showing a means for rotating a plane-parallel plate to remove illuminance unevenness on an object to be irradiated.

[Explanation of symbols]

1: laser light source, 2: reticle, 3: projection optical system, 4:
Wafer, 5: half mirror, 6: photo sensor, 7: sensor amplifier, 8: CPU, 9: optical system.

Front page continuation (58) Fields surveyed (Int.Cl. ⁷ , DB name) H01L 21/027 G03F 7/20

Claims (6)

(57) [Claims] 1. An exposure amount control apparatus for an apparatus that exposes an irradiation target by using a pulse laser as a light source and causing the pulse laser to emit light in a plurality of pulses. Exposure for setting an exposure target energy amount for each pulse of the pulse laser. Energy
Energy amount setting means, output energy control means for controlling the output energy of each pulse of the pulse laser based on the exposure target energy amount, and exposure energy amount detecting means for detecting the exposure energy amount of each pulse of the pulse laser. And the setting value of the exposure energy amount setting means.
Pre-compensator and exposure energy amount detection means
The amount of exposure energy emitted is returned to the predistorter.
And a feedback system including a means for controlling the exposure amount. 2. The predistorter is an integral compensator.
The exposure amount control device according to claim 1, wherein: 3. The exposure amount control apparatus according to claim 1, wherein the precompensator gain of the feedback system is determined from a characteristic relationship between a laser charging voltage and output energy. 4. The exposure according to claim 1, wherein the precompensator gain of the feedback system is determined in a plurality of stages based on the characteristic relationship between the laser charging voltage and the output energy and is always switched to the optimum gain. Quantity control device. Wherein said pulsed laser light quantity attenuation rate due to a change of lighting conditions measured or predicted, claim 1, wherein the switching the predistorter gain of the feedback system it based on 2. The exposure amount control device according to any one of 1. 6. Changing the illumination conditions of the pulsed laser comprises:
The exposure amount control apparatus according to claim 5, including an ND filter, a prism, a diaphragm or a lens inserted in the laser optical path.