GB2155647A

GB2155647A - Controlled exposure

Info

Publication number: GB2155647A
Application number: GB08501764A
Authority: GB
Inventors: Akiyoshi Suzuki
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 1984-02-01
Filing date: 1985-01-24
Publication date: 1985-09-25
Also published as: GB2196440A; GB2196440B; GB8501764D0; DE3503273A1; DE3503273C2; GB8719664D0; GB2155647B

Abstract

A wafer is exposed to an excimer laser beam, the amount of exposure being controlled by using a plurality of pulses of the same preset energy value alone (Figs. 5 and 6) or followed by one or more pulses of lower preset value (Figs. 7 and 8). In the latter case, the required pulse energy of the additional pulse(s) is calculated using photosensor 5b, integrator 6 and comparator 9. The invention substantially overcomes the problem of deviation of individual pulse output from a preset energy value. <IMAGE>

Description

SPECIFICATION Exposure method and apparatus BACKGROUND OF THE INVENTION This invention relates to an exposure method and an exposure apparatus, and, more particularly, relates to exposure method and apparatus for the manufacture of semiconductor circuit devices.

Recent development in the semiconductor technology has enforced higher capacities and further miniaturization of the semiconductor circuit devices. Along such trend, photolithography techniques such as an optical exposure process have become more and more dominant with the development of high resolution lenses. In such exposure systems, a short wavelength of light within the deep UV range has recently been used to transfer and print a circuit pattern of a mask or reticle onto a wafer. This is because the resolution fo the minimum line width of the circuit pattern to be printed on the wafer is proportional to the wavelength of the light.

Conventionally, heavy hydrogen lamps or XeHg lamps have been used as the deep UV light sources. These lamps are featurized in the point of continuous emission in both cases of DC energization and AC energization. In view of such feature, the amount of exposure for the wafer has been achieved by analog-like control systems such as, for example, a timer control system for controlling the exposure time by means of a timer, or an integrating exposure-meter system in which the amount of exposure is integrated and the exposure is continued until the integrated exposure reaches a predetermined value.

Use of such light sources however involves inconveniences, because only a decreased output is obtainable in the deep UV range and the sensitivity of the photoresist material applied to the wafer surface is low. This results in a longer exposure time and a decreased throughput.

It has recently been found that an Excimer laser capable of providing a higher output in the deep UV range may be effective as the light source means for the exposure apparatus. However, the excimer laser is a pulseoscillation type laser, as compared with the conventional heavy hydrogen lamps or Xe-Hg lamps. For this reason, the above-described analog-like control system for controlling the amount of exposure could not be applied, as it is, to exposure apparatuses employing excimer lasers.

SUMMARY OF THE INVENTION It is accordingly a primary object of the present invention to provide a method and an apparatus for performing exposure with a pulsed laser beam, in which a correct amount of exposure can be easily attainable.

It is another object of the present invention to provide a method and an apparatus for performing exposure with the pulsed laser beam, in which the amount of exposure can be very easily and positively controlled.

Briefly, according to the present invention, there are provided a method and an apparatus for exposing an object with a pulsed laser beam. The exposure of one shot (one exposure area) is achieved by a plurality of pulse exposures with the corresponding number of pulses, as the result of which any fluctuations or errors in the outputs of the pulses are substantially compensated so that correct exposure of each shot is assured. In other aspect, the amounts of exposures by the plural pulses for the one shot exposure are integrated and the integrated amount of exposure is compared with a correct or desired amount of exposure. On the basis of the result of comparison, an additional pulse exposure is effected in accordance with the degree of under exposure.

These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic and diagrammatic view showing an exposure apparatus, of reduction projection type, in accordance with an embodiment of the present invention.

Figure 2 is a schematic and diagrammatic view showing an illumination optical system of the exposure apparatus shown in Figure 1.

Figure 3 is a schematic and diagrammatic view showing an exposure apparatus, of a reduction projection type, in accordance with another embodiment of the present invention.

Figure 4 is a flow chart showing an example of control for the amount of exposure in the exposure apparatus of Figure 3.

Figures 5-8 are waveform views showing examples of pulse control.

DESCRIPTION OF THE PREFERRED EMBODI MENTS Figure 1 shows an embodiment of the present invention which is applied to an exposure apparatus of reduction projection type, called a stepper. The exposure apparatus includes a light source 1, such as an excimer laser, providing a pulsed laser beam. Within the light source 1, a gas of, e.g., KrF or XeCI is sealingly contained, so that a wavelength of light in the deep UV range such as 248 nm (in the case of KrF gas) or 308 nm (in the case of XeCI gas) is emitted in the form of pulses.

The laser beam emitted by the light source 1 enters into an illumination optical system 2, the details of which are illustrated in Figure 2.

The illumination optical system 2 includes a beam shaping optical system 21 such as a toric lens, an optical integrator 22 such as a compound-eye lens, a collimator lens 23 and a a mirror 24. Each of the optical systems 21-23 are made of a material such as SiO2, CaF2 or the like which is transmissive to the light of deep UV range. Commercially available excimer lasers usually provide a beam of oblong cross-sectional shape. In view of this, the beam shaping optical system 21 is provided to suitably shape the laser beam, e.g.

into a square shape. The optical integrator 22 is provided to achieve uniform distribution of light.

Referring now back to Figure 1, a reticle or mask M having formed thereon an integrated circuit pattern, a projection optical system 3 and a wafer W are disposed, in the named order, along the optical path defined by the illumination optical system 2. Similarly to the illumination optical system 2, the projection optical system 3 is made of a material which is transmissive to the light of deep UV range.

The reduction projection may of course be achieved by a reflection imaging system, in place of the projection lens system.

Designated by a reference numeral 4 is a control unit for controlling the pulse output of the excimer laser 1. In this laser output control unit 4, the output of each pulse to be provided by the excimer laser 1 can be variably preset.

As described in the foregoing, the excimer laser is capable of providing a higher output.

For this reason, only one pulse exposure can provide a sufficient amount of exposure relative to one shot. Thus, the exposure process can be effected by one pulse emission for each shot. However, it is known that the outputs of the pulses generated by the excimer laser involve fluctuations or errors in the range of j 5% or more. Such fluctuations in the outputs of the pulses would result in irregularities in the amounts of exposures.

Therefore, the one pulse exposure per one shot would not easily ensure a correct exposure for each shot and uniform exposures relative to plural shots.

The above-described problem can be solved by the present invention. Briefly stated, the present invention aims at deleting or suppressing incorrect or uneven exposure which will be otherwise caused by the fluctuations in the outputs of the pulses, without correcting the fluctuations themselves of the outputs of the pulses. In summary, the present invention is based on the following finding made by the inventor: That is, in the case of one pulse exposure per one shot exposure, + 5% fluctuation or error in the output of each pulse would directly leads to + 5% error in the amount of exposure. As compared therewith, if the exposure of one shot is effected by two or more pulses, the probability that the total amount of exposure by these plural pulse exposures becomes 5% over (the maximum error) relative to the correct amount of exposure is remarkably decreased.This is because the total amount of exposure becomes 5% over only when each of the pulses has provided the maximum over-exposure (5% over). On the other hand, there is a substantial probability that the plural times pulse exposures cancel the errors with each other.

According to the present invention, on the basis of such finding, the output of each pulse to be generated by the excimer laser 1 is controlled by the laser output control unit 4 so that an output, suitable to providing an amount of exposure which is smaller than the correct amount of exposure for one shot, is preset for each of the pulses, and the exposure of one shot is effected by a plurality of pulse exposures through the corresponding number of pulses each having the preset output as aforesaid. The term "preset output means an expected average value of the pulse outputs which will be actually generated by the excimer laser 1 if a fixed value is continuously input to the excimer laser 1.That is, in view of that each pulse output from the excimer laser involves an error or fluctuation in the range of approx. j 5%, an expected average value of the pulse outputs which will be actually generated by the excimer laser 1, with respect to a particular input value, is specified at the laser output control unit 4. In this sense, the "preset output can be expressed as a "specified output" or a "target output". In accordance with the present invention, stated strictly, the exposure of one shot is effected by a plurality of pulse exposures with the corresponding number of pulses each containing an error in its output.

Nevertheless, the errors in the pulse outputs are substantially compensated for, as the result of the plural pulse exposures. Accordingly, it is not necessary to provide any specific control means for correcting the fluctuation in the quantity of pulsed laser beam.

As a convenient way, the outputs of the plural pulses for the exposure of one shot are preset at the same value. In other words, when the number of pulse exposures for one shot is N, the output of each pulse is preset at a value suitable to providing an amount of exposure which is equal to or substantially equal to 1 /N of the correct amount of exposure.

Commecially available excimer lasers generally have a high emission-repetition frequency of the order of 200-300 Hz. For this reason, the plural pulse exposures per one shot still improves the throughput, as compared with conventional exposure apparatuses. For example, in a case where averagely ten (10) pulse exposures are effected for each shot and even if the number of pulse exposures varies within a range from nine (8) to eleven (11) due to the errors in the pulse outputs, the required exposure time will be within a range of 0.04#.05 sec. This is a striking contrad istinction to the conventional steppers which generally require approx. 0.3 sec. exposure time.Thus, the exposure time required by the plural pulse exposures is shortened to a value which is smaller, by one "figure", than that required by the conventional steppers. Therefore, both the stabilized exposure and im proved throughput are assured. Even if the exposure of one shot is effected by twenty (20) pulses, the exposure will be completed within approx. 0.1 sec., which is a significant improvement over the conventional exposure apparatuses.

In this specification, the term "one shot" means the exposure which is enough to expose the whole surface of the wafer in a case of global or whole-surface exposure; the exposure which is enough to expose one chip in a case of step-and-repeat type exposure wherein the exposure operation is effected for each chip; and the exposure which is enough to expose one slit width in a case of slit exposure.

While in the Figure 1 embodiment the invention has been described with reference to the lens projection type exposure apparatus, the invention is not limited thereto and it is applicable to a contact type or proximity type exposure apparatus.

In accordance with the present invention as has hitherto- been described, the exposure of one shot is effected by a plurality of pulse exposures with the pulsed laser beam. By this, in spite of the strikingly different feature of the pulsed laser beam as compared wit the continuous emission of conventional light sources, stable exposure is ensured without using any specific control means. Moreover, the throughput is improved.

Figure 3 shows another embodiment of the present invention which is applied to a reduction projection type exposure apparatus, called a stepper, with the elements corresponding to those of Figure 1 embodiment being designated by the same reference numerals. The exposure apparatus includes a light source 1 such as an excimer laser, an illumination optical system 2 and a projection optical system 3, all of which may be the same as those of Figure embodiment. Further, a reticle or mask M and a wafer W may be the same as those shown in Figure 1. Therefore, descriptions thereof will be omitted here for the sake of simplicity of explanation.

Essentially, the Figure 3 embodiment differs from the Figure 1 embodiment in the following poins: In the present embodiment, in addition to effecting a plurality of pulse exposures for each shot, the amounts of pulse exposures are integrated and the integrated total amount of exposure is compared with a correct amount of exposure for one shot. On the basis of the result of comparison, addition of further pulse exposure is controlled, whereby the amount of exposure for each shot is more strictly and precisely controlled.

As shown in Figure 3, a mirror 5a is disposed in the optical path defined by the illumination optical system 2 and a UV photosensor 5b is disposed in the optical path defined by the light beam reflected by the mirror 5a. The photosensor 5b may be disposed near the laser light source 1 or in the optical path extending from the light source 1 to the wafer W. In response to reception of light, the photosensor 5b generates an output which is supplied to a light quantity integration circuit 8 into which the sensitivity of the photoresist has been previously input. Within the integration circuit 6, the amounts of exposures by the pulses from the excimer laser 1 are sequentially integrated. The integrated total amount of exposure is then compared with the correct amount of exposure at a comparator circuit 8.Subsequently, the result of comparison is input to a central processing unit 7.

On the basis of result of comparison at the comparator circuit 8, the central processing unit 7 computes the number of pulses and/or the output of each pulse necessary for sufficiently exposing the photoresist material on the wafer W. The result of computation is then supplied to a laser output control unit 8 which is adapted to drive the excimer laser 1 in accordance with the result of computation at the central processing unit 7, whereby the pattern of the mask M is irradiated by the pulses of the number and/or outputs controlled as required. By this, the mask pattern is printed on the wafer W surface.

If necessary, the efficiency of the illumination optical system (e.g., the diameter of the laser beam, quantity, etc.) may be controlled by an illumination efficiency controlling unit 10 in accordance with the result of computation at the central processing unit 7. Further, the output of each pulse can be controlled by one of or both of the laser output control unit 8 and the illumiantion efficiency controlling unit 10.

Operation of the present embodiment will now be described with reference to the flow chart shown in Figure 4.

As has already been described with reference to the Figure 1 embodiment, the pluralpulse exposure for each shot is effective to substantially compensate for the fluctuations or errors in the pulse outputs and assures stable control for the amount of exposure. In the present embodiment, in addition to such pluralpulse exposure, the amount of pluralpulse exposure is integrated, by real time, and the integrated amount of exposure is compared with the correct amount of exposure. If the result of comparison shows underexpo sure, addition of further pulse exposure is effected in accordance with the amount or degree of "under".

For example, "n" times of pulse exposures are effected by n pulses each having the same preset output, while the amount of exposure is monitored by the integration circuit 8. Alternatively, "m" times of pulse exposures, which are effective to achieve the required amount of exposure provided that the maximum errors (e.g., + 5%) in the outputs are accumulated, are effected by using m pulses each having the same preset output, and subsequently the total amount of exposure is monitored by the integration circuit 6. The integrated amount of exposure by the n or m times of pulse exposures is compared with the correct amount of exposure at the comparator circuit 9. If the difference therebetween is within a predetermined permissible range, the exposure is completed.If, on the other hand, the result of comparison shows a substantial defficiency in the amount of exposure, additional exposure is effected using one or plural additional pulses in accordance with the defficiency, such as shown in Figure 5. The number of such additional pulses is determined so as to minimize the difference between the correct amount of exposure and the final amount of exposure, i.e. to minimize the degree of under-exposure or over-exposure.

Even if the total amount of exposure which is finally determined by the final pulse exposure becomes "under" or "over" relative to the correct amount of exposure, the difference therebetween can be remarkably reduced in contrast with the one shot/one pulse exposure.

If, for any reason, the correct amount of exposure is reached by one or such a number of pulse exposures which is smaller than n or m as aforesaid, the exposure operation is of course completed at that time.

In a case where the output of each pulse is further decreased and the number of pulses for each shot is accordingly increased, such as shown in Figure 6, an amount of exposure which is very close to the correct amount of exposure is attainable. This is because the preset output of each pulse is decreased so that the absolute fluctuation or deviation of each pulse output is smaller even if the last one or ones of the pulses involve + 5% error.

The pulse output can be decreased by decreasing the output of the laser light source itself, as described in the foregoing. Alternatively, the pulse output may be decreased by using an ND filter disposed in the optical path of the laser light source or by changing the efficiency of the illumination optical system.

The latter may be assured by reducing the diameter of the illumination light. As a further alternative, the output of the laser light source may be primarily adjusted while the efficiency of the illumination optical system may be adjusted supplementarily.

While, in the examples shown in Figures 5 and 6, the additional pulse exposure according to the degree of under-exposure is effected by controlling the number of additional pulses, such pulse number control may be replaced by or combined with the control of the preset output of the additional pulse.

More specifically, the amount of exposure by the above-described n or m times of pulse exposures is compared with the correct amount of exposure and, if under-exposure, the additional pulse exposure is effected by an additional pulse having a preset output which is controlled in accordance with the defficiency in the amount of exposure, such as shown in Figure 7. In such case, the preset output of the last pulse, i.e. the additional pulse, is made lower (the case of Figure 7) or higher than those of the preceding pulses, in accordance with the degree of under-exposure. The addition of pulse exposure may be achieved by using a plurality of additional pulses, as described in the foregoing. In such case, the additional pulses may have the same preset output, as shown in Figure 8. Alternatively, only the last pulse may have a different preset output, such as shown in Figure 7.In any case, the amount of exposure which is substantially or approximately equal to the correct amount of exposure is attainable.

The plural pulse exposures may be effected by using the corresponding number of pulses having different preset outputs. When, in such case, the correct amount of exposure is denoted by Eot the preset output of the first pulse nt is denoted by E" the preset output of the next pulse n2 is denoted by E2, and the preset output of the last pulse n3 is denoted by E3, it follows that: Eo = n,E, + n2E2 + n3E3 it follows E a E and E = a3EO it follows E3 = a3EO, it follows that: n,a, + n2a2 + n3a3 = 1 By adjusting the values of a, a2 and a3 in that the value of "n, + n2 + n3" is made small, the exposure time can be shortened while the correct amount of exposure is ensured.

Similarly to the Figure 1 embodiment, the present invention in respect to the Figure 3 embodiment is not limited to the lens projection reduction exposure apparatus or a proximity type exposure apparatus.

In accordance with the present invention, as has hitherto been described, correct exposure is stably attainable with the use of a pulsed laser beam such as an excimer laser beam, in spite of substantial fluctuation in the output of each pulse.

While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.

Claims

1. A method of exposure with a pulsed laser beam, comprising effecting exposure of one shot by a plurality of pulse exposures through a corresponding number of plural pulses each having a preset output suitable to providing an amount of exposure smaller than a correct amount of exposure.

2. A method according to Claim 1, wherein each of said plural pulses has a preset output suitable to providing an amount of exposure which is substantially equal to 1 /N of the correct amount of exposure, where N is the number of said pulse exposures.

3. A method according to Claim 1, wherein an excimer laser is used to provide the pulsed laser beam.

4. A method of exposure with a pulsed laser beam, comprising: effecting, upon exposure of one shot, a plurality of pulse exposures with a correspond- ing number of plural pulses each having a preset output suitable to providing an amount of exposure smaller than a correct amount of exposure; integrating the amount of exposure and comparing the integrated amount of exposure with the correct amount of exposure; and controlling addition of a pulse exposure on the basis of the result of comparison.

5. A method according to Claim 4, wherein said plural pulses have preset outputs substantially equal to each other.

6. A method according to Claim 5, wherein said additional pulse exposure is effected by a pulse having a preset output substantially equal to that of each of said plural pulses.

7. A method according to Claim 5, wherein said additional pulse exposure is effected by a pulse having a preset output different from that of each of said plural pulses.

8. A method according to Claim 5, wherein said additional pulse exposure is efected by a plurality of additional pulses and wherein at least the last one of said additional pulses has a preset output different from that of each of said plural pulses for the firstmentioned pulse exposures.

9. A method according to Claim 4, wherein the pulsed laser beam comprises a laser beam provided by an excimer laser.

10. An apparatus for exposure with a pulsed laser beam, comprising: illumination means for generating pulses of laser beam each having an output suitable to providing an amount of exposure smaller than a correct amount of exposure, said illumination means being adapted to provide a plurality of pulses as aforesaid for effecting a corresponding number of plural pulse exposures for one shot; means for integrating the amount of exposure by the pulsed laser beam generated by said illumination means and for comparing the integrated amount of exposure with the correct amount of exposure; and means for controlling addition of a pulse exposure on the basis of the result of comparison.

11. An apparatus according to Claim 10, wherein said illumination means includes a light source for generating the pulsed laser beam and pulse output presetting means for variably presetting the output of each of said pulses to be generated by said light source at a value suitable to providing an amount of exposure smaller than the correct amount of exposure.

12. An apparatus according to Claim 11, wherein said pulse output presetting means is adapted to preset the same output relative to said plural pulses for said plural of pulse exposures.

13. An apparatus according to Claim 12, wherein said pulse output presetting means is adapted to preset, relative to a pulse for said additional pulse exposure, an output substantially equal to the preset output of each of said plural pulses for said plural pulse exposures.

14. An apparatus according to Claim 12, wherein said pulse output presetting means is adapted to preset, relative to a pulse for said additional pulse exposure, an output different from the preset output of each of said plural pulses for said plural pulse exposures.

15. An apparatus according to Claim 12, wherein said means for controlling addition of pulse exposure is adapted to effect a plurality of additional pulse exposures with a corresponding number of plural additional pulses, and wherein said pulse output presetting means is adapted to preset, relative to at least the last one of said additional pulses, an output different from the preset output of each of said plural pulses for the first-mentioned plural pulse exposures.

16. An apparatus according to Claim 10, wherein said illumination means includes an excimer laser for providing the pulsed laser beam.

17. An apparatus according to Claim 11, wherein said light source comprises an excimer laser.

18. An exposure method substantially as herein described with reference to Figs. 1 and 2 of the accompanying drawings.

19. An exposure method substantially as herein described with reference to Figs. 3 and 4 of the accompanying drawings.

20. An exposure method substantially as herein described with reference to Figs. 5 and 8 of the accompanying drawings.

21. Exposure apparatus substantially as herein described with reference to Figs. 1 and 2 of the accompanying drawings.

22. Exposure apparatus substantially as herein described with reference to Figs. 3 and 4 of the accompanying drawings.