GB2196440A

GB2196440A - Controlled exposure

Info

Publication number: GB2196440A
Application number: GB08719664A
Authority: GB
Inventors: Akiyoshi Suzuki
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 1984-02-01
Filing date: 1987-08-20
Publication date: 1988-04-27
Also published as: GB2155647B; DE3503273C2; GB8719664D0; DE3503273A1; GB8501764D0; GB2155647A; GB2196440B

Description

GB2196440A 1 SPECIFICATION It is another object of the present invention

to provide a method and an apparatus for per Exposure method and apparatus forming exposure with the pulsed laser beam, in which the amount of exposure can be very

BACKGROUND OF THE INVENTION 70 easily and positively controlled.

This invention relates to an exposure method Briefly, according to the present invention, and an exposure apparatus, and, more particu- there are provided a method and an apparatus larly, it relates to exposure method and appa- for exposing an object with a pulsed laser ratus for the manufacture of semiconductor beam. The exposure of one shot (one expo circuit devices. 75 sure area) is achieved by a plurality of pulse Recent development in the semiconductor exposures with the corresponding number of technology has enforced higher capacities and pulses, as the result of which any fluctuations further miniaturization of the semiconductor or errors in the outputs of the pulses are sub circuit devices. Along such trend, photolitho- stantially compensated so that correct expo graphy techniques such as an optical exposure 80 sure of each shot is assured. In other aspect, process have become more and more domi- the amounts of exposures by the plural pulses nant with the development of high resolution for the one shot exposure are integrated and lenses. In such exposure systems, a short the integrated amount of exposure is com wavelength of light within the deep UV range pared with a correct or desired amount of has recently been used to transfer and print a 85 exposure. On the basis of the result of com circuit pattern of a mask or reticle onto a parison, an additional pulse exposure is ef wafer. This is because the resolution for the fected in accordance with the degree of under minimum line width of the circuit pattern to be exposure.

printed on the wafer is proportional to the These and other objects, features and ad- wavelength of the light. 90 vantages of the present invention will become Conventionally, heavy hydrogen lamps or more apparent upon a consideration of the XeHg lamps have been used as the deep UV following description of the preferred embodi light sources. These lamps are featurized in merits of the present invention taken in con the point of continuous emission in both junction with the accompanying drawings.

cases of DC energization and AC energization. 95 In view of such feature, the amount of expo- BRIEF DESCRIPTION OF THE DRAWINGS sure for the wafer has been achieved by an- Figure 1 is a schematic and diagrammatic alog-like control systems such as, for view showing an exposure apparatus, of re example, a timer control system for controlling duction projection type, in accordance with an the exposure time by means of a timer, or an 100 embodiment of the present invention.

integrating exposure-meter system in which Figure 2 is a schematic and diagrammatic the amount of exposure is integrated and the view showing an illumination optical system of exposure is continued until the integrated ex- the exposure apparatus shown in Figure 1.

posure reaches a predetermined value. Figure 3 is a schematic and diagrammatic Use of such light sources however involves 105 view showing an exposure apparatus, of a re- inconveniences, because only a decreased out- duction projection type, in accordance with put is obtainable in the deep UV range and another embodiment of the present invention.

the sensitivity of the photoresist material ap- Figure 4 is a flow chart showing an example plied to the wafer surface is low. This results of control for the amount of exposure in the in a longer exposure time and a decreased 110 exposure apparatus of Figure 3.

throughput. Figures 5-8 are waveform views showing It has recently been found that an Excimer examples of pulse control.

laser capable of providing a higher output in the deep UV range may be effective as the DESCRIPTION OF THE PREFERRED EMBODI light source means for the exposure appara- 115 MENTS tus. However, the excimer laser is a pulse- Figure 1 shows an embodiment of the pre- oscillation type laser, as compared with the sent invention which is applied to an exposure conventional heavy hydrogen lamps or Xe-Hg apparatus of reduction projection type, called lamps. For this reason, the above-described a stepper. The exposure apparatus includes a analog-like control system for controlling the 120 light source 1, such as an excimer laser, pro amount of exposure could not be applied, as viding a pulsed laser beam. Within the light it is, to exposure apparatuses employing ex- source 1, a gas of, e.g., KrF or XeCI is sealin cimer lasers. gly contained, so that a wavelength of light in the deep UV range such as 248 rim (in the SUMMARY OF THE INVENTION 125 case of KrF gas) or 308 rim (in the case of

It is accordingly a primary object of the pre- XeCI gas) is emitted in the form of pulses.

sent invention to provide a method and an The laser beam emitted by the light source apparatus for performing exposure with a 1 enters into an illumination optical system 2, pulsed laser beam, in which a correct amount the details of which are illustrated in Figure 2.

of exposure can be easily attainable. 130 The illumination optical system 2 includes a 2 GB2196440A 2 beam shaping optical system 21 such as a to the correct amount of exposure is remarka toric lens, an optical integrator 22 such as a bly decreased. This is because the total compound-eye lens, a collimator lens 23 and amount of exposure becomes 5% over only a mirror 24. Each of the optical systems when each of the pulses has provided the 21-23 are made of a material,such as Si02, 70 maximum over-exposure (5% over). On the CaF, or the like which is transmissive to the other hand, there is a substantial probability light of deep UV range. Commercially available that the plural times pulse exposures cancel excimer lasers usually provide a beam of ob- the errors with each other.

long cross-sectional shape. In view of this. the According to the present invention, on the beam shaping optical system 21 is provided 75 basis of such finding, the output of each pulse to suitably shape the laser beam, e.g. into a to be generated by the excimer laser 1 is square shape. The optical integrator 22 is pro- controlled by the laser output control unit 4 vided to achieve uniform distribution of light. so that an output, suitable to providing an Referring now back to Figure 1, a reticle or amount of exposure which is smaller than the mask M having formed thereon an integrated 80 correct amount of exposure for one shot, is circuit pattern, a projection optical system 3 preset for each of the pulses, and the expo and a wafer W are disposed, in the named sure of one shot is effected by a plurality of order, along the optical path defined by the pulse exposures through the corresponding illumination optical system 2. Similarly to the number of pulses each having the preset out illumination optical system 2, the projection 85 put as aforesaid. The term -preset output optical system 3 is made of a material which means an expected average value of the pulse is transmissive to the light of deep UV range. outputs which will be actually generated by The reduction projection may of course be the excimer laser 1 if a fixed value is continu achieved by a reflection imaging system, in ously input to the excimer laser 1. That is, in place of the projection lens system. 90 view of that each pulse output from the ex- Designated by a reference numeral 4 is a cimer laser involves an error or fluctuation in control unit for controlling the pulse output of the range of approx. 5%, an expected aver the excimer laser 1. In this laser output con- age value of the pulse outputs which will be trol unit 4, the output of each pulse to be actually generated by the excimer laser 1, with provided by the excimer laser 1 can be vari- 95 respect to a particular input value, is specified ably preset. at the laser output control unit 4. In this As described in the foregoing, the excimer sense, the -preset output- can be expressed laser is capable of providing a higher output. as a -specified output- or a -target output" For this reason, only one pulse exposure can In accordance with the present invention, provide a sufficient amount of exposure rela- 100 stated strictly, the exposure of one shot is tive to one shot. Thus, the exposure process effected by a plurality of pulse exposures with can be effected by one pulse emission for the corresponding number of pulses each con each shot. However, it is known that the out- taining an error in its output. Nevertheless, the puts of the pulses generated by the excimer errors in the pulse outputs are substantially laser involve fluctuations or errors in the range 105 compensated for, as the result of the plural of 5% or more. Such fluctuations in the out- pulse exposures. Accordingly, it is not neces puts of the pulses would result in irregularities sary to provide any specific control means for in the amounts of exposures. Therefore, the correcting the fluctuation in the quantity of one pulse exposure per one shot would not pulsed laser beam.

easily ensure a correct exposure for each shot 110 As a convenient way, the outputs of the and uniform exposures relative to plural shots. plural pulses for the exposure of one shot are The above-described problem can be solved preset at the same value. In other words, by the present invention. Briefly stated, the when the number of pulse exposures for one present invention aims at deleting or sup- shot is N, the output of each pulse is preset pressing incorrect or uneven exposure which 115 at a value suitable to providing an amount of will be otherwise caused by the fluctuations in exposure which is equal to or substantially the outputs of the pulses, without correcting equal to 1 /N of the correct amount of expo the fluctuations themselves of the outputs of sure.

the pulses. In summary, the present invention Commecially available excimer lasers gener- is based on the following finding made by the 120 ally have a high emission-repetition frequency inventor: of the order of 200-300 Hz. For this reason, ThaOs, in the case of one pulse exposure the plural pulse exposures per one shot still per one shot exposure, 5% fluctuation or improves the throughput, as compared with error in the output of each pulse would di- conventional exposure apparatuses. For rectly leads to 5% error in the amount of 125 example, in a case where averagely ten (10) exposure. As compared therewith, if the expo- pulse exposures are effected for each shot sure of one shot is effected by two or more and even if the number of pulse exposures pulses, the probability that the total amount of varies within a range from nine (9) to eleven exposure by these plural pulse exposures be- (11) due to the errors in the pulse outputs, comes 5% over (the maximum error) relative 130 the required exposure time will be within a 3 GB2196440A 3 range of 0.04-0.05 see. This is a striking further pulse exposure is controlled, whereby contradistinction to the conventional steppers the amount of exposure for each shot is more which generally require approx. 0.3 sec. expo- strictly and precisely controlled.

sure time. Thus, the exposure time required As shown in Figure 3, a mirror 5a is dis- by the plural pulse exposures is shortened to 70 posed in the optical path defined by the illumi- a value which is smaller, by one---figure-,nation optical system 2 and a LIV photosensor than that required by the conventional step- 5b is disposed in the optical path defined by pers. Therefore, both the stabilized exposure the light beam reflected by the mirror 5a. The and improved throughput are assured. Even if photosensor 5b may be disposed near the the exposure of one shot is effected by 75 laser light source 1 or in the optical path ex twenty (20) pulses, the exposure will be corn- tending from the light source 1 to the wafer plated within approx. 0.1 sec., which is a sig- W. In response to reception of light, the pho nificant improvement over the conventional extosensor 5b generates an output which is posure apparatuses. supplied to a light quantity integration circuit 6 In this specification, the term---oneshot- 80 into which the sensitivity of the photoresist means the exposure which is enough to ex- has been previously input. Within the integra pose the whole surface of the wafer in a case tion circuit 6, the amounts of exposures by of global or wholesurface exposure; the expo- the pulses from the excimer laser 1 are se sure which is enough to expose one chip in a quentially integrated. The integrated total case of step-and-repeat type exposure 85 amount of exposure is then compared with wherein the exposure operation is effected for the correct amount of exposure at a compara each chip; and the exposure which is enough tor circuit 9. Subsequently, the result of corn to expose one slit width in a case of slit parison is input to a central processing unit 7.

exposure. On the basis of result of comparison at the While in the Figure 1 embodiment the inven- 90 comparator circuit 9, the central processing tion has been described with reference to the unit 7 computes the number of pulses and/or lens projection type exposure apparatus, the the output of each pulse necessary for suffici invention is not limited thereto and it is appli- ently exposing the photoresist material on the cable to a contact type or proximity type ex- wafer W. The result of computation is then posure apparatus. 95 supplied to a laser output control unit 8 which In accordance with the present invention, as is adapted to drive the excimer laser 1 in has hitherto been described, the exposure of accordance with the result of computation at one shot is effected by a plurality of pulse the central processing unit 7, whereby the exposures with the pulsed laser beam. By pattern of the mask M is irradiated by the this, in spite of the strikingly different feature 100 pulses of the number and/or outputs con of the pulsed laser beam as compared wit the trolled as required. By this, the mask pattern continuous emission of conventional light is printed on the wafer W surface.

sources, stable exposure is ensured without If necessary, the efficiency of the illumina- using any specific control means. Moreover, tion optical system (e.g., the diameter of the the throughput is improved. 105 laser beam, quantity, etc.) may be controlled Figure 3 shows another embodiment of the by an illumination efficiency controlling unit 10 present invention which is applied to a reduc- in accordance with the result of computation tion projection type exposure apparatus, called at the central processing unit 7. Further, the a stepper, with the elements corresponding to output of each pulse can be controlled by one those of Figure 1 embodiment being desig- 110 of or both of the laser output control unit 8 nated by the same reference numerals. The and the illumiantion efficiency controlling unit exposure apparatus includes a light source 1 10.

such as an excimer laser, an illumination opti- Operation of the present embodiment will cal system 2 and a projection optical system now be described with reference to the flow 3, all of which may be the same as those of 115 chart shown in Figure 4.

Figure 1 embodiment. Further, a reticle or As has already been described with refer- mask M and a wafer W may be the same as ence to the Figure 1 embodiment, the plural those shown in Figure 1. Therefore, descrip- pulse exposure for each shot is effective to tions thereof will be omitted here for the sake substantially compensate for the fluctuations of simplicity of explanation. 120 or errors in the pulse outputs and assures Essentially, the Figure 3 embodiment differs stable control for the amount of exposure. In from the Figure 1 embodiment in the following the present embodiment, in addition to such poins: plural-pulse exposure, the amount of plural- In the present embodiment, in addition to pulse exposure is integrated, by real time, and effecting a plurality of pulse exposures for 125 the integrated amount of exposure is com each shot, the amounts of pulse exposures pared with the correct amount of exposure. If are integrated and the integrated total amount the result of comparison shows underexpo of exposure is compared with a correct sure, addition of further pulse exposure is ef amount of exposure for one shot. On the ba- fected in accordance with the amount or de sis of the result of comparison, addition of 130 gree of "under".

4 GB2196440A 4 For example, -n- times of pulse exposures to the degree of under-exposure is effected by -are effected by n pulses each having the same controlling the number of additional pulses, preset output, while the amount of exposure such pulse number control may be replaced is monitored by the integration circuit 6. Alter- by or combined with the control of the preset natively, -m- times of pulse exposures, which 70 output of the additional pulse. More specifi are effective to achieve the required amount cally, the amount of exposure by the above of exposure provided that the maximum errors described n or m times of pulse exposures is (e.g., 5%) in the outputs are accumulated, compared with the correct amount of expo are effected by using m pulses each having sure and, if under-exposure, the additional the same preset output, and subsequently the 75 pulsd exposure is effected by an additional total amount of exposure is monitored by the pulse having a preset output which is con integration circuit 6. trolled in accordance with the defficiency in The integrated amount of exposure by the n the amount of exposure, such as shown in or m times of pulse exposures is compared Figure 7. In such case, the preset output of with the correct amount of exposure at the 80 the last pulse, i.e. the additional pulse, is comparator circuit 9. If the difference there- made lower (the case of Figure 7) or higher between is within a predetermined permissible than those of the preceding pulses, in accor range, the exposure is completed. If, on the dance with the degree of underexposure. The other hand, the result of comparison shows a addition of pulse exposure may be achieved substantial defficiency in the amount of expo- 85 by using a plurality of additional pulses, as sure, additional exposure is effected using one described in the foregoing. In such case, the or plural additional pulses in accordance with additional pulses may have the same preset the defficiency, such as shown in Figure 5. output, as shown in Figure 8. Alternatively, The number of such additional pulses is deter- only the last pulse may have a different preset mined so as to minimize the difference be- 90 output, such as shown in Figure 7. In any tween the correct amount of exposure and the case, the amount of exposure which is sub final amount of exposure, i.e. to minimize the stantially or approximately equal to the correct degree of under-exposure or over-exposure. amount of exposure is attainable.

Even if the total amount of exposure which The plural pulse exposures may be effected is finally determined by the final pulse expo- 95 by using the corresponding number of pulses sure becomes---under-or---over-relative to having different preset outputs. When, in such the correct amount of exposure, the difference case, the correct amount of exposure is de therebetween can be remarkably reduced in noted by EO, the preset output of the first contrast with the one shot/one pulse expo- pulse n, is denoted by E, the preset output sure. 100 of the next pulse n, is denoted by E, and the If, for any reason, the correct amount of preset output of the last pulse n3 is denoted exposure is reached by one or such a number by E31 it follows that:

of pulse exposures which is smaller than n or m as aforesaid, the exposure operation is of E, = n1E, + n2E2 + n3E3 course completed at that time. 105 In a case where the output of each pulse is If E, = alEO, E2 = a2E, and E3 = a3E0, it further decreased and the number of pulses follows that:

for each shot is accordingly increased, such as shown in Figure 6, an amount of exposure nla, + n202 + n3a3 which is very close to the correct amount of 110 exposure is attainable. This is because the By adjusting the values Of Cl, 4912 and a3 in preset output of each pulse is decreased so the manner that the value of "n, + n2 + n3 that the absolute fluctuation or deviation of is made small, the exposure time can be shor each pulse output is smaller even if the last tened while the correct amount of exposure is one or ones of the pulses involve +5% error. 115 ensured.

The pulse output can be decreased by de- Similarly to the Figure 1 embodiment, the creasing the output of the laser light source present invention in respect to the Figure 3 itself, as described in the foregoing. Alterna- embodiment is not limited to the lens projec tively, the pulse output may be decreased by tion reduction exposure apparatus but is also using an ND filter disposed in the optical path 120 applicable to a mirror projection type exposure of the laser light source or by changing the apparatus. a contact type exposure apparatus efficiency of the illumination optical system. or a proximity type exposure apparatus.

The latter may be assured by reducing the In accordance with the present invention, as diameter of the illumination light. As a further has hitherto been described, correct exposure alternative, the output of the laser light source 125 is stably attainable. with the use of a pulsed may be primarily adjusted while the efficiency laser beam such as an excimer laser beam, in of the illumination optical system may be ad- spite of substantial fluctuation in the output of justed supplementarily. each pulse.

While, in the examples shown in Figures 5 While the invention has been described with and 6, the additional pulse exposure according 130 reference to the structures disclosed herein, it GB2196440A 5 is not confined to the details set forth and ing number of plural pulse exposures for one this application is intended to cover such mo- shot; difications or changes as may come within the means for integrating the amount of expo- purposes of the improvements or the scope sure by the pulsed laser beam generated by of the following claims. 70 said illumination means and for comparing the integrated amount of exposure with the cor

Claims

CLAIMS rect amount of exposure; and

1. A method of exposure with a pulsed means for controlling addition of a pulse ex- laser beam, comprising effecting exposure of posure on the basis of the result of compari one shot by a plurality of pulse exposures 75 son.

through a corresponding number of plural 11. An apparatus according to Claim 10, pulses each having a preset output suitable to wherein said illumination means includes a providing an amount of exposure smaller than light source for generating the pulsed laser a correct amount of exposure. beam and pulse output presetting means for 2. A method according to Claim 1, wherein 80 variably presetting the output of each of said each of said plural pulses has a preset output pulses to be generated by said light source at suitable to providing an amount of exposure a value suitable to providing an amount of which is substantially equal to 1/N of the corexposure smaller than the correct amount of rect amount of exposure, where N is the num- exposure.

ber of said pulse exposures. 85 12. An apparatus according to Claim 11, 3. A method according to Claim 1, wherein wherein said pulse output presetting means is an excimer laser is used to provide the pulsed adapted to preset the same output relative to laser beam. said plural pulses for said plural of pulse expo- 4. A method of exposure with a pulsed sures.

laser beam, comprising: 90 13. An apparatus according to Claim 12, effecting, upon exposure of one shot, a plu- wherein said pulse output presetting means is rality of pulse exposures with a corresponding adapted to preset, relative to a pulse for said number of plural pulses each having a preset additional pulse exposure, an output substan output suitable to providing an amount of ex- tially equal to the preset output of each of posure smaller than a correct amount of expo- 95 said plural pulses for said plural pulse expo sure; sures.

integrating the amount of exposure and 14. An apparatus according to Claim 12, comparing the integrated amount of exposure wherein said pulse output presetting means is with the correct amount of exposure; and adapted to preset, relative to a pulse for said controlling addition of a pulse exposure on 100 additional pulse exposure, an output different the basis of the result of comparison. from the preset output of each of said plural 5. A method according to Claim 4, wherein pulses for said plural pulse exposures.

said plural pulses have preset outputs sub- 15. An apparatus according to Claim 12, stantially equal to each other. wherein said means for controlling addition of 6. A method according to Claim 5, wherein 105 pulse exposure is adapted to effect a plurality said additional pulse exposure is effected by a of additional pulse exposures with a corre pulse having a preset output substantially sponding number of plural additional pulses, equal to that of each of said plural pulses. and wherein said pulse output presetting -,i 7. A method according to Claim 5, wherein means is adapted to preset, relative to at said additional pulse exposure is effected by a 110 least the last one of said additional pulses, an pulse having a preset output different from output different from the preset output of that of each of said plural pulses. each of said plural pulses for the first-men- 8. A method according to Claim 5, wherein tioned plural pulse exposures.

said additional pulse exposure is efected by a 16. An apparatus according to Claim 10, plurality of additional pulses and wherein at 115 wherein said illumination means includes an least the last one of said additional pulses has excimer laser for providing the pulsed laser a preset output different from that of each of beam.

said plural pulses for the first-mentioned pulse 17. An apparatus according to Claim 11, exposures. wherein said light source comprises an ex- 9. A method according to Claim 4, wherein 120 cimer laser.

the pulsed laser beam comprises a laser beam 18. An exposure method substantially as provided by an excimer laser. herein described with reference to Figs. 1 and 10. An apparatus for exposure with a 2 of the accompanying drawings.

pulsed laser beam, comprising: 19. An exposure method substantially as illumination means for generating pulses of 125 herein described with reference to Figs. 3 and laser beam each having an output suitable to 4 of the accompanying drawings.

providing an amount of exposure smaller than 20. An exposure method substantially as a correct amount of exposure, said illumination herein described with reference to Figs. 5 and means being adapted to provide a plurality of 8 of the accompanying drawings.

pulses as aforesaid for effecting a correspond- 130 21. Exposure apparatus substantially as 6 GB2196440A 6 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.

Published 1988 at-rhe Patent Office, State House, 66/71 High Holborn, London WC 1 R 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD.

Printed by Burgess & Son (Abingdon) Ltd. Con. 1/87.