Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
1999-06-17
2002-07-09
Davie, James W. (Department: 2828)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S025000, C372S030000
Reexamination Certificate
active
06418155
ABSTRACT:
TECHNICAL FIELD
This invention pertains to laser devices for using laser beams to output prescribed light to expose semiconductors, polymer materials, and inorganic materials and for outputting laser beams for machining apparatuses that perform machining processes, and more particularly to improvements therein in order to obtain constantly uniform pulse energy values when implementing burst mode operations wherein a continuous oscillation action for generating pulses of laser light a prescribed number of times continuously is alternated over and over with a stopping action for stopping the pulse generation for a prescribed time.
BACKGROUND ART
In the field of semiconductor exposure devices which employ UV light beams, it is found necessary to implement fine precision exposure light quantity control in order to maintain circuit pattern resolution above a certain level. However, with the excimer lasers that are used as the light sources in these semiconductor exposure devices, there is variation in the pulse energy from one pulse to the next because they are so-called discharge excitation gas lasers. There is a need to reduce this variation in order to improve the precision of exposure light quantity control.
Semiconductor exposure devices, on the other hand, alternately repeat exposures with stage movements. More specifically, in
FIG. 9
is diagrammed a semiconductor wafer W whereon are arranged a plurality of IC chips
90
, and, when performing exposures with a stepper, upon the completion of exposure processing wherein one IC chip
90
on the semiconductor wafer W has been irradiated with a plurality of continuous light pulses, either the wafer W or the optical system is shifted so that the next unirradiated IC chip
90
can be irradiated with continuous light pulses, whereupon, after this stage shift, light irradiation is performed as before. Upon the completion of this exposure operation on all of the IC chips
90
on the semiconductor wafer W, alternating the exposures and the stage shifts in this manner, the completely exposed wafer W is carried away and the next wafer W is placed in the irradiation position so that the same light irradiation process can be repeated again.
Thus a semiconductor exposure device is fashioned so that exposures and stage shifts are alternately repeated. The operation of an excimer laser constituting the light source in such an exposure device, therefore, as diagrammed in
FIG. 10
, involves a burst mode operation wherein a continuous pulse generation operation for pulse-generating laser beams continuously a prescribed number of times is repeated together with an oscillation stop interval t during which pulse generation is suspended for a prescribed time interval.
More specifically, in the burst mode operation diagrammed in
FIG. 10
, the oscillation stop interval t corresponds to the time required to move the stage in the semiconductor exposure device. This oscillation stop interval t, however, for various reasons, is not necessarily constant. When one wafer is being exchanged with another, for example, the oscillation stop interval will be much longer than when moving the stage between IC chips. Also, the oscillation stop interval needed when shifting between IC chips in the same row will be very different from the oscillation stop interval needed when shifting from one IC chip to another IC chip in a different row. When the number or arrangement of the IC chips on the wafer changes, moreover, that will also cause the oscillation stop intervals to change. There are various other factors that cause changes in the oscillation stop interval. It should be noted also that, in
FIG. 10
, the energy intensity of each pulse is represented for a case where the excitation intensity (discharge voltage) is fixed at a constant value.
In such burst operations as this, when the length of the oscillation stop interval t varies, these variations cause large changes in the output of individual laser pulses, as diagrammed in FIG.
11
. More specifically, when the oscillation stop interval t is short, the effects of past laser generations remain in the form of rises in gas temperature, disruption of the gas or gases inside the laser chamber, and localized rises in electrode temperature, etc. When the oscillation stop interval t is long, on the other hand, the effects of past laser generations on the laser disappear. For this reason, even if the laser discharge voltage is held constant, as is diagrammed in
FIG. 11
, when the stop time is short the output energy will be smaller, and when the stop time is long the output energy will be larger. Thus the laser output will change greatly in response to the oscillation stop interval.
In the meantime, as noted above, an excimer laser is a pulse discharge excitation gas laser, for which reason it is very difficult to continue oscillations so as to produce a pulse energy that is always at a constant level. There are at least two reasons for this, namely (1) density commotion in the laser gas inside the discharge space develops due to the discharges, making the next discharge uneven and unstable, and (2) localized temperature rises occur in the surface of the discharge electrodes due to these uneven discharges, etc., which result in deterioration in the next discharge and cause discharges to be uneven and unstable.
This tendency is particularly pronounced during the initial stage of the continuous pulse generation interval described above. As diagrammed in
FIG. 12
, in the spike region that contains the first several pulses after the completion of the oscillation stop interval t, at first comparatively high pulse energy is obtained, but thereafter the pulse energy gradually falls. This is the so-called spiking phenomenon. When this spike region is finished, the pulse energy passes through a plateau region wherein a stable value continues at a comparatively high level, and then enters a steady region.
Thus, with an excimer laser device operated in burst mode, the energy variation between pulses described above causes the precision of quantitative exposure control to decline, and the spiking phenomenon makes this variation even more pronounced, resulting in a large decline in quantitative exposure control, which is a problem.
In the face of this problem, the applicant has filed for patents on various inventions pertaining to so-called spike prevention control wherein, using the property whereby the energy of pulses generated increases as the excitation intensity (charging voltage, discharge voltage) increases, the discharge voltage (charging voltage) for the first pulse in continuous pulse generation in the burst mode is made smaller, and the discharge voltage for the following pulses is made gradually larger, thereby changing the discharge voltage for each pulse and preventing the initial energy rise due to the spiking phenomenon (Japanese Patent Application No. 4-191056, Japanese Patent Application Laid-open No. 7-106678 (Japanese Patent Application No. 5-49483), etc.).
More specifically, as based on the prior art cited above, discharge voltage data for causing the energy of each pulse in continuous pulse generation to be at a desired target value Pr, taking various parameters such as oscillation stop interval t and power lock voltage (the power supply voltage determined in response to the deterioration of the laser gas) into consideration, are stored beforehand in a table for each pulse in the continuous pulse generation, the pulse energy Pi (where i=1, 2, . . . ) for the current continuous pulse generation is detected, this detected value Pi is compared against the pulse energy target value Pd, and, based on the results of this comparison, the discharge voltage data for each pulse previously stored, as noted above, are corrected and updated. These corrected voltage data are used as the discharge voltage data during the next burst cycle.
In the discharge voltage correction control described above, the pulse energy Pi resulting from laser oscillation is detected using the discharge voltage datum Vi stored in the
Ariga Tatsuya
Igarashi Toru
Sakanishi Shoichi
Takahashi Tomokazu
Wakabayashi Osamu
Davie James W.
Komatsu Ltd.
Varndell & Varndell PLLC
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