Miscellaneous active electrical nonlinear devices – circuits – and – Specific signal discriminating without subsequent control – By polarity
Reexamination Certificate
2000-10-16
2004-04-27
Jackson, Jerome (Department: 2815)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific signal discriminating without subsequent control
By polarity
C372S025000, C372S029015, C372S030000, C372S038070, C372S038020
Reexamination Certificate
active
06727731
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to energy control for excimer and molecular fluorine gas lasers, and particularly to control and feedback software algorithms and gas replenishment for maintaining constant laser output emission pulse energies and/or application process energy doses.
2. Discussion of the Related Art
The energy of output emission pulses of an excimer or molecular fluorine laser will decrease continuously unless certain input parameters or conditions are controlled during the operation of the laser. This is due to halogen consumption by reactions of the halogen gas within the gas vessel and halogen burn up by the gas discharge. Additionally, the output power will decrease due to build up of gas contamination.
An excimer laser can be operated for a certain time at a constant energy level if the charging voltage is continuously increased to compensate these factors which cause energy losses. For demanding applications like lithography or TFT annealing, it is desired to maintain control of the charging voltage, as well as other beam parameters, in addition to pulse energy or laser output power. Therefore, more sophisticated processes were developed. When a maximum applicable charging voltage is reached, gas replenishment actions can be performed to further extend the operation time at constant energy of the laser. Such gas replenishment actions may be performed to compensate halogen depletion and for contamination reduction. Halogen depletion is typically compensated by halogen injections (HI). Contamination reduction is achieved by partial gas replacements (PGR).
Gas replenishment was introduced around 1986 for excimer lasers (see U.S. Pat. No. 4,997,573, which is hereby incorporated by reference). Gas replenishment actions may be triggered when the charging voltage exceeds a preset level. Gas replenishment actions have been characterized in the past by significant reductions in charging voltage. Large variations in charging voltage during long constant energy operation periods are a disadvantage, however, because such large variations in charging voltage can affect various beam parameters other than beam energy or power. In other words, large variations in charging voltage for stabilizing the output energy serve to destabilize other important beam parameters.
In order to maintain both the charging voltage and the output energy or power of the laser at substantially constant levels, large gas replenishment actions were replaced by smaller gas actions such as micro halogen injections (HI) and micro partial gas replacements or mini gas replacements (GR or mGR) (see U.S. patent application Ser. No. 09/447,882 and No. 60/171,717, each of which is assigned to the same assignee and is hereby incorporated by reference). These micro or mini gas replenishment actions preferably result in little or no disturbance in charging voltage that is detectable with sufficient precision under industrial operation conditions. Therefore, it is desired to use another parameter other than changes in the charging voltage to trigger the micro or mini gas replenishment actions. One parameter that may be used is the number of laser pulses, or pulse count, as a suitable trigger for micro or mini gas replenishment actions. This was disclosed in U.S. Pat. No. 5,097,291 and later in U.S. Pat. No. 5,337,215, each of which is hereby incorporated by reference. For example, a gas replenishment action may be performed periodically approximately every 100,000 pulses.
For microlithography scanner systems, it is desired to maintain constant energy dose when scanning over a die site on a wafer. The scanning speed, the exposure slit width and the laser repetition rate determine the number of pulses overlaid at each site on the wafer. The number of overlaid pulses is dependent on the application process. For example, approximately 40 pulses may be overlaid at a die site, whereas a typical length of a burst may be between 100 and 500 pulses.
The constant energy dose for each site on a wafer may be specified by a moving energy average. Precise dose control may then be observed as low fluctuation in moving energy average. The output energy of the laser may be controlled by changing the high voltage (HV) that is used for a discharge in the laser tube. The output energy can be and typically is measured for each pulse, and also the HV can be changed for each individual pulse.
Excimer and molecular fluorine lasers may be typically operated in burst mode. This means that the laser generates a “burst” of pulses, such as 100 to 500 pulses as mentioned above at a constant repetition rate, followed by a burst break or pause of from a few milliseconds up to a few seconds while the stepper/scanner does some wafer positioning. A burst break may be a short burst break such as may occur when the beam spot is moved to a different location on a same wafer, or may be a long burst break such as would occur when the stepper/scanner changes the wafer.
When an excimer or molecular fluorine laser is operated in burst mode, the first few pulses of each burst will have a higher pulse energy than later pulses if left uncompensated. Therefore, the moving average at the beginning of a burst would be higher than later in the burst. It is desired to compensate this overshoot in order to achieve a constant energy dose. Overshoot compensation may be achieved by reducing the charging voltage for the first few pulses (see U.S. Pat. Nos. 5,463,650, 5,710,787 and 6,084,897, each of which is hereby incorporated by reference). This allows the energy dose to be kept constant at the beginning of a burst sequence. The charging voltage is adjusted for each laser pulse at the beginning of a burst below that which is applied to pulses later in the burst.
The problem that the first few pulses after a burst break (at the beginning of a burst) have a higher ratio of energy to HV than the pulses in the middle or at the end of a burst, can be understood by observing what happens when the HV is kept constant during the burst, as illustrated in the sketch of FIG.
1
. In the sketch of
FIG. 1
, the first 5 to 10 pulses have a high energy, and then the energy first decays rapidly, and then more slowly until after 20 to 100 pulses the energy reaches a constant level. This phenomenon is called overshoot or spiking.
In order to keep the pulse energy or energy dose constant (which is desired during laser operation), one uses a low HV for the first pulses of a burst and then increases the HV to a constant level during the burst. This is done in response to the overshoot of the pulse energy that will otherwise occur as just described.
The exact behavior of the energy is affected by various parameters in a way that is difficult to predict. It is desired to have a technique for predicting the HV for the next pulse so that the energy of the next pulse or the energy dose at the application process will meet the target energy or target energy dose.
RECOGNIZED IN THE INVENTION
There are short-term effects and long-term effects that influence the behavior associated with the energies of pulses during burst and from burst to burst. Short-term effects may last for only a few seconds or less. Long term effects include gas aging (several days), tube aging (several months) and maybe optical effects (years). These effects may be taken into account by changing controller parameters. The parameter adaptation may be advantageously performed automatically.
The energy behavior changes, depending on the length of the burst break, the repetition rate of the laser, the energies of the most recent pulses and other effects. It is more difficult to control the energies of the first pulses in a burst than it is to keep the energy or energy-dose constant for pulses at the middle and end of a burst because gas conditions do not change as rapidly with time over the duration of the burst. It is thus desired to have pulse energy or energy dose control algorithm that produces high pulse energy or energy dose stability at the beginning of a burst, and al
Nowinski Guenter
Rebhan Ulrich
Jackson Jerome
Lambda Physik AG
Landau Matthew C.
Stallman & Pollock LLP
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