Patent Application
20080059096
The following detailed description is made with reference to the figures. Preferred embodiments are described to illustrate the present invention, not to limit its scope, which is defined by the claims. Those of ordinary skill in the art will recognize a variety of equivalent variations on the description that follows.
Further, the preferred embodiments are described with reference to an analogue SLM. It will be obvious to one with ordinary skill in the art that there may be situations when other SLMs than analogue ones will be equally applicable; for example digital SLMs like a digital micromirror device (DMD) made by Texas instruments. Additionally, SLMs may be comprised of reflective or transmissive pixels. Even further, the preferred embodiments are described with reference to an excimer lacer source. It will be obvious to one with ordinary skill in the art that a pulsed electromagnetic radiation source other than an excimer laser may be equally compensated by the inventive method, for instance a Nd-YAG laser, ion laser, Ti sapphire laser, free electron laser or other pulsed fundamental frequency lasers, flash lamps, laser plasma sources, synchrotron light sources etc.
The invention relates to a method to compensate for time jitter in a pulsed electromagnetic radiation source. Such a method is inter alia useful when patterning a workpiece using a spatial light modulator (SLM), where a pulsed electromagnetic radiation is impinged onto said SLM and relays images of said spatial light modulator, which images are stitched together at a continuously moving stage.
The laser source 110 may be an excimer laser emitting for instance 308 nm, 248 nm 193 nm, 156 nm, or 126 nm pulses. Said pulses are homogenized and shaped by the homogenizing and shaping lenses 120, 140. Said lenses 120, 140 comprise optics such that plane waves are exposing the Surface of the SLM 150. The temporal pulse length of the laser may be 0.1 μs or smaller, for instance 10 ns. The pulse repetition rate of the laser may be 0.5-5 kHz, for instance 2 kHz.
The third lens 160 determines the demagnification of the system. When using an analogue spatial light modulator, a spatial filter and a Fourier lens (not illustrated in the figure) are arranged between the third lens 160 and the semitransparent mirror 130.
The computer 185 generates the pattern to be imaged onto the workpiece. Said workpiece may be a transparent substrate covered with a layer with chrome which in turn is covered with a layer of photosensitive material. This is an example of a workpiece used in the manufacturing of masks and reticles. The workpiece may also be a semi-conducting wafer onto which the pattern is directly generated without a mask. This pattern may be generated by conventional software used in the lithography industry. Said pattern is transformed into a bitmap representation by the pattern bitmap generator 180. Said bitmap representation is in its turn transformed into drive signals for the spatial light modulator by said bitmap generator 180. Said drive signals will set individual pixel elements in said spatial light modulator 150 into a desired modulation state. In case of an analogue spatial light modulator a specific drive signal will correspond to a specific deflection state of a particular pixel element. Deflection states of an analogue pixel element such as a micro mirror operated in an analogue mode may be set to any number of states between fully deflected and non-deflected, for instance 64, 128 or 256 states.
The interferometer 170 continuously detects the position of the workpiece. The workpiece may move with a constant speed when patterning a strip of stamps. The workpiece may also move with a variable speed. When the workpiece moves with a variable speed, it is necessary to detect the actual speed a short time period before illuminating the SLM in order to be sure that a stamp of the SLM will be printed at a requested position on the workpiece. The stamp is a reproduction of the pattern of the SLM onto the workpiece. A reduction of the pattern of the SLM may be performed through one or a plurality of lenses before being reproduced onto the workpiece. Several stamps stitched together will form a strip. Strips stitched together will form a complete image. The interferometer 170 transmits and receives signals 165 for detecting said position of the workpiece. When the workpiece is at a given position detected by said interferometer a trigger signal is sent to the laser. One way of generating said trigger signal is to compare a detected value of position of the workpiece with a stored value of position. When there is a match between a stored value of position, in for example a look up table, and a detected value of position a trigger signal will be generated. Said trigger signal will eventually cause the laser to pulse.
In
At a pulse rate of 1 kHz and a demagnified SLM size in the direction of movement of the workpiece of 50 μm, the writing speed will be 50 mm/s. At 1 kHz pulse rate and with a temporal pulse length of each pulse of 10 ns, there is a time space of 0.99 μs between each pulse. In one embodiment the interferometer is calibrated to generate a trigger signal in the middle of said time space, i.e., 0.495 μs after the end of the laser pulse. With no time jitter the time shift device holds said trigger signal for another 0.495 μs before sending said trigger signal to the laser. However, other holding times for said trigger signal is also possible, which holding time may be greater or smaller than said 0.495 μs. For instance, in one embodiment the trigger signal is held a time period being equal to the 100 ns. In another embodiment said trigger signal is held a time period of 25 ns.
The light delay from said trigger signal depends on several factors, inter alia the charging voltage in a laser chamber and the actual temperature of the laser chamber. Without knowing the charging voltage and said temperature this invention compensates for errors in pulse timing, i.e., jitter, by using the knowledge of a delay of at least one earlier laser pulse.
The trigger signal is detected by a detector 230 and starts a clock in the flash measure device 235. The same trigger signal is transmitted to said time shift device 250. When the laser pulse is detected by said detector 230 said clock is stopped. A time-period between the trigger signal and the laser pulse, denoted A, is compared with a requested value, which is denoted with B. A difference between said requested value B and said actual time period A is calculated. Said difference can be used to compensate the jitter in the next pulse by delaying said trigger signal more or less, where increasing the delay correspond to a positive difference and decreasing the delay corresponds to a negative difference. The information about said difference is sent to the time shift device 250 which will increase or decrease the delay by said difference.
In another embodiment of the present invention, which may be used when starting up the laser, one call detect a plurality, i.e., two, three, four or more pulses to see variations in said pulse timing. By doing so it is possible to detect a trend or an oscillation of the error of pulse timing around a certain value. Said variation or oscillation can be used to predict the next error in pulse timing. After a certain number of pulses one can fit a curve represented by a given formula to said variations of error in pulse timing or jitter. Said curve can be used when predicting the timing of the following laser pulse. A curve fit may be performed using numerical approximation or other statistical methods for making a prediction of a next coming pulse.
Before using the laser for pattern generation, i.e., during start up procedure a number of pulses is ran through a test loop to determine the characteristics of the particular laser at a particular event, when this has been determined said laser may be used our pattern formation, i.e., flashes will be allowed to impinge on the SLM 150 to expose the workpiece 190.
The trigger signal is detected by a detector 330 and starts a clock in the flash measure device 235. The same trigger signal is transmitted to said time shift device 350. When the laser pulse is detected by said detector 330 said clock is stopped. A time-period between the trigger signal and the laser pulse, denoted A, is compared with a requested value, which is denoted with B. A difference between said requested value B and said actual time period A is calculated. Said difference can be used to compensate the jitter in the next pulse by delaying said trigger signal more or less, where increasing the delay correspond to a positive difference and decreasing the delay corresponds to a negative difference. The information about said difference is sent to the time shift device 250 which will increase or decrease the delay by said difference.
This embodiment further comprises a speed detector 360 which detects a speed of a workpiece onto which said pulsed laser will impinge. The speed of the workpiece is regulated by an analogue or digital servo. The actual speed of tile workpiece, denoted C, is measured in the time interval between laser pulses. The time shift device may compensate a difference between the actual speed C of the workpiece from a requested speed of the same, denoted D, by an increase or decrease of the delay of the trigger signal. A too slow movement of the workpiece, i.e., the difference between requested speed D and actual speed C is a positive value, will result in an increase of a delay time of the trigger signal and a too rapid movement of the workpiece, i.e., the difference between requested speed D and actual speed C is a negative value, will result in a decrease of a delay time of said trigger signal.
The speed of the workpiece may be measured at any moment between laser pulses. In one embodiment the speed of the workpiece is measured just before sending the trigger signal to the laser. By doing so, compensation of speed variations of the workpiece by altering the delay time of the trigger signal will use as fresh detected speed as possible and thereby possibly enhancing the accuracy.
In yet another embodiment a combination of adjusting the speed of the workpiece and a variation of the delay of the trigger signal is performed for said jitter reduction.
When the trigger pulse is sent to the light source 520 it will take a certain time until the light pulse is generated. The light delay may vary from one pulse to another. The resulting time jitter or drift translates into jitter and drifts of the stamp positions onto the workpiece, which may result in printed pattern line widths going out of predetermined specifications. In this embodiment the requested position from the look up table is corrected to obtain a new position against which the stage position is compared. The correction is obtained from the last measured delay (or statistics from several earlier measurements) optionally together with a speed measurement, where said measured delay and said optionally speed measurement is subtracted from the requested position. In this manner, speed variations over the strip are also corrected for. This embodiment differs to the ones in
As in the previously described embodiments one can use information about jitter from only one earlier pulse to reduce jitter in a following pulse or information about jitter from a plurality of pulses to reduce jitter in a following pulse by altering the delay in the time shifter or correcting the trigger position. When using information from more than one earlier pulse a better prediction about the jitter in a following pulse may be performed due to the fact that the jitter may follow a characteristic variation pattern. By knowing the jitter variations one can make a better prediction about jitter in a next coming pulse.
In a pattern generator, a test loop may be performed between each strip of stamps. Said test loop generates statistical material at said location in order to flash the pattern of the SLM onto a requested position of the workpiece as accurate as possible.
While the present invention is disclosed by reference to the preferred embodiments and examples detailed above, it is understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art, which modifications and combinations will be within the spirit of the invention and the scope of the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0300138-5 | Jan 2003 | SE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/SE04/00069 | 1/22/2004 | WO | 00 | 10/17/2007 |
