Electric heating – Metal heating – By arc
Reexamination Certificate
1999-02-10
2001-01-09
Heinrich, Samuel M. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121820
Reexamination Certificate
active
06172325
ABSTRACT:
TECHNICAL FIELD
This invention relates to lasers and more particularly to a method and an apparatus for providing uniform energy laser pulses at a high pulse repetition frequency (“PRF”) in on-the-fly specimen processing applications.
BACKGROUND OF THE INVENTION
Lasers are typically employed in a variety of industrial operations including inspecting, processing, and micro-machining substrates, such as electronic materials. For example, to repair a dynamic random access memory (“DRAM”), a first laser pulse is used to remove an electrically conductive link to a faulty memory cell of a DRAM device, and then a second laser pulse is used to remove a resistive link to a redundant memory cell to replace the faulty memory cell. Because faulty memory cells needing link removals are randomly located, workpiece positioning delay times typically require that such laser repair processes be performed over a wide range of PRFs, rather than at a constant PRF. This production technique is referred to in the industry as on-the-fly (“OTF”) link processing and allows for greater efficiency in the rate at which links on a given wafer can be repaired, thereby improving the efficiency of the entire DRAM production process.
However, it is well known that the laser energy per pulse typically decreases with increasing PRF, a characteristic that is particularly true for Q-switched, solid-state lasers. This energy per pulse roll-off limits the upper PRF range for many laser memory repair processes. Moreover, memories and other electronic components are manufactured with various processes each requiring processing by a particular range of pulse energies, often referred to as a “process window.” For many memory devices, the “process window” requires that laser pulse energy vary by less than 5% from a selected pulse energy value.
Prior workers have taken various approaches for ensuring operation within a process window or for opening up the process window. For example, U.S. Pat. No. 5,590,141 for METHOD AND APPARATUS FOR GENERATING AND EMPLOYING A HIGH DENSITY OF EXCITED IONS IN A LASANT, which is assigned to the assignee of this application, describes solid-state lasers having lasants exhibiting a reduced pulse energy drop off as a function of PRF and, therefore, a higher usable PRF. Such lasers are, therefore, capable of generating more stable pulse energy levels when operated below their maximum PRF.
It is also known that laser processing applications typically employ positioners to rapidly move target locations on a workpiece through a sequence of programmed processing positions. The movements of the positioner and the laser pulse timing are asynchronous, requiring lasers in such applications to operate in an OTF mode having an inherently wide range of PRFs. The resulting wide range of interpulse periods causes corresponding pulse to pulse energy variations and indefinite pulse firing timing, which leads to inaccurate laser pulse positioning on a workpiece.
Accordingly, U.S. Pat. No. 5,453,594 for RADIATION BEAM POSITION AND EMISSION COORDINATION SYSTEM, which is assigned to the assignee of this application, describes a technique for synchronizing a clock that controls the positioner with a variable clock that controls OTF laser pulse emission. The synchronized clocks allow the laser to emit pulses in synchronism with positioner movements across target locations on the workpiece, thereby improving laser pulse positioning accuracy.
The above-described laser processing applications typically employ infrared (“IR”) lasers having 1,047 nanometer (“nm”) or 1,064 nm fundamental wavelengths. Applicants have discovered that many laser processing applications are improved by employing ultraviolet (“UV”) energy wavelengths, which are typically less than about 500 nm. Such UV wavelengths may be generated by subjecting an IR laser to a harmonic generation process that stimulates the second, third, or fourth harmonics of the IR laser. Unfortunately, the pulse to pulse energy levels of such UV lasers are particularly sensitive to PRF and interpulse period variations.
What is needed, therefore, is an apparatus and a method for generating stable UV laser processing pulse energies at a high PRF in high-accuracy OTF laser processing applications.
SUMMARY OF THE INVENTION
An object of this invention is, therefore, to provide an apparatus and a method for generating stable laser processing pulse energy at a high PRF in high-accuracy OTF laser processing applications.
Another object of this invention is to satisfy the above object for OTF. specimen processing applications employing UV wavelengths.
A Q-switched solid state laser operates in cooperation with a pulse processing control system that employs an autopulse mode and a pulse-on-position mode to stabilize the pulse-to-pulse laser energy delivered to target locations on a workpiece that is moved by a positioner. In the autopulse mode, laser pulses are emitted at a near maximum PRF, but the pulses are blocked from reaching the workpiece by an external modulator, such as an acousto-optic modulator (“AOM”) or electro-optic modulator (also referred to as a Pockels cell). In the pulse-on-position mode, the laser emits a pulse each time the positioner moves a workpiece location through coordinates that coincide with a commanded laser beam coordinate. The processing control system moves the positioner at a near constant velocity that causes triggering of the laser at about the maximum PRF in response to the workpiece passing through a regularly spaced apart set of commanded laser beam coordinates. The pulse processing control system sets the AOM to a transmissive state whenever a location to be processed is commanded and sets the AOM to a blocking state whenever a location not to be processed is commanded. This pulse timing technique provides a nearly constant interpulse period for the laser, thereby stabilizing its pulse-to-pulse energy level at the near maximum PRF.
This invention is advantageous for generating stable pulse-to-pulse laser pulse energy when processing features that ordinarily require near random interpulse periods. This invention is also advantageous for stabilizing the pulse-to-pulse energy of a Q-switched solid state laser generating frequency-doubled, -tripled, or quadrupled laser pulses by employing a nonlinear harmonic generation process.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof that proceed with reference to the accompanying drawings.
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patent: 226198 (1986-10-01), None
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Baird Brian W.
Jordens William J.
Swaringen Stephen N.
Electro Scientific Industries Inc.
Heinrich Samuel M.
Stoel Rives LLP
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