Abstract:
A programmable laser pulse combines electrical modulation of the pulse frequency and optical modulation of the pulse shape to form laser pulses of prescribed pulse shapes. A prescribed pulse shape features high peak power and low average power. The laser system disclosed also allows for power-scaling and nonlinear conversions to other (shorter or longer) wavelengths. The system provides an economical reliable alternative to using a laser source with high repetition rates to achieve shaped pulses at a variety of wavelengths. The combinatorial scheme disclosed is inherently more efficient than existing subtractive methods.

Description:
COPYRIGHT NOTICE 
     © 2008 Electro Scientific Industries, Inc. A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 37 CFR §1.71(d). 
     TECHNICAL FIELD 
     The present disclosure relates to pulsed fiber lasers and solid-state laser amplifiers from which tailored laser pulses propagate for use in laser micromachining applications and, in particular, to a highly efficient laser pulse-shaping generator emitting tailored laser pulses with prescribed pulse shapes programmed through a combination of separate electrical and optical modulators. 
     BACKGROUND INFORMATION 
     After manufacture of a semiconductor memory array chip is complete, integrated circuit (IC) patterns on an exposed surface of the chip are sealed with an electrically insulating layer of passivating material. Typical passivating materials include resins or thermoplastic polymers such as, for example, polyimide. The purpose of this final “passivation” layer is to prevent the surface of the chip from reacting chemically with ambient moisture, to protect the surface from environmental particulates, and to absorb mechanical stress. Following passivation, the chip is mounted in an electronic package embedded with metal interconnects that allow probing and functional testing of the memory cells. When one of many redundant memory cells is determined to be faulty, the cell is disabled by severing the conductive interconnects, or wires, linking that cell to its neighbors in the array. Disabling individual memory cells by “link processing” or “link blowing” is accomplished by laser micromachining equipment that is capable of directing laser beam energy so as to selectively remove the link material in a highly localized region without imparting damage to the materials adjacent to, below, or above the target. Selectively processing a designated link may be achieved by varying the laser beam wavelength, spot size, pulse repetition rate, pulse shape, or other spatial or temporal beam parameters that influence energy delivery. 
     Laser micromachining processes that entail post-processing of conductive links in memory arrays or other types of IC chips use sharp pulses with a fast rising front edge (e.g., with a 1-2 ns rise time) to achieve desired quality, yield, and reliability. To cleanly sever a conductive link, the laser pulse penetrates the overlying passivation layer before cutting through the metal interconnect. The rising edge of a typical pulse from an existing solid state laser varies with pulse width. Use of a traditional Gaussian-shaped laser pulse having a 5-20 ns pulse width and a sloped, gradually rising front edge in link processing tends to cause an “over crater” in the passivation layer, especially if its thickness is too large or is uneven. 
     Rupture behavior of overlying passivation layers has been well analyzed by Yunlong Sun in his PhD dissertation entitled, “Laser processing optimization of semiconductor based devices” (Oregon Graduate Institute, 1997). Because passivation layer thickness is an important parameter, the optimal thickness of a particular passivation layer material may be determined by simulations based on Sun&#39;s analysis. Difficulty in maintaining wafer-level process control of the passivation layer during IC fabrication may result in non-optimal thickness and poor cross-wafer or wafer-to-wafer thickness uniformity. Therefore, optimizing characteristics of laser pulses used in post-processing may help to compensate for mis-targeted dimensions and sources of variation in the passivation layer. 
     U.S. Pat. No. 6,281,471 of Smart proposes using substantially square-shaped laser pulses for link processing. Such a sharp-edged pulse may be generated by coupling a master oscillator laser with a fiber amplifier (MOPA). This low power master oscillator employs a diode laser that is capable of generating a square-shaped pulse with a fast rise time. On the other hand, in U.S. Pat. No. 7,348,516 of Yunlong Sun et al., which patent is assigned to the assignee of this patent application, states that, despite a vertical rising edge, a substantially square-shaped laser pulse is not the best laser pulse shape for link processing. Instead, Sun, et al. describes use of a specially tailored laser pulse shape that, in one embodiment, resembles a chair, with a fast rising peak or multiple peaks to most effectively process links, followed by a drop-off in signal strength that remains relatively flat at a lower power level before shutting off. Such a tailored laser pulse, with high peak power but low average power, has been successfully generated by what is called pulse slicing technology, which can be implemented by either electro-optical modulation (EOM) or acousto-optical modulation (AOM). For example, a conventional active Q-switched solid state laser provides nanosecond seed pulses with high intensity and high pulse energy, and then a light-loop slicing device transforms a standard laser pulse into a desired tailored pulse shape. 
     It is possible to obtain high efficiency and high peak power output directly from laser diodes. In other words, it is possible to generate high peak power and high pulse energy using exclusively electrically modulated seed pulses. The simplicity of this scheme is advantageous, and it also may be implemented with fewer amplifier stages. However, the center wavelength of the semiconductor diode tends to drift with small changes in temperature resulting from the change in pulse shape, which temperature drift may adversely affect downstream solid-state amplifiers and harmonic generation. 
     Alternatively, a specially tailored laser pulse may be generated by a MOPA that employs a gain fiber as the power amplifier. Using a MOPA is advantageous in that it constitutes a stable signal source at a specified constant frequency. 
     U.S. Patent Application No. 2006/0159138 of Pascal Deladurantaye describes a shaped-pulse laser in which two modulators shape a continuous wave (CW) light beam to generate various shaped pulses. However, generating a pulsed laser from a CW light beam is fairly inefficient, and thus requires more amplification. Because such a low peak-power signal may be influenced by noise, which causes pulse-to-pulse instability, the two modulators are preferably synchronized to maintain pulse stability and energy stability, thereby adding further complexity and cost. 
     SUMMARY OF THE DISCLOSURE 
     A programmable laser pulse-shaping generator combines electrical modulation of laser pulse frequency with optical modulation of laser pulse shape to produce tailored laser pulses of a prescribed shape with pulse widths on the order of a few nanoseconds to tens of nanoseconds and fast rise times on the order of a few nanoseconds to less than a nanosecond. A preferred laser pulse-shaping generator includes a modulated pulsed laser source in the form of a seed laser diode, which has as its input a frequency-modulated electrical signal. The system produces a series of high power tailored laser pulses that are shaped by a high speed optical modulator and optical power amplifiers. The pulse-shaping generator allows for power-scaling and generating harmonics at shorter wavelengths and provides an economical, reliable alternative to using a laser source operating at high repetition rates to achieve shaped pulses at a variety of wavelengths. The combinatorial scheme implemented by the pulse-shaping generator is inherently more efficient than existing subtractive methods that form a tailored pulse by optically slicing a seed pulse. 
     Additional aspects and advantages will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing the components of a laser pulse-shaping generator capable of producing a tailored pulse by a combination of electrical and optical modulations.  FIG. 1  also shows, at outputs of certain pulse-shaping generator components, the temporal profiles of laser pulses formed at various stages of development of the tailored pulse produced. 
         FIG. 2  presents a set of three waveform diagrams of diode laser output, pulse-shaping circuit output for electrical drive modulation, and optical output of a modulated pulsed laser source of the pulse-shaping generator of  FIG. 1 , the three waveforms exhibiting corresponding pulse shapes formed in the production of four examples of tailored laser pulse profiles. 
         FIGS. 3A ,  3 B, and  3 C are schematic diagrams showing three gain fiber pre-amplifier and output amplifier configurations for use with the modulated pulsed laser source of  FIG. 1 . 
         FIG. 4  is a block diagram showing the electrical circuit components of an analog implementation of the programmable pulse-shaping circuit of  FIG. 1 . 
         FIG. 5  is a waveform diagram showing pulse shapes of electrical signals that correspond to different stages of the programmable pulse-shaping circuit of  FIG. 4 . 
         FIG. 6  is a simulated high resolution (1 ns) pulse train output from the diode clamp circuitry of the programmable pulse-shaping circuit of  FIG. 4 . 
         FIG. 7  is a plot of a 1 ns resolution, gating electrical control signal pulse constructed by the programmable pulse-shaping circuit of  FIG. 4 , in which the shape of the gating electrical control signal pulse approximates that of a desired “chair” tailored laser pulse profile. 
         FIG. 8  is a block diagram showing the electrical circuit components of a digital implementation of the programmable pulse-shaping circuit of  FIG. 1 . 
         FIG. 9  is a diagram showing a semiconductor wafer having semiconductor link structures on its work surface. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows a laser pulse-shaping generator  90  with the desired operating and performance characteristics discussed above. Pulse-shaping generator  90  is constructed of a modulated pulsed laser source  100  that produces a prescribed tailored laser pulse for amplification by a set of optical power amplifiers  102  and for wavelength conversion by a harmonic generator  104 . 
     Modulated pulsed laser source  100 , such as a semiconductor laser, is preferably composed of a high-speed distributed feedback (DFB) seed laser diode  110  that is modulated by a seed pulse signal  112  produced by an electrical modulator  114  to provide at a high repetition rate a series of seed laser pulses  116 . In a preferred implementation, seed pulse signal  112  represents a series of seed pulses  116 . An optical modulator  120  receives and, in response to gating electrical control signal pulses  122  that are produced by a high-speed programmable pulse-shaping circuit  124  and are synchronized to seed pulse signal  112 , modulates seed laser pulses  116  to produce a series of prescribed laser pulses  126 . Optical modulator  120  and pulse-shaping circuit  124  cooperate to reconfigure each of seed laser pulses  116  to the desired temporal profile of laser pulse  126  for optimal material processing. 
     Optical power amplifiers  102  produce amplified tailored laser pulses  132  that are generally faithful replicas of tailored laser pulses  126 . Harmonic generator  104  converts amplified tailored laser pulses  132  to output laser pulses  134  in a different wavelength range, such as green, ultraviolet (UV), or deep ultraviolet (DUV), and, as a consequence of the nonlinear conversion process, with an accentuated tailored pulse profile. 
     Skilled persons will appreciate that a Q-switched solid state laser pulse source or fiber laser source could be substituted for semiconductor seed laser  110 , but the latter is preferred because of the following advantages. Laser pulse-shaping generator  90  configured with DFB seed laser diode  110  offers wide tunability and narrow linewidth in a compact, rugged setup. Such DFB seed laser diode  110  equipped with polarization maintaining (PM) fiber couplers (not shown) may be obtained from Toptica Photonics, AG of Munich, Germany. 
       FIG. 2  shows three waveform diagrams that present four examples (separated by dashed vertical lines) of the formation of different tailored laser pulse profiles produced at the output of optical modulator  120  (and modulated pulsed laser source  100 ).  FIG. 2 , line A represents a series of four similar seed laser pulses  116   1 ,  116   2 ,  116   3 , and  116   4  emitted by laser diode  110 .  FIG. 2 , line B represents four different gating control signal pulses  122   1 ,  122   2 ,  122   3 , and  122   4  of programmable pulse-shaping circuit  124 ; and  FIG. 2 , line C represents four different tailored laser pulses  126   1 ,  126   2 ,  126   3 , and  126   4  of optical modulator  120  to which the respective gating control signal pulses  122   1 ,  122   2 ,  122   3 , and  122   4  correspond. (The reference numerals of the seed laser pulses and gating control signal pulses contributing to a same one of the examples of the tailored laser pulses share common subscripts.) 
     In each of the four examples, a gating control signal pulse modulates a seed laser pulse to form a tailored laser pulse, the shape of which is a substantially faithful replica of the shape of the gating control signal pulse. Tailored laser pulses  126   1 ,  126   2 ,  126   3 , and  126   4  represent, respectively, chair-, reverse chair-, double peak-, and double spike-shaped laser pulses, each of which provides a high peak power level and a low average power level. 
       FIGS. 3A ,  3 B, and  3 C show respective alternative embodiments  102   a ,  102   b , and  102   c  implementing different configurations of optical power amplifiers  102  that are suitable for amplifying tailored laser pulses  126  appearing at the output of optical modulator  120 . Modulated pulsed laser source  100  produces at its output laser pulses  126  of any one of a variety of pulse shapes (as demonstrated in  FIG. 2 , line C). Each embodiment  102   a ,  102   b , and  102   c  includes a gain fiber pre-amplifier  138  that contains optical gain fibers such as Ytterbium (Yb), Erbium (Er), or Neodymium (Nd) glass to produce an intermediate shaped laser pulse  140  with increased peak power. Amplifier stages may be added to produce at least 1 kW of peak power output. Embodiments  102   a ,  102   b , and  102   c  employ as amplifier stages fiber amplifiers  142 , solid state amplifiers  144 , or a combination of both of them, respectively, to produce a high power amplified tailored laser pulse  132 .  FIG. 3A  presents a simple and efficient all-fiber optic configuration (without any solid state components) that may, however, be subject to damage and some undesired nonlinear effects under high peak power operation with a single mode, polarized laser.  FIGS. 3B and 3C  present two hybrid or “tandem” configurations that are more robust at peak power levels greater than 1 kW because they include solid state amplifiers  144 . By programmable pulse shaping and successive amplification, high peak power amplified, prescribed laser pulse  132  may thus be constructed gradually, by progressively building a desired pulse shape and accumulating a desired power output in a combinatorial fashion. Such gradual building of a pulse shape and accumulating laser output power constitute an inherently more efficient process than generating a high power pulse and selectively subtracting or absorbing energy to achieve a desired pulse shape. 
       FIG. 4  is a block diagram showing the electrical components of an analog implementation  124   a  of programmable pulse-shaping circuit  124  in greater detail. Electrical signal waveforms produced at intermediate stages within pulse-shaping circuit  124   a  of  FIG. 4  are shown and identified with corresponding reference numerals in  FIG. 5 . Electrical modulator  114  drives DFB seed laser diode  110  to produce seed laser pulses  116 , as described above with reference to  FIG. 1 . A host control computer or microcontroller  160  provides on a universal serial bus (USB), R232, or similar external data bus connection  162  signals that coordinate and control the operation of a Complex Programmable Logic Device (CPLD)  164 . A suitable CPLD is an Altera Max II EPM240T100C3N, which is available from Altera Corporation, San Jose, Calif. Host control computer  160  coordinates the operations of electrical modulator  114  and pulse shaping circuit  124  so that seed pulse signal  112  and gating control signal pulses  122  are in synchronism. CPLD  164  includes an internal pulse generator  166  that produces a series of square pulses  168 . Pulses  168  are applied to the inputs of N number of delay line circuits  170  (four shown in  FIG. 4 ) to produce time-displaced, conditioned output pulses  172  that are combined to form electrical control signal  122 . 
     All of the N number of delay line circuits  170  are nominally the same and are identified by reference numeral  170  and a different one of subscripts  1 ,  2 ,  3 , . . . , N. Corresponding components of delay line circuits  170  share common reference numerals with a subscript identifying the delay line circuit in which the components reside. The following description of the construction and operation of an individual delay line circuit is directed, therefore, to only delay line circuit  170   1 . Delay line circuit  170   1  includes a programmable time delay element  180   1 , having a signal input that receives square pulses  168  and a delay time input that receives a time delay control signal  182   1  from CPLD  164  to produce a delayed pulsed signal  184   1 . A suitable programmable time delay element  180   1  is a DS 1020, which is available from Maxim Integrated Products, Inc., of Sunnyvale, Calif. A capacitor C 1  blocks the direct current (DC) portion of delayed pulsed signal  184   1 , thereby producing signal pulses with positive- and negative-going voltage portions. A diode clamp circuit  188   1  blocks the negative-going voltage portions to provide a series of peaked pulses  190   1 . A gain-controllable operational amplifier  192   1  has a signal input that receives peaked pulses  190   1  and a gain control input that receives a gain control signal  194   1  from CPLD  164  to produce a series of output pulses  172   1  of programmable voltage levels. 
     A suitable high bandwidth, fast slew rate operational amplifier  192  is a THS3201, which is available from Texas Instruments of Dallas, Tex. and features a 2.2 GHz bandwidth at unity gain. Suitable alternative operational amplifiers include a digital programmed differential amplifier LMH6518, which is available from National Semiconductor of Santa Clara, Calif. and features an 825 MHz bandwidth and a 500 picosecond rise/fall time. 
     The N number of delay line circuits  170  is programmed to produce time-delayed peaked pulses  172   1 - 172   N  that are combined by a summing operational amplifier  196  to form gating electrical control signal  122  of the desired shape. More specifically, time delay control signals  182   1 - 182   N  applied to their respective programmable time delay elements  180   1 - 180   N  impart programmed amounts of delay relative to leading edges  200  of square pulses  168  to produce delayed pulse signals  184   1 - 184   N . The delay amounts imparted enable formation of a desired composite wave shape of gating electrical control signal  122 .  FIG. 5  shows as an example a sequence of delayed pulse signals  184   1 ,  184   2 , and  184   3  that represent zero, one delay unit, d, and two delay units,  2   d , imparted by their corresponding time delay control signals  182   1 ,  182   2 , and  182   3 . The zero, one delay unit, d, and two delay units,  2   d , imparted produce a sequence of temporally nonoverlapping delayed pulse signals  184   1 ,  184   2 , and  184   3  that are progressively delayed by a uniform amount, as illustrated. A realizable delay unit, d, is 0.15 ns, for example. Diode clamp circuits  188   1 ,  188   2 , and  188   3  produce the respective peaked pulses  190   1 ,  190   2 , and  190   3 , as illustrated. Each of operational amplifiers  192   1 - 192   N  features a fast slew rate and broad bandwidth to amplify its associated one of peaked pulses  190   1 - 190   N  to a different, separately programmable, voltage level. Thus, for the example of  FIG. 5 , first pulse  190   1  may be amplified more than subsequent pulses  190   2  and  190   3 , as illustrated. Such differential amplification produces a leading high energy peak  172   1  that, upon combination with the subsequent time-displaced amplified peaked pulses  172   2  and  172   3 , forms the back of the “chair” of resultant gating electrical control signal  122  of the type shown as Example Pulse Profile  1  in  FIG. 2 . 
     Two methods of programming pulse-shaping circuit  124   a  of  FIG. 4  may be used either separately or in combination to specify different tailored shapes such as those of gating electrical control signal pulses  122   1 ,  122   2 ,  122   3 , and  122   4  of  FIG. 2 , line B. A first method entails specifying fixed gain values of operational amplifiers  192  so that each of them has a binary weighted gain value. In this case, time delay elements  180  would simply be pre-programmed by time delay control signal  182  to different values to form a new prescribed electrical control signal pulse shape. A second method entails using a time delay element  180  programmed to a fixed delay value and controlling the gain level of each operational amplifier  192  by its associated gain control signal  194  to achieve a desired prescribed electrical control signal pulse shape. Alternatively, a combination of the two programming methods may be used. 
     If the desired temporal pulse width of electrical control signal pulses  190  is longer than the time delay between them, or if operational amplifiers  192  overshoot or undershoot the target power level, adjacent pulse overlap results, as indicated by overlap  202  between peaked pulses  190   2  and  190   3  in  FIG. 6 . It is therefore preferable to custom-adjust delay times programmed in time delay elements  180  or amplifier gain values set in operational amplifiers  192  to achieve the prescribed electrical control signal pulse shape, and it is desirable to choose values of capacitors C 1 , . . . , C N  that allow for shorter pulse durations to maximize accuracy. 
     Electrical control signal  122  shown in greater detail in  FIG. 7  represents the sum of output pulses  172   1 , . . . ,  172   N  that appears at the output of summing operational amplifier  196  and approximates the prescribed “chair” shape of control signal  122   1  shown in  FIG. 2 , line B. A “ringing” produced by trailing pulses  204  following the leading peak pulse  206  averages to a low power value  208 . A high peak power  210  of leading peak pulse  206  has a short rise time  212  on the order of 1 ns, as compared with an overall pulse width  214 , which is on the order of 10 ns. Referring again to  FIG. 1 , electrical control signal  122  generated by pulse-shaping circuit  124  modulates seed laser pulse  116  to form tailored laser pulse  126  at the output of optical modulator  120 . The set of high bandwidth optical power amplifiers  102  then amplifies tailored laser pulse  126  to produce high powered tailored laser output pulse  132 , having a shape that faithfully represents that of the prescribed tailored laser pulse  126 . 
       FIG. 8  is a block diagram showing the electrical components of an alternative, digital implementation of programmable pulse-shaping circuit  124 . Alternative, digital implementation  124   d  includes programmable digital pulse-shaping circuitry as a substitute for the programmable time delay elements  180 , diode clamp circuits  188 , and operational amplifiers  192  of the analog implementation of  FIG. 4 . The programmable digital pulse-shaping circuitry includes a high speed field programmable gate array (FPGA)  220 , such as a Xilinx Virtex 5, available from Xilinx, Inc., San Jose, Calif., for generating a stream of binary data  222  specifying a desired tailored pulse shape such as the chair example used in the analog implementation described above. FPGA  220  is shown in  FIG. 8  in combination with CPLD  164 . Stream of binary data  222  appearing at the output of FPGA  220  is applied to the input of a digital-to-analog converter (DAC)  224 , which produces at its output an analog control signal that has the shape of electrical control signal  122 . A suitable DAC  224  is a DAC 5681, available from Texas Instruments, Inc., Dallas, Tex. The output of DAC  224  is applied to the input of an operational amplifier  226 , at the output of which electrical control signal  122  appears. 
     The analog implementation of  FIG. 4  is simpler in that it has fewer component parts than the number of them in the digital implementation of  FIG. 8 . Both of the circuits of  FIGS. 4 and 8  have strict requirements in the circuit layout so that the data arrive at the same time or a prescribed delay with respect to the other signals. At this time, the delay line-based circuit of  FIG. 4  has greater resolution, but it is expected that faster DACs will be available in the near future. 
     An illustrative example of a useful application of output laser pulses  134  having a tailored temporal profile is the severing of semiconductor link structures  230  on a wafer specimen  232 , which is shown in  FIG. 9 . 
     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of the present invention should, therefore, be determined only by the following claims.