Patent Publication Number: US-2003222324-A1

Title: Laser systems for passivation or link processing with a set of laser pulses

Description:
RELATED APPLICATIONS  
     [0001] This is a continuation-in-part of U.S. patent application Ser. No. 10/361,206, filed Feb. 7, 2003, which claims priority from U.S. Provisional Application No. 60/355,151, filed Feb. 8, 2002; is a continuation-in-part of U.S. patent application Ser. No. 10/322,347, filed Dec. 17, 2002, which claims priority from U.S. Provisional Application No. 60/341,744, filed Dec. 17, 2001; and is a continuation-in-part of U.S. patent application Ser. No. 09/757,418, filed Jan. 9, 2001, which claims priority from both U.S. Provisional Application No. 60/223,533, filed Aug. 4, 2000 and U.S. Provisional Application No. 60/175,337, filed Jan. 10, 2000. 
    
    
     
       TECHNICAL FIELD  
       [0002] The present invention relates to laser processing of memory or other IC links and, in particular, to laser systems and methods employing a set of laser pulses to sever an IC link and/or remove the passivation over the IC link on-the-fly.  
       BACKGROUND OF THE INVENTION  
       [0003] Yields in IC device fabrication processes often incur defects resulting from alignment variations of subsurface layers or patterns or particulate contaminants. FIGS. 1, 2A, and  2 B show repetitive electronic circuits  10  of an IC device or work piece  12  that are commonly fabricated in rows or columns to include multiple iterations of redundant circuit elements  14 , such as spare rows  16  and columns  18  of memory cells  20 . With reference to FIGS. 1, 2A, and  2 B, circuits  10  are also designed to include particular laser severable conductive links  22  between electrical contacts  24  that can be removed to disconnect a defective memory cell  20 , for example, and substitute a replacement redundant cell  26  in a memory device such as a DRAM, an SRAM, or an embedded memory. Similar techniques are also used to sever links  22  to program a logic product, gate arrays, or ASICs.  
       [0004] Links  22  are about 0.3-2 microns (μm) thick and are designed with conventional link widths  28  of about 0.4-2.5 μm, link lengths  30 , and element-to-element pitches (center-to-center spacings)  32  of about 2-8 μm from adjacent circuit structures or elements  34 , such as link structures  36 . Although the most prevalent link materials have been polysilicon and like compositions, memory manufacturers have more recently adopted a variety of more conductive metallic link materials that may include, but are not limited to, aluminum, copper, gold, nickel, titanium, tungsten, platinum, as well as other metals, metal alloys, metal nitrides such as titanium or tantalum nitride, metal suicides such as tungsten silicide, or other metal-like materials.  
       [0005] Circuits  10 , circuit elements  14 , or cells  20  are tested for defects, the locations of which may be mapped into a database or program. Traditional 1.047 μm or 1.064 μm infrared (IR) laser wavelengths have been employed for more than 20 years to explosively remove conductive links  22 . Conventional memory link processing systems focus a single pulse of laser output having a pulse width of about 4 to 30 nanoseconds (ns) at a selected link  22 . FIGS. 2A and 2B show a laser spot  38  of spot size (area or diameter)  40  impinging a link structure  36  composed of a polysilicon or metal link  22  positioned above a silicon substrate  42  and between component layers of a passivation layer stack including an overlying passivation layer  44  (shown in FIG. 2A but not in FIG. 2B), which is typically 500-10,000 angstrom (Å) thick, and an underlying passivation layer  46 . Silicon substrate  42  absorbs a relatively small proportional quantity of IR laser radiation, and conventional passivation layers  44  and  46  such as silicon dioxide or silicon nitride are relatively transparent to IR laser radiation. The links  22  are typically processed “on-the-fly” such that the beam positioning system does not have to stop moving when a laser pulse is fired at a selected link  22 , with each selected link  22  being processed by a single laser pulse. The on-the-fly process facilitates a very high link-processing throughput, such as processing several tens of thousands of links  22  per second.  
       [0006]FIG. 2C is a fragmentary cross-sectional side view of the link structure of FIG. 2B after the link  22  is removed by the prior art laser pulse. To avoid damage to the substrate  42  while maintaining sufficient laser energy to process a metal or nonmetal link  22 , Sun et al. in U.S. Pat. No. 5,265,114 and U.S. Pat. No. 5,473,624 proposed using a single 9 to 25 ns laser pulse at a longer laser wavelength, such as 1.3 μm, to process memory links  22  on silicon wafers. At the 1.3 μm wavelength, the laser energy absorption contrast between the link material and silicon substrate  20  is much larger than that at the traditional 1 μm laser wavelengths. The much wider laser processing window and better processing quality afforded by this technique has been used in the industry for about five years with great success.  
       [0007] The 1 μm and 1.3 μm laser wavelengths have disadvantages however. The energy coupling efficiency of such IR laser beams  12  into a highly electrically conductive metallic link  22  is relatively poor; and the practical achievable spot size  40  of an IR laser beam for link severing is relatively large and limits the critical dimensions of link width  28 , link length  30  between contacts  24 , and link pitch  32 . This conventional laser link processing relies on heating, melting, and evaporating link  22 , and creating a mechanical stress build-up to explosively open overlying passivation layer  44  with a single laser pulse. Such a conventional link processing laser pulse creates a large heat affected zone (HAZ) that could deteriorate the quality of the device that includes the severed link. For example, when the link is relatively thick or the link material is too reflective to absorb an adequate amount of the laser pulse energy, more energy per laser pulse has to be used. Increased laser pulse energy increases the damage risk to the IC chip. However, using a laser pulse energy within the risk-free range on thick links often results in incomplete link severing.  
       [0008] U.S. Pat. No. 6,057,180 of Sun et al. describe a method of using ultraviolet (UV) laser output to sever links with the benefit of a smaller beam spot size. However, removal of the link itself by such a UV laser pulse entails careful consideration of the underlying passivation structure and material to protect the underlying passivation and silicon wafer from being damaged by the UV laser pulse.  
       [0009] U.S. Pat. No. 5,329,152 of Janai et al. describes coating a metal layer with a laser absorbing resist material (and an anti-reflective material), blowing away the coatings with a high-powered YAG, excimer, or pulsed laser diode with fluences of 0.2-10 J/cm 2  at a 350-nm wavelength, and then etching the uncovered metal with a chemical or plasma etch process. In an alternative to blowing away the resist, Janai describes using laser pulses that travel through a resist material so that the laser pulses can react with the underlying metal and integrate it into the resist material to make the resist material etchable along with the metal (and/or partially blowing away the resist material).  
       [0010] U.S. Pat. No. 5,236,551 of Pan teaches providing metalization portions, covering them with a photoabsorptive polymeric dielectric, ablating the dielectric to uncover portions of the metal, etching the metal, and then coating the resulting surface with a polymeric dielectric. Pan discloses only excimer lasers having wavelengths of less than 400 nm and relies on a sufficiently large energy fluence per pulse (10 mJ/cm 2  to 350 mJ/cm 2 ) to overcome the ablative photodecomposition threshold of the polymeric dielectric.  
       [0011] U.S. Pat. No. 6,025,256 of Swenson et al. describes methods of using ultraviolet (UV) laser output to expose or ablate an etch protection layer, such as a resist or photoresist, coated over a link that may also have an overlying passivation material, to permit link removal (and removal of the overlying passivation material) by different material removal mechanisms, such as by chemical etching. This process enables the use of an even smaller beam spot size. However, expose and etch removal techniques employ additional coating steps and additional developing and/or etching steps. The additional steps typically entail sending the wafer back to the front end of the manufacturing process for extra step(s).  
       [0012] U.S. Pat. No. 5,656,186 of Mourou et al. discloses a general method of laser induced breakdown and ablation at several wavelengths by high repetition rate ultrafast laser pulses, typically shorter than 10 ps, and demonstrates creation of machined feature sizes that are smaller than the diffraction limited spot size.  
       [0013] U.S. Pat. No. 5,208,437 of Miyauchi et al. discloses a method of using a single “Gaussian”-shaped pulse of a subnanosecond pulse width to process a link.  
       [0014] U.S. Pat. No. 5,742,634 of Rieger et al. discloses a simultaneously Q-switched and mode-locked neodymium (Nd) laser device with diode pumping. The laser emits a series of pulses each having a duration time of 60 to 300 picoseconds (Ps), under an envelope of a time duration of 100 ns.  
       SUMMARY OF THE INVENTION  
       [0015] An object of the present invention is to provide a method or apparatus for improving the processing quality for removal of IC links.  
       [0016] Another object of the invention is to process a link and/or the passivation layer above it with a set of low energy laser pulses.  
       [0017] A further object of the invention is to provide a method and apparatus for employing a much smaller laser beam spot size for passivation and/or link removal techniques.  
       [0018] Yet another object of the invention is to deliver such sets of laser pulses to process passivation and/or links on-the-fly.  
       [0019] Still another object of the invention is to avoid or minimize substrate damage and undesirable damage to the passivation structure.  
       [0020] Still another object of the invention is to avoid numerous extra processing steps while removing links with an alternative method to that of explosive laser blowing.  
       [0021] The present invention employs a set of at least two laser pulses, each with a laser pulse energy within a safe range, to sever an IC link  22 , instead of using a single laser pulse of conventional link processing systems. This practice does not, therefore, entail either a long dwell time or separate duplicative scanning passes of repositioning and refiring at each selected link  22  that would effectively reduce the throughput by factor of about two or more. The duration of the set is preferably shorter than 1,000 ns, more preferably shorter than 500 ns, most preferably shorter than 300 ns and preferably in the range of 5 to 300 ns; and the pulse width of each laser pulse within the set is generally in the range of 100 femtoseconds (fs) to 30 ns. Each laser pulse within the set has an energy or peak power per pulse that is less than the damage threshold for the (silicon) substrate  42  supporting the link structure  36 . The number of laser pulses in the set is controlled such that the last pulse cleans off the bottom of the link  22  leaving the underlying passivation layer  46  and the substrate  42  intact. Because the whole duration of the set is shorter than 1,000 ns, the set is considered to be a single “pulse” by a traditional link-severing laser positioning system. The laser spot of each of the pulses in the set encompasses the link width  28 , and the displacement between the laser spots  38  of each pulse is less than the positioning accuracy of a typical positioning system, which is typically+±0.05 to 0.2 μm. Thus, the laser system can still process links  22  on-the-fly, i.e. the positioning system does not have to stop moving when the laser system fires a set of laser pulses at each selected link  22 .  
       [0022] In one embodiment, a continuous wave (CW) mode-locked laser at high laser pulse repetition rate, followed by optical gate and an amplifier, generates sets having two or more ultrashort laser pulses that are preferably from about 100 fs to about 10 ps. In another one embodiment, a Q-switched and CW mode-locked laser generates sets having ultrashort laser pulses that are preferably from about 100 fs to about 10 ps. Because each laser pulse within the set is ultrashort, its interaction with the target materials (metallic link  22  and/or passivation layers  44  and  46 ) is substantially not thermal. Each laser pulse breaks off a thin sublayer of about 100-2,000 Å of material, depending on the laser energy or peak power, laser wavelength, and type of material, until the link  22  is severed. This substantially nonthermal process may mitigate certain irregular and inconsistent link processing quality associated with thermal-stress explosion behavior of passivation layers  44  of links  22  with widths  28  narrower than about 1 μm or links  22  thicker (depthwise) than about 1 μm. In addition to the “nonthermal” and well-controllable nature of ultrashort-pulse laser processing, the most common ultrashort-pulse laser source emits at a wavelength of about 800 nm and facilitates delivery of a small-sized laser spot. Thus, the process may facilitate greater circuit density.  
       [0023] In another embodiment, the sets have laser pulses that are preferably from about 25 ps to about 20 ns or 30 ns. These sets of laser pulses can be generated from a CW mode-locked laser system including an optical gate and an optional down stream amplifier, from a step-controlled acousto-optic (A-O) Q-switched laser system, from a laser system employing a beam splitter and an optical delay path, or from two or more synchronized but offset lasers that share a portion of an optical path.  
       [0024] In alternative embodiments, the present invention employs the laser processing methods and apparatus to produce laser output including sets of two or more laser pulses, each with a laser pulse energy in a very safe range, to remove or “open” a target area of passivation layer  44  overlying a target IC link  22  such that the target link  22  is exposed and then can be etched by a separate process and such that the passivation layer  46  and silicon wafer  42  underlying the link  22  are not subjected to the amount of laser output energy used in a traditional link-processing technique. The pulse width of each laser pulse within the set is generally shorter than 30 ns, preferably in the range of 0.05 ps to 5 ns, and more preferably shorter than 10 ps. Each laser pulse within the set has an energy or peak power per pulse that is less than the damage threshold for the substrate  42  supporting the link structure. The number of laser pulses in the set is controlled such that the laser output cleans off the bottom of the passivation layer  44 , but leaves at least some of the link  22  such that the underlying passivation layer  46  and the substrate  42  are not subjected to the laser energy induced damage and are completely intact. In some embodiments, the passivation removal sets include only a single laser pulse, particularly a laser pulse having a pulse width in the range of 0.05 ps to 5 ns, and more preferably shorter than 10 ps.  
       [0025] After the passivation layer  44  is removed above all of the links  22  that are to be severed, chemical etching can be employed to cleanly clear the exposed link  22  without the debris, splash, or other common material residue problems that plague direct laser link severing. Because the set of laser pulses ablates only the overlying passivation layer  44  and the whole link  22  is not heated, melted, nor vaporized, there is no opportunity to thermally or physically damage connected or nearby circuit structures or to cause cracks in the underlying passivation layer  44  or the neighboring overlying passivation layer  46 . Chemical etching of the links  22  is also relatively indifferent to variations in the link structures  36  from work piece  12  to work piece  12 , such as the widths  28  and thicknesses of the links  22 , whereas conventional link processing parameters should be tailored to suit particular link structure characteristics. The chemical etching of the links  22  entails only a single extra process step that can be performed locally and/or in-line such that the work pieces  12  need not be sent back to the front end of the processing line to undergo the etching step.  
       [0026] Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0027]FIG. 1 is a schematic diagram of a portion of a DRAM showing the redundant layout of and programmable links in a spare row of generic circuit cells.  
     [0028]FIG. 2A is a fragmentary cross-sectional side view of a conventional, large semiconductor link structure receiving a laser pulse characterized by a prior art pulse parameters.  
     [0029]FIG. 2B is a fragmentary top view of the link structure and the laser pulse of FIG. 2A, together with an adjacent circuit structure.  
     [0030]FIG. 2C is a fragmentary cross-sectional side view of the link structure of FIG. 2B after the link is removed by the prior art laser pulse.  
     [0031]FIG. 3 shows a power versus time graph of exemplary sets of constant amplitude laser pulses employed to sever links in accordance with the present invention.  
     [0032]FIG. 4 shows a power versus time graph of alternative exemplary sets of modulated amplitude laser pulses employed to sever links in accordance with the present invention.  
     [0033]FIG. 5 shows a power versus time graph of other alternative exemplary sets of modulated amplitude laser pulses employed to sever links in accordance with the present invention.  
     [0034]FIG. 6 is a partly schematic, simplified diagram of an embodiment of an exemplary green laser system including a work piece positioner that cooperates with a laser processing control system for practicing the method of the present invention.  
     [0035]FIG. 7 is a simplified schematic diagram of one laser configuration that can be employed to implement the present invention.  
     [0036]FIG. 8 is a simplified schematic diagram of an alternative embodiment of a laser configuration that can be employed to implement the present invention.  
     [0037]FIG. 9 shows a power versus time graph of alternative exemplary sets of modulated amplitude laser pulses employed to sever links in accordance with the present invention.  
     [0038]FIG. 10A shows a power versus time graph of a typical single laser pulse emitted by a conventional laser system to sever a link.  
     [0039]FIG. 10B shows a power versus time graph of an exemplary set of laser pulses emitted by a laser system with a step-controlled Q-switch to sever a link.  
     [0040]FIG. 11 is a power versus time graph of an exemplary RF signal applied to a step-controlled Q-switch.  
     [0041]FIG. 12 is a power versus time graph of exemplary laser pulses that can be generated through a step-controlled Q-switch employing the RF signal shown in FIG. 11.  
     [0042]FIG. 13 is a simplified schematic diagram of an alternative embodiment of a laser system that can be employed to implement the present invention.  
     [0043] FIGS.  14 A- 14 D show respective power versus time graphs of an exemplary laser pulses propagating along separate optical paths of the laser system shown in FIG. 14.  
     [0044]FIG. 15 is a simplified schematic diagram of an alternative embodiment of a laser system that employs two or more lasers to implement the present invention.  
     [0045] FIGS.  16 A- 16 C show respective power versus time graphs of exemplary laser pulses propagating along separate optical paths of the laser system shown in FIG. 16.  
     [0046]FIG. 17A is a fragmentary cross-sectional side view of a target structure, covered by a passivation layer, receiving a laser output characterized by laser output parameters in accordance with the present invention.  
     [0047]FIG. 17B is a fragmentary cross-sectional side view of the target structure of FIG. 17A subsequent to a passivation-removing laser processing step.  
     [0048]FIG. 17C is a fragmentary cross-sectional side view of the target structure of FIG. 17B subsequent to an etch processing step. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
     [0049] FIGS.  3 - 5 ,  9 ,  10 B,  12 ,  14 D, and  16 C show power versus time graphs of exemplary sets  50   a,    50   b,    50   c,    50   d,    50   e,    50   f,  and  50   g  (generically sets  50 ) of laser pulses  52   a,    52   b   1 - 52   b   8 ,  52   c   1 - 52   c   5 ,  52   d   1 - 52   d   3 ,  52   e   1 - 52   e   4 ,  52   f   1 - 52   f   2 , and  52   g   1 - 52   g   2  (generically laser pulses  52 ) employed to sever links  22  in accordance with the present invention. Preferably, each set  50  severs a single link  22 . Preferred sets  50  include 2 to 50 pulses  52 . The duration of each set  50  is preferably shorter than about 1000 ns, more preferably shorter than 500 ns, and most preferably in the range of about 5 ns to 300 ns. Sets  50  are time-displaced by a programmable delay interval that is typically shorter than 0.1 millisecond and may be a function of the speed of the positioning system  62  and the distance between the links  22  to be processed. The pulse width of each laser pulse  52  within set  50  is in the range of about 30 ns to about 100 fs or shorter.  
     [0050] During a set  50  of laser pulses  52 , each laser pulse  52  has insufficient heat, energy, or peak power to fully sever a link  22  or damage the underlying substrate  42  but removes a part of link  22  and/or any overlying passivation layer  44 . At a preferred wavelength from about 150 nm to about 2000 nm, preferred ablation parameters of focused spot size  40  of laser pulses  52  include laser energies of each laser pulse between about 0.005 μJ to about 10 μJ (and intermediate energy ranges between 0.01 μJ to about 0.1 μJ) and laser energies of each set between 0.01 μJ to about 10 μJ at greater than about 1 Hz and preferably 10 kHz to 50 kHz or higher. The focused laser spot diameter is preferably 50% to 100% larger than the width of the link  22 , depending on the link width  28 , link pitch size  32 , link material and other link structure and process considerations.  
     [0051] Depending on the wavelength of laser output and the characteristics of the link material, the severing depth of pulses  52  applied to link  22  can be accurately controlled by choosing the energy of each pulse  52  and the number of laser pulses  52  in each set  50  to clean off the bottom of any given link  22 , leaving underlying passivation layer  46  relatively intact and substrate  42  undamaged. Hence, the risk of damage to silicon substrate  42  is substantially eliminated, even if a laser wavelength in the UV range is used.  
     [0052] The energy density profile of each set  50  of laser pulses  52  can be controlled better than the energy density profile of a conventional single multiple nanosecond laser pulse. With reference to FIG. 3, each laser pulse  52   a  can be generated with the same energy density to provide a pulse set  50   a  with a consistent “flat-top” energy density profile. Set  50   a  can, for example, be accomplished with a mode-locked laser followed by an electro-optic (E-O) or acousto-optic (A-O) optical gate and an optional amplifier (FIG. 8).  
     [0053] With reference to FIG. 4, the energy densities of pulses  52   b   1 - 52   b   8  (generically  52   b ) can be modulated so that sets  50   b  of pulses  52   b  can have almost any predetermined shape, such as the energy density profile of a conventional link-blowing laser pulse with a gradual increase and decrease of energy densities over pulses  52   b   1 - 52   b   8 . Sets  50   b  can, for example, be accomplished with a simultaneously Q-switched and CW mode-locked laser system  60  shown in FIG. 6. Sequential sets  50  may have different peak power and energy density profiles, particularly if links  22  and/or passivation layers  44  with different characteristics are being processed.  
     [0054]FIG. 5 shows an alternative energy density profile of pulses  52   c   1 - 52   c   5  (generically  52   c ) that have sharply and symmetrically increasing and decreasing over sets  50   c.  Sets  50   c  can be accomplished with a simultaneously Q-switched and CW mode-locked laser system  60  shown in FIG. 6.  
     [0055] Another alternative set  50  that is not shown has initial pulses  52  with high energy density and trailing pulses  52  with decreasing energy density. Such an energy density profile for a set  50  would be useful to clean out the bottom of the link without risk of damage to a particularly sensitive work piece.  
     [0056]FIG. 6 shows a preferred embodiment of a simplified laser system  60  including a Q-switched and/or CW mode-locked laser  64  for generating sets  50  of laser pulses  52  desirable for achieving link severing in accordance with the present invention. Preferred laser wavelengths from about 150 nm to about 2000 nm include, but are not limited to, 1.3, 1.064, or 1.047, 1.03- 1.05, 0.75-0.85  μm or their second, third, fourth, or fifth harmonics from Nd:YAG, Nd:YLF, Nd:YVO 4 , Yb:YAG, or Ti:Sapphire lasers  64 . Skilled persons will appreciate that lasers emitting at other suitable wavelengths are commercially available, including fiber lasers, and could be employed.  
     [0057] Laser system  60  is modeled herein only by way of example to a second harmonic (532 nm) Nd:YAG laser  64  since the frequency doubling elements can be removed to eliminate the harmonic conversion. The Nd:YAG or other solid-state laser  64  is preferably pumped by a laser diode  70  or a laser diode-pumped solid-state laser, the emission  72  of which is focused by lens components  74  into laser resonator  82 . Laser resonator  82  preferably includes a lasant  84 , preferably with a short absorption length, and a Q-switch  86  positioned between focusing/folding mirrors  76  and  78  along an optic axis  90 . An aperture  100  may also be positioned between lasant  84  and mirror  78 . Mirror  76  reflects light to mirror  78  and to a partly reflective output coupler  94  that propagates laser output  96  along optic axis  98 . Mirror  78  is adapted to reflect a portion of the light to a semiconductor saturable absorber mirror device  92  for mode locking the laser  64 . A harmonic conversion doubler  102  is preferably placed externally to resonator  82  to convert the laser beam frequency to the second harmonic laser output  104 . Skilled persons will appreciate that where harmonic conversion is employed, a gating device  106 , such as an E-O or A-O device can be positioned before the harmonic conversion apparatus to gate or finely control the harmonic laser pulse energy.  
     [0058] Skilled person will appreciate that a Q-switched laser  64  without CW mode-locking is preferred for several embodiments, particularly for applications employing pulse widths greater than 1 ns. Such laser systems  60  do not employ a saturable absorber  92 , and optical paths  90  and  98  of such systems are collinear. Such alternative laser systems  60  are commercially available and well known to skilled practitioners.  
     [0059] Skilled persons will appreciate that any of the second, third, or fourth harmonics of Nd:YAG (532 nm, 355 nm, 266 nm); Nd:YLF (524 nm, 349 nm, 262 nm) or the second harmonic of Ti:Sapphire (375-425 nm) can be employed to preferably process certain types of links  22  and/or passivation layers  44  using appropriate well-known harmonic conversion techniques. Harmonic conversion processes are described in pp. 138-141, V. G. Dmitriev, et. al., “Handbook of Nonlinear Optical Crystals”, Springer-Verlag, New York, 1991 ISBN 3-540-53547-0.  
     [0060] An exemplary laser  64  can be a mode-locked Ti-Sapphire ultrashort pulse laser with a laser wavelength in the near IR range, such as 750-850 nm. Spectra Physics makes a Ti-Sapphire ultra fast laser called the MAI TAI™ which provides ultrashort pulses  52  having a pulse width of 150 femtoseconds (fs) at 1 W of power in the 750 to 850 nm range at a repetition rate of 80 MHz. This laser  64  is pumped by a diode-pumped, frequency-doubled, solid-state green YAG laser (5W or 10 W). Other exemplary ultrafast Nd:YAG or Nd:YLF lasers  64  include the JAGUAR-QCW-1000™ and the JAGUAR-CW-250™ sold by Time-Bandwidth® of Zurich, Switzerland.  
     [0061]FIG. 7 shows a schematic diagram of a simplified alternative configuration of a laser system  108  for implementing the present invention. FIG. 8 shows a schematic diagram of another simplified alternative configuration of a laser system  110  that employs an amplifier  112 .  
     [0062] Laser output  104  (regardless of wavelength or laser type) can be manipulated by a variety of conventional optical components  116  and  118  that are positioned along a beam path  120 . Components  116  and  118  may include a beam expander or other laser optical components to collimate laser output  104  to produce a beam with useful propagation characteristics. One or more beam reflecting mirrors  122 ,  124 ,  126  and  128  are optionally employed and are highly reflective at the laser wavelength desired, but highly transmissive at the unused wavelengths, so only the desired laser wavelength will reach link structure  36 . A focusing lens  130  preferably employs an F1, F2, or F3 single component or multicomponent lens system that focuses the collimated pulsed laser system output  140  to produce a focused spot size  40  that is greater than the link width  28 , encompasses it, and is preferably less than 2 μm in diameter or smaller depending on the link width  28  and the laser wavelength.  
     [0063] A preferred beam positioning system  62  is described in detail in U.S. Pat. No. 4,532,402 of Overbeck. Beam positioning system  62  preferably employs a laser controller  160  that controls at least two platforms or stages (stacked or split-axis) and coordinates with reflectors  122 ,  124 ,  126 , and  128  to target and focus laser system output  140  to a desired laser link  22  on IC device or work piece  12 . Beam positioning system  62  permits quick movement between links  22  on work piece  12  to effect unique link-severing operations on-the-fly based on provided test or design data.  
     [0064] The position data preferably direct the focused laser spot  38  over work piece  12  to target link structure  36  with one set  50  of laser pulses  52  of laser system output  140  to remove link  22 . The laser system  60  preferably severs each link  22  on-the-fly with a single set  50  of-laser pulses  52  without stopping the beam positioning system  62  over any link  22 , so high throughput is maintained. Because the sets  50  are less than about 1,000 ns, each set  50  is treated like a single pulse by positioning system  62 , depending on the scanning speed of the positioning system  62 . For example, if a positioning system  62  has a high speed of about 200 mm per second, then a typical displacement between two consecutive laser spots  38  with an interval time of 1,000 ns between them would be typically less than 0.2 μm, and preferably less then 0.06 μm during a preferred time interval of 300 ns of set  50 , so two or more consecutive spots  38  would substantially overlap, and each of the spots  38  would completely cover the link width  28 . In addition to control of the repetition rate, the time offset between the initiation of pulses  52  within a set  50  is typically less than 1,000 ns and preferably between about 5 ns and 500 ns and can also be programmable by controlling Q-switch stepping, laser synchronization, or optical path delay techniques as later described.  
     [0065] Laser controller  160  is provided with instructions concerning the desired energy and pulse width of laser pulses  52 , the number of pulses  52 , and/or the shape and duration of sets  50  according to the characteristics of link structures  36 . Laser controller  160  may be influenced by timing data that synchronizes the firing of laser system  60  to the motion of the platforms such as described in U.S. Pat. No. 5,453,594 of Konecny for Radiation Beam Position and Emission Coordination System. Alternatively, skilled persons will appreciate that laser controller  160  may be used for extracavity modulation of laser energy via an E-O or an A-O device  106  and/or may optionally instruct one or more subcontrollers  164  that control Q-switch  86  or gating device  106 . Beam positioning system  62  may alternatively or additionally employ the improvements or beam positioners described in U.S. Pat. No. 5,751,585 of Cutler et al. or U.S. Pat. No. 6,430,465 B2 of Cutler, which are assigned to the assignee of this application. Other fixed-head, fast positioner-head such as galvanometer-, piezoelectrically-, or voice coil-controlled mirrors, or linear motor-driven conventional positioning systems or those employed in the 9300 or 9000 model series manufactured by Electro Scientific Industries, Inc. (ESI) of Portland, Oreg. could also be employed.  
     [0066] With reference again to FIGS.  3 - 5 , in some embodiments, each set  50  of laser pulses  52  is preferably a burst of ultrashort laser pulses  52 , which are generally shorter than 25 ps, preferably shorter than or equal to 10 ps, and most preferably from about 10 ps to 100 fs or shorter. The laser pulse widths are preferably shorter than 10 ps because material processing with such laser pulses  52  is believed to be a nonthermal process unlike material processing with laser pulses of longer pulse widths. Skilled persons will also appreciate that due to the ultrashort laser pulse width and the higher laser intensity, a higher laser frequency conversion efficiency can be readily achieved and employed. When laser output  140  comprises ultrashort pulses  52 , the duration of each set  50  can be less than 1,000 ns as previously described, but the set duration is preferably less than 300 ns and more preferably in the range of 10 ns to 200 ns.  
     [0067] During a set  50  of ultrashort laser pulses  52 , each laser pulse  52  pits off a small part or sublayer of the passivation layer  44  and/or link material needed to be removed without generating significant heat in link structure  36  or an IC device of work piece  12 . Due to its extremely short pulse width, each pulse  52  exhibits high laser energy intensity that causes dielectric breakdown in conventionally transparent passivation materials. Each ultrashort laser pulse  12  breaks off a thin sublayer of, for example, about 500-2,000 Å of overlying passivation layer  44  until overlying passivation layer  44  is removed. Consecutive ultrashort laser pulses  52  ablate metallic link  22  in a similar layer by layer manner. For conventionally opaque material, each ultrashort pulse  52  ablates a sublayer having a thickness comparable to the absorption depth of the material at the wavelength used. The absorption or ablation depth per single ultrashort laser pulse for most metals is about 100-300 Å.  
     [0068] Although in many circumstances a wide range of energies per ultrashort laser pulse  52  will yield substantially similar severing depths, in a preferred embodiment, each ultrashort laser pulse  52  ablates about a 0.02-0.2 μm depth of material within spot size  40 . When ultrashort pulses are employed, preferred sets  50  include 2 to 20 ultrashort pulses  52 .  
     [0069] In addition to the “nonthermal” and well-controllable nature of ultrashort laser processing, some common ultrashort laser sources are at wavelengths of around 800 nm and facilitate delivery of a small-sized laser spot. Skilled persons will appreciate, however, that the substantially nonthermal nature of material interaction with ultrashort pulses  52  permits IR laser output be used on links  22  that are narrower without producing an irregular unacceptable explosion pattern. Skilled persons will also appreciate that due to the ultrashort laser pulse width and the higher laser intensity, a higher laser frequency conversion efficiency can be readily achieved and employed.  
     [0070] With reference FIGS.  9 - 16 , in some embodiments, each set  50  preferably includes 2 to 10 pulses  52 , which are preferably in the range of about 0.1 ps to about 30 ns and more preferably from about 25 ps to 30 ns or ranges in between such as from about 100 ps to 10 ns or from 5 ns to 20 ns. These typically smaller sets  50  of laser pulses  52  may be generated by additional methods and laser system configurations. For example, with reference to FIG. 9, the energy densities of pulses  52   d  of set  50   d  can accomplished with a simultaneously Q-switched and CW mode-locked laser system  60  (FIG. 6).  
     [0071]FIG. 10A depicts an energy density profile of typical laser output from a conventional laser used for link blowing. FIG. 10B depicts an energy density profile of a set  50   e  of laser pulses  52   e   1  and  52   e   2  emitted from a laser system  60  (with or without mode-locking) that has a step-controlled Q-switch  86 . Skilled persons will appreciate that the Q-switch can alternatively be intentionally misaligned for generating more than one laser pulse  52 . Set  50   e  depicts one of a variety of different energy density profiles that can be employed advantageously to sever links  22  of link structures  36  having different types and thicknesses of link or passivation materials. The shape of set  50   c  can alternatively be accomplished by programming the voltage to an E-O or A-O gating device or by employing and changing the rotation of a polarizer.  
     [0072]FIG. 11 is a power versus time graph of an exemplary RF signal  54  applied to a step-controlled Q-switch  86 . Unlike typical laser Q-switching which employs an all or nothing RF signal and results in a single laser pulse (typically elimination of the RF signal allows the pulse to be generated) to process a link  22 , step-controlled Q-switching employs one or more intermediate amounts of RF signal  54  to generate one or more quickly sequential pulses  52   e   3  and  52   e   4 , such as shown in FIG. 12, which is a power versus time graph.  
     [0073] With reference to FIGS. 11 and 12, RF level  54   a  is sufficient to prevent generation of a laser pulse  52   e.  The RF signal  54  is reduced to an intermediate RF level  54   b  that permits generation of laser pulse  52   e   3 , and then the RF signal  54  is eliminated to RF level  54   c  to permit generation of laser pulse  52   e   4 . The step-control Q-switching technique causes the laser pulse  52   e   3  to have a peak power that is lower than that of a given single unstepped Q-switched laser pulse and allows generation of additional laser pulse(s)  52   e   4  of peak powers that are also lower than that of the given single unstepped Q-switched laser pulse. The amount and duration of RF signal  54  at RF level  54   b  can be used to control the peak powers of pulses  52   e   3  and  52   e   4  as well as the time offset between the laser pulses  52  in each set  50 . More that two laser pulses  52   e  can be generated in each set  50   e,  and the laser pulses  52   e  may have equal or unequal amplitudes within or between sets  50   e  by adjusting the number of steps and duration of the RF signal  54 .  
     [0074]FIG. 13 is a simplified schematic diagram of an alternative embodiment of a laser system  60   b  employing a Q-switched laser  64   b  (with or without CW-mode-locking) and having an optical delay path  170  that diverges from beam path  120 , for example. Optical delay path  170  preferably employs a beam splitter  172  positioned along beam path  120 . Beam splitter  172  diverts a portion of the laser light from beam path  120  and causes a portion of the light to propagate along beam path  120   a  and a portion of the light to propagate along optical delay path  170  to reflective mirrors  174   a  and  174   b,  through an optional half wave plate  176  and then to combiner  178 . Combiner  178  is positioned along beam path  120  downstream of beam splitter  172  and recombines the optical delay path  170  with the beam path  120   a  into a single beam path  120   b.  Skilled persons will appreciate that optical delay path  170  can be positioned at a variety of other locations between laser  64   b  and link structure  36 , such as between output coupling mirror  78  and optical component  116  and may include numerous mirrors  174  spaced by various distances.  
     [0075] FIGS.  14 A- 14 D show respective power versus time graphs of exemplary laser pulses  52   f  propagating along optical paths  120 ,  120   a,    120   b,  and  170  of the laser system  60   b  shown in FIG. 13. With reference to FIGS. 13 and 14A- 14 D, FIG. 14A shows the power versus time graph of a laser output  96  propagating along beam path  120 . Beam splitter  172  preferably splits laser output  96  into equal laser pulses  52   f   1  of FIG. 14B and 52 f   2  of FIG. 14C (generically laser pulses  52   f ), which respectively propagate along optical path  120   a  and optical delay path  170 . After passing through the optional half wave plate  176 , laser pulse  52   f   2  passes through combiner  178  where it is rejoined with laser pulse  52   f   1  propagate along optical path  120   b.  FIG. 14D shows the resultant power versus time graph of laser pulses  52   f   1  and  52   f   2  propagating along optical path  120   b.  Because optical delay path  170  is longer than beam path  120   a,  laser pulse  52   f   2  occurs along beam path  120   b  at a time later than  52   f   1 .  
     [0076] Skilled persons will appreciate that the relative power of pulses  52  can be adjusted with respect to each other by adjusting the amounts of reflection and/or transmission permitted by beam splitter  172 . Such adjustments would permit modulated profiles such as those discussed or presented in profiles  50   c.  Skilled persons will also appreciate that the length of optical delay path  170  can be adjusted to control the timing of respective pulses  52   f.  Furthermore, additional delay paths of different lengths and/or of dependent nature could be employed to introduce additional pulses at a variety of time intervals and powers.  
     [0077] Skilled persons will appreciate that one or more optical attenuators can be positioned along common portions of the optical path or along one or both distinct portions of the optical path to further control the peak-instantaneous power of the laser output pulses. Similarly, additional polarization devices can be positioned as desired along one or more of the optical paths. In addition, different optical paths can be used to generate pulses  52  of different spot sizes within a set  50 .  
     [0078]FIG. 15 is a simplified schematic diagram of an alternative embodiment of a laser system  60   c  that employs two or more lasers  64   c   1  and  64   c   2  (generally lasers  64 ) to implement the present invention, and FIGS.  16 A- 16 C show respective power versus time graphs of an exemplary laser pulses  52   g   1  and  52   g   2  (generically  52   g ) propagating along optical paths  120   c,    120   d,  and  120   e  of laser system  60   c  shown in FIG. 15. With reference to FIGS. 15 and 16A- 16 C, lasers  64  are preferably Q-switched (preferably not CW mode-locked) lasers of types previously discussed or well-known variations and can be of the same type or different types. Skilled persons will appreciate that lasers  64  are preferably the same type and their parameters are preferably controlled to produce preferred, respectively similar spot sizes, pulse energies, and peak powers. Lasers  64  can be triggered by synchronizing electronics  180  such that the laser outputs are separated by a desired or programmable time interval. A preferred time interval includes about 5 ns to about 1,000 ns.  
     [0079] Laser  64   c   1  emits laser pulse  52   g   1  that propagates along optical path  120   c  and then passes through a combiner  178 , and laser  64   c   2  emits laser pulse  52   g   2  that propagates along optical path  120   d  and then passes through an optional half wave plate  176  and the combiner  178 , such that both laser pulses  52   g   1  and  52   g   2  propagate along optical path  120   e  but are temporally separated to produce a set  50   g  of laser pulses  52   g  having a power versus time profile shown in FIG. 16C.  
     [0080] With respect to all the embodiments, preferably each set  50  severs a single link  22 . In most applications, the energy density profile of each set  50  is identical. However, when a work piece  12  includes different types (different materials or different dimensions) of links  22 , then a variety of energy density profiles (heights and lengths and as well as the shapes) can be applied as the positioning system  62  scans over the work piece  12 .  
     [0081] In view of the foregoing, link processing with sets  50  of laser pulses  52  offers a wider processing window and a superior quality of severed links than does conventional link processing without sacrificing throughput. The versatility of pulses  52  in sets  50  permits better tailoring to particular link characteristics.  
     [0082] Because each laser pulse  52  in the laser pulse set  50  has less laser energy, there is less risk of damaging the neighboring passivation and the silicon substrate  42 . In addition to conventional link blowing IR laser wavelengths, laser wavelengths shorter than the IR can also be used for the process with the added advantage of smaller laser beam spot size, even though the silicon wafer&#39;s absorption at the shorter laser wavelengths is higher than at the conventional IR wavelengths. Thus, the processing of narrower and denser links is facilitated. This better link removal resolution permits links  22  to be positioned closer together, increasing circuit density. Although link structures  36  can have conventional sizes, the link width  28  can, for example, be less than or equal to about 0.5 μm.  
     [0083] Similarly, passivation layers  44  above or below the links  22  can be made with material other than the traditional materials, or can be modified if desirable to be other than a typical height since the sets  50  of pulses  52  can be tailored and since there is less damage risk to the underlying or neighboring passivation structure. In addition, because wavelengths much shorter than about 1.06 μm can be employed to produce critical spot size diameters  59  of less than about 2 μm and preferably less than about 1.5 μm or less than about 1 μm, center-to-center pitch  32  between links  22  processed with sets  50  of laser pulses  52  can be substantially smaller than the pitch  32  between links  22  blown by a conventional IR laser beam-severing pulse. Link  22  can, for example, be within a distance of 2.0 μm or less from other links  22  or adjacent circuit structures  34 .  
     [0084]FIGS. 17A, 17B, and  17 C (collectively FIG. 17) are fragmentary cross-sectional side views of target structure  56  undergoing sequential stages of target processing in accordance with alternative embodiments of the present invention employed to remove only the passivation layer  44  overlying the selected links  22  to be removed. Target structure  56  can have dimensions as large as or smaller than those blown by laser spots  38  of conventional link-blowing laser output  48 . For convenience, certain features of target structure  56  that correspond to features of target structure  36  of FIG. 2A have been designated with the same reference numbers.  
     [0085] With reference to FIG. 17, target structure  56  comprises an overlying passivation layer  44  that covers an etch target such as link  22  that is formed upon an optional underlying passivation layer  46  above substrate  42 . The passivation layer  44  may include any conventionally used passivation materials such as silicon dioxide and silicon nitride. The underlying passivation layer  46  may include the same or different passivation material(s) as the overlying passivation layer  44 . In particular, underlying passivation layer  46  in target structures  56  may comprise fragile materials, including but not limited to, materials formed from low K materials, low K dielectric materials, low K oxide-based dielectric materials, orthosilicate glasses (OSGs), flourosilicate glasses, organosilicate glasses, tetraethylorthosilicate (TEOS), methyltriethoxyorthosilicate (MTEOS), propylene glycol monomethyl ether acetate (PGMEA), silicate esters, hydrogen silsesquioxane (HSQ), methyl silsesquioxane (MSQ), polyarylene ethers, benzocyclobutene (BCB), “SiLK” sold by Dow, or “Black Diamond” sold by AMAT. Underlying passivation layers  46  made from some of these materials are more prone to crack when their targeted links  22  are blown or ablated by conventional single laser-pulse link-removal operations.  
     [0086]FIG. 17A shows a target area  51  of overlying passivation layer  44  of a target structure  56  receiving a laser spot  55  of laser output  140  characterized by an energy distribution adapted to achieve removal of overlying passivation layer  44  in accordance with the present invention. The laser output  140  can have a much lower power than a conventional pulse of laser output  48  because the power necessary for removing overlying passivation layer  44  can be significantly lower than the power needed to blow link  22  (and passivation layer  44 ) as shown in FIGS. 2A and 2C. The lower powers facilitated by the passivation layer-removing and target-etch process substantially increase the processing window for the parameters of the laser output. Therefore, passivation layer removal provides more choices for laser sources that can be selected based on other criteria such as wavelength, spot size, and availability.  
     [0087]FIG. 17B shows target structure  56  after an impinged portion  58  of target area  51  of overlying passivation layer  44  (indicated by an arrow where removed) has been removed by laser output  140 .  
     [0088]FIG. 17C shows target structure  56  of FIG. 17B after an exposed portion  61  of link  22  has been removed by etching. Skilled persons will recognize that etching, particularly chemical and plasma etching, is well known from photolithography and other circuit fabrication processes.  
     [0089] The passivation removal technique described with respect to FIG. 17 is far less likely to generate debris of link material common to link-blowing processes. Even if the passivation ablation process dips into a link  22  and generates some link material debris, such debris would be cleaned off during the following chemical etch process. Thus, for some applications removal a small portion of the top of link  22  may be desirable to insure that enough of passivation layer  44  is removed so as not to impede the subsequent link etch process, nevertheless it is desirable to minimize laser impingement on link  22  to minimize redeposit of link material and avoid cracking the surrounding passivation. In circumstances where link impingement is desirable, the differential removal rate between materials of passivation layer  44  and the materials of link  22  permit the passivation layer  44  to be completely be removed with comparatively little penetration into the metal of link  22 . In an exemplary embodiment, each ultrashort laser pulse  52  removes about a 0.02-0.2 μm depth of material within spot size  59 . The substrate protection, smaller critical dimensions, and reduced risk of causing cracks in the underlying passivation afforded by the passivation removal and link-etching process are, therefore, significant improvements over the conventional link-blowing process.  
     [0090] The embodiments described with respect to FIG. 17 permit IC manufacturers to laser process on-the-fly unique positions on circuit elements  14  having minimum pitch dimensions limited primarily by the emission wavelength of the laser output  140 . Links  22  can, for example, be within less than a couple of microns of other links or adjacent circuit structures  34 . Skilled persons will also appreciate that because etching can remove thicker links more effectively than traditional link blowing can, memory manufacturers can decrease link widths  28  and link by designing thicker links to maintain or increase signal propagation speed or current carrying capacity.  
     [0091] With respect to passivation removal, any of the previously-described laser techniques and embodiments can be used. Preferred sets  50  for passivation removal include 1 to 20 pulses  52 , more preferably 1 to 5 pulses  52 , and most preferably 1 to 2 pulses  52 , and preferred pulse widths are in the range of about 30 ns to about 50 fs or shorter. Depending on the wavelength of laser output and the characteristics of the passivation layer  44 , the removal depth of pulses  52  applied to passivation layer  44  can be accurately controlled by choosing the energy of each pulse  52  and the number of laser pulses  52  in each set  50  to completely expose any given link  22  by cleaning off the bottom of passivation layer  44 , leaving at least the bottom portion of the link  22 , if not the whole link  22 , relatively intact and thereby not exposing the underlying passivation layer  44  or the substrate  42  to any high laser energy. It is preferred, but not essential, that a major portion of the thickness of a given link  22  remains intact in any passivation removal process. Hence, the risk of cracking even a fragile passivation layer  46  or damaging the silicon substrate  42  is substantially eliminated, even if a laser wavelength in the UV range is used.  
     [0092] Skilled persons will appreciate that when the longer pulse widths are employed for passivation removal at laser wavelengths not absorbed by the passivation layer  44 , sufficient energy must be supplied to the top of the link  22  so that it causes a rupture in the passivation layer  44 . In such embodiments, a large portion of the top of links  22  may be removed. However, subsequently etching the remaining portions of exposed links  22  still provides better quality and tighter tolerances than removing the entire link  22  with a conventional link-blowing laser pulse.  
     [0093] In some preferred embodiments, the laser output  140  for removing the passivation layer  44  over each link  22  to be severed comprises a single laser output pulse  52 . Such single laser output pulse  52  preferably has a pulse width that is shorter than about 20 ns, preferably shorter than about 1 ns, and most preferably shorter than about 10 to 25 ps. An exemplary laser pulse  52  of a single pulsed set  50  has laser pulse energies ranging between about 0.005 μJ to about 2 μJ, or even up to 10 μJ, and intermediate energy ranges between 0.01 μJ to about 0.1 μJ. Although these ranges of laser pulse energies largely overlap those for laser pulses  52  in multiple sets, skilled persons will appreciate that a laser pulse  52  in a single pulse set  50  will typically contain a greater energy than a laser pulse  52  in a multiple set employed to process similar passivation materials of similar thicknesses. Skilled persons will appreciate that laser sets  50  of one or more sub-nanosecond laser pulses  52  may be generated by the laser systems  60  already described but may also be generated by a laser having a very short resonator.  
     [0094] Skilled persons will appreciate that for some embodiments, the links  22  and the bond pads are be made from the same material, such aluminum, and such bond pads can be (self-) passivated to withstand etching of exposed links  22 . In other embodiments, the links  22  and the bond pads are made from different materials, such as links  22  made of copper and bond pads made of aluminum. In such cases, the nonexistence of passivation over the bond pads may be irrelevant because etch chemistries may be employed that do not adversely affect the bond pads. In some circumstances, it may be desirable to protect the bond pads by coating the surface of the work piece  12  with a protection layer that is easy to remove with the overlying passivation layer  44  during the aforementioned laser processes and, if desirable, easy to remove from the remaining work piece surfaces once link etching is completed. Material for such a protection layer may include, but is not limited to, any protective coating such as any resist material with or without photosensitizers, particularly materials having a low laser ablation threshold for the selected wavelength of laser pulses  52 .  
     [0095] In view of the foregoing, passivation processing with sets  50  of laser pulses  52  and subsequent etching of links  22  offers a wider processing window and a superior quality of severed links than does conventional link processing, and the processing of narrower and denser links  22  is also facilitated. The versatility of laser pulses  52  in sets  50  permits better tailoring to particular passivation characteristics. Link passivation processing is described in detail in U.S. patent application Ser. No. 10/361,206 of Sun et al., which is herein incorporated by reference.  
     [0096] It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiment of this invention without departing from the underlying principles thereof. The scope of the present invention should, therefore, be determined only by the following claims.