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  Processing a memory link with a set of at least two laser pulsesUSPTO Application #: 20060138110Title: Processing a memory link with a set of at least two laser pulses Abstract: A set (50) of laser pulses (52) is employed to sever a conductive link (22) in a memory or other IC chip. The duration of the set (50) is preferably shorter than 1,000 ns; and the pulse width of each laser pulse (52) within the set (50) is preferably within a range of about 0.1 ps to 30 ns. The set (50) can be treated as a single “pulse” by conventional laser positioning systems (62) to perform on-the-fly link removal without stopping whenever the laser system (60) fires a set (50) of laser pulses (52) at each link (22). Conventional IR wavelengths or their harmonics can be employed. (end of abstract)
Agent: Stoel Rives LLP - Portland, OR, US Inventors: Yunlong Sun, Edward J. Swenson, Richard S. Harris USPTO Applicaton #: 20060138110 - Class: 219121690 (USPTO) Related Patent Categories: Electric Heating, Metal Heating (e.g., Resistance Heating), By Arc, Using Laser, Cutting, Etching Or Trimming, Methods The Patent Description & Claims data below is from USPTO Patent Application 20060138110. Brief Patent Description - Full Patent Description - Patent Application Claims
RELATED APPLICATIONS [0001] This patent application is a continuation 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, now U.S. Pat. No. 6,574,250, 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 a laser system and method employing a set of at least two laser pulses to sever an 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 2B 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 2B, 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 to program a logic product, gate arrays, or ASICs. [0004] Links 22 are about 0.3-2 microns (.mu.m) thick and are designed with conventional link widths 28 of about 0.4-2.5 .mu.m, link lengths 30, and element-to-element pitches (center-to-center spacings) 32 of about 2-8 .mu.m 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 silicides 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 .mu.m or 1.064 .mu.m 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 each 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 (.DELTA.) thick, and an underlying passivation layer 46. Silicon substrate 42 absorbs a relatively small proportional quantity of IR radiation, and conventional passivation layers 44 and 46 such as silicon dioxide or silicon nitride are relatively transparent to IR 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 link 22, with each 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 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 pulse at a longer laser wavelength, such as 1.3 .mu.m, to process memory links 22 on silicon wafers. At the 1.3 .mu.m laser wavelength, the absorption contrast between the link material and silicon substrate 42 is much larger than that at the traditional 1 .mu.m 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 1.0 .mu.m and 1.3 .mu.m laser wavelengths have disadvantages however. The 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. and U.S. Pat. No. 6,025,256 of Swenson et al. more recently describe methods of using ultraviolet (UV) laser output to sever or expose links that "open" the overlying passivation by different material removal mechanisms and have 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. 5,656,186 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. [0010] 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. [0011] 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 [0012] An object of the present invention is to provide a method or apparatus for improving the quality of laser processing of IC links. [0013] Another object of the invention is to process a link with a set of low energy laser pulses. [0014] A further object of the invention is to process a link with a set of low energy laser pulses at a shorter wavelength. [0015] Yet another object of the invention is to employ such sets of laser pulses to process links on-the-fly. [0016] 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, instead of using a single laser pulse of conventional link processing systems. This practice does not, however, entail either a long dwell time or separate duplicative scanning passes of repositioning and refiring at each link that would effectively reduce the throughput by factor of about two. 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 supporting the link structure. The number of laser pulses in the set is controlled such that the last pulse cleans off the bottom of the link leaving the underlying passivation layer and the substrate 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 and the displacement between the laser spots of each pulse is less than the positioning accuracy of a typical positioning system, which is typically +-0.05 to 0.2 .mu.m. Thus, the laser system can still process links 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. [0017] 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 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 burst set is ultrashort, its interaction with the target materials (passivation layers and metallic link) is substantially not thermal. Each laser pulse breaks off a thin sublayer of about 100-2,000 .ANG. of material, depending on the laser energy or peak power, laser wavelength, and type of material, until the link 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 narrower than about 1 .mu.m. 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. [0018] 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. [0019] 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. 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