EP1365880A1 - Realisation des motifs de microstructures de semiconducteurspar ablation au laser ultraviolet - Google Patents

Realisation des motifs de microstructures de semiconducteurspar ablation au laser ultraviolet

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Publication number
EP1365880A1
EP1365880A1 EP02707453A EP02707453A EP1365880A1 EP 1365880 A1 EP1365880 A1 EP 1365880A1 EP 02707453 A EP02707453 A EP 02707453A EP 02707453 A EP02707453 A EP 02707453A EP 1365880 A1 EP1365880 A1 EP 1365880A1
Authority
EP
European Patent Office
Prior art keywords
laser
laser system
substrate
chuck
silicon
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP02707453A
Other languages
German (de)
English (en)
Other versions
EP1365880A4 (fr
Inventor
Brian W. Baird
Michael J. Wolfe
Richard J. Harris
Kevin P. Fahey
Lian-Cheng Zou
Thomas R. Mcneil
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Electro Scientific Industries Inc
Original Assignee
Electro Scientific Industries Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from US09/803,382 external-priority patent/US20020033558A1/en
Application filed by Electro Scientific Industries Inc filed Critical Electro Scientific Industries Inc
Publication of EP1365880A1 publication Critical patent/EP1365880A1/fr
Publication of EP1365880A4 publication Critical patent/EP1365880A4/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/06—Shaping the laser beam, e.g. by masks or multi-focusing
    • B23K26/062—Shaping the laser beam, e.g. by masks or multi-focusing by direct control of the laser beam
    • B23K26/0626—Energy control of the laser beam
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/08—Devices involving relative movement between laser beam and workpiece
    • B23K26/083—Devices involving movement of the workpiece in at least one axial direction
    • B23K26/0853—Devices involving movement of the workpiece in at least in two axial directions, e.g. in a plane
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/08—Devices involving relative movement between laser beam and workpiece
    • B23K26/0869—Devices involving movement of the laser head in at least one axial direction
    • B23K26/0876—Devices involving movement of the laser head in at least one axial direction in at least two axial directions
    • B23K26/0884—Devices involving movement of the laser head in at least one axial direction in at least two axial directions in at least in three axial directions, e.g. manipulators, robots
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/36—Removing material
    • B23K26/40—Removing material taking account of the properties of the material involved
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2101/00—Articles made by soldering, welding or cutting
    • B23K2101/36—Electric or electronic devices
    • B23K2101/40—Semiconductor devices
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2103/00—Materials to be soldered, welded or cut
    • B23K2103/50—Inorganic material, e.g. metals, not provided for in B23K2103/02 – B23K2103/26
    • B23K2103/52—Ceramics

Definitions

  • This invention relates to a method and/or apparatus for high-speed formation of micron-scale features by ablation of semiconductors, and particularly silicon, using pulsed output of an ultraviolet (UN) laser.
  • UN ultraviolet
  • Lee report use of Nd:YAG laser wavelengths at 1064 nm and 532 nm to create vias throughout the surface of a silicon wafer for the purpose of enabling production of multichip modules.
  • Lee reports that when laser drilling through holes in silicon wafers at 1064 nm, molten material frequently condensed onto the walls of the holes once an appreciable depth was reached. This apparent redeposition of silicon made the holes unsuitable for further processing.
  • Lee reports employment of a double drilling process at 1064 nm to improve hole quality.
  • Lee describes employing 532 nm frequency- doubled pulsed laser output from a lamp-pumped, Q-switched Nd: YAG in a trepanning process using rotating lenses that are offset with respect to the incoming laser beam to cut 4 mil (approximately 100 micron ( ⁇ m)) diameter holes in silicon. He reports the processing parameters used as 833 ⁇ J per pulse at a pulse repetition frequency of 3 kHz with a pulsewidth of 70 ns. Redeposition of silicon around the perimeter and along the walls of the laser drilled via was still observed and a chemical etching process was used to clean the holes.
  • Lee further reports on using an excimer laser at a wavelength of 248 nm to drill holes in silicon. Holes with very smooth sidewalls were reported due to the very high pulse energies employed. He reports using an energy per pulse of 290 mJ at a pulse repetition frequency of 250 Hz and a focused spot size of 5 mils (approximately 125 ⁇ m) to drill a hole through a silicon wafer in 30 seconds. He compared drilling time to the 3 seconds required for the holes drilled using his 532 nm Nd:YAG trepanning technique. Lee suggests a method for reducing the drilling time required for silicon holes by a 248 nm excimer laser through use of a projection technique. As those skilled in the art will recognize, such a technique is reliant upon an appropriate aperture mask for each pattern of holes to be formed using such a technique.
  • Dahm discusses the problem of use of near infrared lasers for the purpose of dicing silicon wafers. He teaches that the primary cause for poor cut quality resulting from redeposition when employing near infrared lasers is use of laser pulsewidths exceeding about 1 ns. Dahm teaches the use of near infrared lasers with short pulsewidths of less than about 1 ns to solve the deep absorption depth of near infrared wavelengths in silicon, stating that such short pulsewidths may produce surface plasmas which can act as highly absorbing layers.
  • Cutler et al. describe a high speed, high accuracy multi-stage positioning system for accurately and rapidly positioning a wide variety of tools, such as a laser beam relative to targets on a workpiece. They employ a multi-rate positioner system which processes workpiece target positioning commands and converts them to commands to slow and fast positioners. These positioners move without necessarily stopping in response to a stream of positioning data. In one embodiment, this technique enables the laser micromachining of a pattern of small features across a large workpiece, thereby allowing increased throughput of laser micromachined parts.
  • An object of the invention is to provide an improved method for using a laser to create microfeatures in semiconductors, including silicon, gallium arsenide (GaAs), silcon carbide (SiC), silicon nitride (SiN), and/or Ge:Si, and/or also including such semiconductors subsequently treated in semiconductor processes, including but not limited to photolithography and etching, well known to those skilled in the art to contain additional layers for the purpose of creating useful electronic and optoelectronic circuits on semiconductor substrates, including semiconductor wafers.
  • GaAs gallium arsenide
  • SiC silcon carbide
  • SiN silicon nitride
  • Ge:Si Ge:Si
  • Another object of the invention is to provide such a method that employs a highly reliable nonexcimer UV laser capable of operating with high pulse energy output at high pulse repetition frequencies.
  • the present invention provides a method for rapidly and directly forming patterns with feature sizes of less than 50 ⁇ m in semiconductor workpieces using ultraviolet laser ablation.
  • a compound beam positioner is used to rapidly position at the workpiece the focused output of a nonexcimer UV laser capable of emitting high energy per pulse output at high pulse repetition frequencies.
  • These patterns may include: formation of very high-aspect cylindrical openings, such as through-holes or blind vias, for integrated circuit connections; curvilinear or rectilinear singulation of processed dies contained on silicon wafers; microtab cutting to separate microcircuits formed in semiconductor workpieces from parent wafer; formation of curvilinear or rectilinear features in optical waveguides, such as arrayed waveguide gratings (AWGs) or microelectronic machine systems (MEMS); and scribing alignment, identification, or other markings into the wafer surface.
  • AMGs arrayed waveguide gratings
  • MEMS microelectronic machine systems
  • a Q-switched frequency-tripled Nd:YAG, Nd:YVO 4 , or Nd:YLF diode-pumped laser provides the preferred source of the ablative ultraviolet output.
  • the laser's optical system produces a Gaussian spot size of about 10 ⁇ m.
  • an optical system producing a top hat beam profile may be used.
  • Exemplary pulse energy for high-speed ablative processing of silicon using this focused spot size is greater than 200 ⁇ J per pulse at pulse repetition frequencies of greater than 5 kHz and preferably above 15 kHz.
  • the laser pulsewidth measured at the full width half-maximum points is preferably less than 80 ns.
  • UV wavelengths are the capability of producing spot sizes significantly smaller than those achievable with longer wavelength sources. This small spot size capability enables the production of micron-scale feature sizes in silicon. Also, for a fixed spot size achievable with conventional Gaussian focusing techniques, a shorter wavelength allows for formation of features with improved aspect ratios due to the greater depth of focus afforded at ultraviolet wavelengths.
  • the present invention also provides methods for reducing damage or marring of processed semiconductor workpieces resulting from stray reflections of the ultraviolet processing beam from workpiece supporting structures, such as wafer chucks, by use of substantially nonreflecting materials and novel fabrication techniques for workpiece supporting structures.
  • FIG. 1 is a graph displaying the optical absorption coefficient of silicon as a function of wavelength.
  • FIG. 2 is a graph displaying the optical absorption coefficient of gallium arsenide (GaAs) as a function of wavelength.
  • GaAs gallium arsenide
  • FIG. 3 is a simplified pictorial diagram of a preferred laser system for ultraviolet laser ablative patterning of microstructures in semiconductors in accordance with the present invention.
  • FIG. 4 is a simplified pictorial diagram of an alternative preferred laser system for ultraviolet laser ablative patterning of microstructures in semiconductors.
  • FIG. 5 is a simplified pictorial diagram of an optional imaged optics module that may be used in a laser system for ultraviolet laser ablative patterning of microstructures in semiconductors.
  • FIG. 6 is a graph displaying the characteristic relationship between pulse energy and pulse repetition frequency of the laser employed during practice of the invention.
  • FIG. 7 is a representative illustration of ultraviolet ablative patterning of a cylindrical opening in silicon.
  • FIG. 8 is a representative illustration of ultraviolet ablative patterning of a trench pattern in silicon.
  • FIG. 9 is a simplified representation of an exemplary segmented cutting profile for making long cuts in semiconductor materials.
  • FIG. 10 is a simplified representation of an alternative segmented cutting profile for making long cuts in semiconductor materials.
  • FIG. 11 is a representative illustration of ultraviolet ablative patterning of a
  • FIG. 12 is a representative illustration of ultraviolet ablative patterning of an
  • AWG device fabricated on a semiconductor wafer.
  • FIG. 13 is a representative illustration of an ultraviolet transparent chuck on which semiconductor workpieces are placed for through processing using the ultraviolet ablative patterning method.
  • FIG. 1 displays the optical absorption coefficient of silicon as a function of wavelength.
  • silicon exhibits a very sharp rise in the optical absorption at wavelengths in the ultraviolet.
  • the present invention advantageously utilizes laser wavelengths shorter than 390 nm and takes advantage of the increased absorption of silicon in the ultraviolet to efficiently ablate silicon and thereby form a variety of useful patterns or features directly in silicon.
  • the absorption behavior facilitates strongly ablative removal of silicon in the ultraviolet with a greatly reduced thermally affected zone in comparison with features formed using either 532 nm or 1064 nm pulsed output as taught by the prior art.
  • FIG. 2 displays the optical absorption coefficient of GaAs as a function of wavelength.
  • GaAs exhibits a very sharp rise in the optical absorption at wavelengths in the ultraviolet.
  • the absorption coefficient at 355 nm of GaAs and silicon are quite close.
  • GaAs is a key material in optoelectronic devices, such as diode lasers and detectors.
  • FIGS. 3 and 4 illustrate alternative preferred embodiments of respective laser processing systems 10a and 10b (generically 10) utilizing a compound beam positioning system 40 equipped with a wafer chuck assembly 140 that can be employed for ultraviolet laser ablative patterning of microstructures in semiconductor workpieces 12 in accordance with the present invention.
  • a preferred embodiment of a laser system 10 includes a Q-switched, diode-pumped (DP), solid-state (SS) UV laser 14 that preferably includes a solid-state lasant such as Nd: YAG, Nd: YLF, or Nd: YNO 4 .
  • DP diode-pumped
  • Laser 14 preferably provides harmonically generated UN laser output 16 of one or more laser pulses at a wavelength such as 355 nm (frequency tripled ⁇ d:YAG), 266 nm (frequency quadrupled Nd:YAG), or 213 nm (frequency quintupled Nd:YAG) with primarily a TEMoo spatial mode profile.
  • Gaussian is used herein to include profiles where M 2 is less than or equal to about 1.5, even though M 2 values of less than 1.3 or 1.2 are preferred.
  • laser 14 includes a Model 210-N06 Q-switched, frequency-tripled ⁇ d:YAG laser, operating at about 355 nm, and commercially available from Lightwave Electronics. This laser has been employed in the ESI Model 2700 microvia drilling system available from Electro Scientific Industries, Inc. of Portland, Oregon.
  • a Lightwave Model 210-N09 Q-switched, frequency-tripled ⁇ d: YAG laser, operating at about 355 nm may be employed in order to employ high energy per pulse at a high pulse repetition frequency (PRF).
  • PRF pulse repetition frequency
  • Skilled persons will appreciate that other lasers could be employed and that other wavelengths are available from the other listed lasants.
  • laser cavity arrangements, harmonic generation, and Q-switch operation, and positioning systems are all well known to persons skilled in the art, certain details of some of these components will be presented within the discussions of the preferred embodiments.
  • UN laser output 16 is optionally passed through a variety of well-known expansion and/or collimation optics 18, propagated a long an optical path 20, and directed by a beam positioning system 30 to impinge laser system output pulse(s) 32 on a desired laser target position 34 on workpiece 12.
  • Beam positioning system 30 preferably includes a translation stage positioner that preferably employs at least two transverse stages 36 and 38 that support, for example, X, Y, and/or Z positioning mirrors 42 and 44 and permit quick movement between target positions 34 on the same or different workpieces 12.
  • the translation stage positioner is a split-axis system where a Y stage 36, typically moved by linear motors along rails 46, supports and moves workpiece 12, and an X stage 38, typically moved by linear motors along rails 48, supports and moves a fast positioner 50 and associated focusing lens(es).
  • the Z dimension between X stage 38 and Y stage 36 may also be is adjustable.
  • the positioning mirrors 42 and 44 align the optical path 20 through any turns between laser 14 and fast positioner 50, which is positioned along the optical path 20.
  • the fast positioner 50 may for example employ high resolution linear motors or a pair of galvanometer mirrors that can effect unique or repetitive processing operations based on provided test or design data.
  • the stages 36 and 38 and positioner 50 can be controlled and moved independently or coordinated to move together in response to panelized or unpanelized data.
  • Fast positioner 50 preferably also includes a vision system that can be aligned to one or more fiducials on the surface of the workpiece 12.
  • Beam positioning system 30 can employ conventional vision or beam to work alignment systems that work through objective lens 36 or off axis with a separate camera and that are well known to skilled practitioners.
  • an HRNX vision box employing Freedom Library software in a positioning system 30 sold by Electro Scientific Industries, Inc. is employed to perform alignment between the laser system 10 and the target locations 34 on the workpiece 12.
  • Other suitable alignment systems are commercially available.
  • the alignment systems preferably employ bright-field, on-axis illumination, particularly for specularly reflecting workpieces like lapped or polished wafers.
  • beam positioning system 30 also preferably employs non- contact, small-displacement sensors to determine Abbe errors due to the pitch, yaw, or roll of stages 36 and 38 that are not indicated by an on-axis position indicator, such as a linear scale encoder or laser interferometer.
  • the Abbe error correction system can be calibrated against a precise reference standard so the corrections depend only on sensing small changes in the sensor readings and not on absolute accuracy of the sensor readings.
  • Such an Abbe error correction system is described in detail in International Publication No. WO 01/52004 Al published on July 19, 2001 and U.S. Publication No. 2001-0029674 Al published on October 18, 2001.
  • the relevant portions of the disclosure of the corresponding U.S. Pat. Appl. No. 09/755,950 of Cutler are herein incorporated by reference.
  • positioning systems 30 are well known to skilled practitioners and some embodiments of positioning system 30 are described in detail in U.S. Pat. No. 5,751,585 of Cutler et al.
  • the ESI Model 5320 microvia drilling system available from Electro Scientific Industries, Inc. of Portland, Oregon is a preferred implementation of positioning system 30 and has been used for laser drilling of resin coated copper packages for the electronics industry.
  • Other preferred positioning systems such as a Model series numbers 27xx, 43xx, 44xx, or 53xx, manufactured by Electro Scientific Industries, Inc. in Portland, Oregon, can also be employed.
  • Some of these systems which use an X-Y linear motor for moving the workpiece 12 and an X-Y stage for moving the scan lens are cost effective positioning systems for making long straight cuts.
  • An optional laser power controller 52 such as a half wave plate polarizer, may be positioned along optical path 20.
  • one or more beam detection devices 54 such as photodiodes, may be downstream of laser power controller 52, such as aligned with a positioning mirror 44 that is adapted to be partly transmissive to the wavelength of laser output 16.
  • Beam detection devices 54 are preferably in communication with beam diagnostic electronics that convey signals to modify the effects of laser power controller 54.
  • laser system 10b preferably employs at least two lasers 14a and 14b that emit respective laser outputs 16a and 16b that are linearly polarized in transverse directions and propagate along respective optical paths 20a and 20b toward respective reflecting devices 42a and 42b.
  • An optional waveplate 56 may be positioned along optical path 20b.
  • Reflecting device 42a is preferably a polarization sensitive beam combiner and is positioned along both optical paths 20a and 20b to combine laser outputs 16a and 16b to propagate along the common optical path 20.
  • Lasers 14a and 14b may be the same or different types of lasers and may produce laser outputs 16a and 16b that have the same or different wavelengths.
  • laser output 16a may have a wavelength of about 266 nm
  • laser output 16b may have a wavelength of about 355nm. Skilled persons will appreciate that lasers 14a and 14b may be mounted side by side or one on top of the other and both attached to one of the translation stages 36 or 38.
  • Laser system 10b is capable of producing very high energy laser output pulses 32b.
  • a particular advantage of the arrangement shown in FIG. 4 is to produce a combined laser output 32 impinging on the work surface having an increased energy per pulse which could be difficult to produce from a conventional single laser head. Such an increased energy per pulse can be particularly advantageous for ablating deep trenches or deep cylindrical openings in thick silicon wafers.
  • image optics module 62 preferably includes an optical element 64, a lens 66, and an aperture mask 68 placed at or near the beam waist created by the optical element 64 to block any undesirable side lobes and peripheral portions of the beam so that a precisely shaped spot profile is subsequently imaged onto the work surface.
  • optical element 64 is a focusing lens
  • lens 66 is a collimating lens to add flexibility to the configuration of laser system 48.
  • Varying the size of the aperture can control the edge sharpness of the spot profile to produce a smaller, sharper-edged intensity profile that should enhance the alignment accuracy.
  • the shape of the aperture can be precisely circular or also be changed to rectangular, elliptical, or other noncircular shapes that can be aligned parallel or perpendicular to a cutting direction.
  • the aperture of mask 68 may optionally be flared outwardly at its light exiting side.
  • mask 68 may comprise a UV reflective or UV absorptive material, but is preferably made from a dielectric material such as UV grade fused silica or sapphire coated with a multilayer highly UV reflective coating other UV resistant coating.
  • aperture mask 68 can be used without optical elements 64 and 66.
  • optical element 64 includes one or more beam shaping components that convert laser pulses having a raw Gaussian irradiance profile into shaped (and focused) pulses that have a near-uniform "top hat" profile, or particularly a super-Gaussian irradiance profile, in proximity to an aperture mask 68 downstream of optical element 64.
  • beam shaping components may include aspheric optics or diffractive optics.
  • lens 66 comprises imaging optics useful for controlling beam size and divergence. Skilled persons will appreciate that a single imaging lens component or multiple lens components could be employed.
  • the beam shaping components include a diffractive optic element (DOE) that can perform complex beam shaping with high efficiency and accuracy.
  • DOE diffractive optic element
  • the beam shaping components not only transform the Gaussian irradiance profile to a near-uniform irradiance profile, but they also focus the shaped output to a determinable or specified spot size.
  • the DOE may include multiple separate elements such as the phase plate and transform elements disclosed in U.S. Pat. No. 5,864,430 of Dickey et al., which also discloses techniques for designing DOEs for the purpose of beam shaping.
  • a fast response amplitude control mechanism such as an acousto-optic modulator or electro-optic modulator may be employed to modulate the pulse energy of successive pulses.
  • the pulse repetition frequency may be increased or decreased to effect a change in the pulse energy of successive pulses.
  • FIG. 6 displays the characteristic relationship between pulse energy and pulse repetition frequency (PRF) of a laser 14 employed during practice of the invention. As FIG. 6 indicates, pulse energies of greater than 200 ⁇ J can be obtained from the Model 210-V06.
  • UV laser system 10 can be used for high-speed formation of micron-scale features by ablation of semiconductors, and particularly silicon.
  • These features may include, but are not limited to, formation of very high aspect cylindrical openings 100 through or partially through silicon wafers or other silicon workpieces 12; formation of through or partially through trenches of complex geometry for the purpose of singulation of processed die on silicon wafers or silicon workpieces 12; formation of microtab features to separate microcircuits formed in silicon from parent wafers; formation of features on and/or singulation of AWGs; and formation of features in MEMS.
  • the present invention facilitates feature formation without significant melt lip formation, without significant slag formation, and without significant peel back of the feature edge.
  • FIG. 7 presents a representative illustration of a cylindrical opening 100 formed by ultraviolet ablative patterning in a silicon workpiece 12 such as a wafer having 500 ⁇ m- thick intrinsic silicon substrate 70 overlaid with a 0.5 ⁇ m-thick passivation layer of SiO 2 (not shown).
  • a silicon workpiece 12 such as a wafer having 500 ⁇ m- thick intrinsic silicon substrate 70 overlaid with a 0.5 ⁇ m-thick passivation layer of SiO 2 (not shown).
  • the cylindrical opening 100 is preferably patterned by positioning the target position 34 of the silicon workpiece 12 at the focal plane of laser system 10 and directing a series of laser system output pulses 32 at target position 34 on silicon workpiece 12.
  • laser system 10 is directed to move the silicon workpiece 12 in X and Y axes to a computer programmed centroid target position 34 of the desired location for cylindrical opening 100.
  • the sequential laser system output pulses 32 are each incident on the programmed centroid target position 34.
  • sequential laser system output pulses 32 are each incident on the programmed centroid target position 34 while workpiece 12 is fixed in X and Y axes positions.
  • the preferred energy per pulse range is about 100 ⁇ J to 1500 ⁇ J, more preferably, a energy per pulse range of about 200 ⁇ J to 1000 ⁇ J, even more preferably from about 300 ⁇ J to 800 ⁇ J, and most preferably over about 360 ⁇ J.
  • the preferred PRF range is about 5 kHz to 100 kHz, and more preferably, a PRF range from about 7 kHz to 50 kHz, and most preferably, a PRF range from about 10 kHz to 30 kHz.
  • the preferred focused spot size range is about 1 ⁇ m to 25 ⁇ m, and more preferably, a focused spot size range from about 3 ⁇ m to 20 ⁇ m, and most preferably, a focused spot size from about 8 ⁇ m to 15 ⁇ m.
  • a focused spot size range is about 1 ⁇ m to 25 ⁇ m, and more preferably, a focused spot size range from about 3 ⁇ m to 20 ⁇ m, and most preferably, a focused spot size from about 8 ⁇ m to 15 ⁇ m.
  • the Z-height of the laser focus position is simultaneously moved coincident with each succeeding laser system output pulse.32 to place the laser focus at a sequentially deeper position in the silicon workpiece 12, thereby maintaining the focused spot at a position more coincident with the remaining silicon surface.
  • cylindrical opening 100 completely penetrates the entire thickness 102 of the workpiece 12 using an output pulse energy from the laser 14 of greater than 300 microjoules ( ⁇ J) using about 100 sequential laser system output pulses 32.
  • Laser system output pulses 32 are incident at the work surface with a focused spot size (1/e 2 ) diameter of about 12 ⁇ m.
  • the cylindrical opening 100 produced in this embodiment will typically have a top surface opening diameter (dt) 104 of about 20 ⁇ m and an exit diameter (db) 106 of about 13 ⁇ m, thereby producing an aspect ratio for this through hole cylindrical opening of about 30: 1 and an opening taper angle of 0.4° .
  • the precise values of energy per pulse, focused spot size, and number of pulses required to efficiently produce high quality cylindrical openings 100 through silicon may vary according to the thickness 102 of the silicon workpiece 12, relative thickness and composition of overlayers, of which SiO 2 is only one example, and the precise ultraviolet wavelength employed.
  • the silicon may be only 50 ⁇ m thick, for example.
  • as few as about ten pulses may be employed to produce the desired through-hole cylindrical opening 100.
  • Those skilled in the art will realize that cylindrical openings, which do not completely penetrate through the entire thickness 102 of the silicon, (blind vias) can be produced through the correct selection of the parameters described.
  • cylindrical openings 100 through silicon with high aspect ratio and very low taper angle are very advantageous for electronic packaging and interconnect applications.
  • one or more groups of these small through-hole cylindrical openings 100 can be positioned on the top side near the periphery of workpieces 12, circuits or dies, or within scribing, slicing, or dicing streets or their intersections such that the back or bottom side of workpiece 12 can be precisely aligned to with respect to features on the top side.
  • Such alignment facilitates backside processing such as laser scribing or sawing to enhance processing speed or quality.
  • Techniques for front and/or backside wafer slicing or dicing are discussed in more detail in U.S. Pat Appl. No.
  • FIG. 8 is a representative illustration of ultraviolet ablative patterning of a trench 110 in a silicon workpiece 12.
  • the trench 110 is preferably patterned by positioning the silicon workpiece 12 at the focal plane of the laser system 10 and directing a string of successively overlapping laser system output pulses 32 at the silicon workpiece 12 as the laser positioning system 30 moves workpiece 12 along the X-and/or Y-axes of the workpiece 12.
  • the preferred energy per pulse range is about 100 ⁇ J to 1500 ⁇ J, and more preferably, a energy per pulse range of about 200 ⁇ j to 1000 ⁇ J, even more preferably from about 300 ⁇ J to 800 ⁇ J, and most preferably over about 360 ⁇ J.
  • the preferred PRF range is about 5 kHz to 100 kHz, and more preferably, a PRF range from about 7 kHz to 50 kHz, and most preferably, a PRF range from about 10 kHz to 30 kHz.
  • the preferred focused spot size range is about 1 ⁇ m to 25 ⁇ m, and more preferably, a focused spot size range from about 3 ⁇ m to 20 ⁇ m, and most preferably, a focused spot size from about 8 ⁇ m to 15 ⁇ m.
  • the preferred bite size range is about 0.1 ⁇ m to 10 ⁇ m, and more preferably, a bite size range from about 0.3 ⁇ m to 5 ⁇ m, and most preferably, a bite size from about 0.5 ⁇ m to 3 ⁇ m.
  • the bite size can be adjusted by controlling the speed of either or both of the stages of the laser beam positioning system and coordinating the movement speed(s) with the repetition rate and firing of the laser.
  • a linear trench 110 is cut completely through 750 ⁇ m-thick intrinsic silicon overlaid with a 2.0 ⁇ m passivation layer of SiO ⁇ using an output pulse energy from the laser 14 of about 360 ⁇ J and using a bite size of 1 ⁇ m with a stage velocity of 10 mm/s in 180 passes over workpiece 12. These laser pulses are incident at the work surface with a focused spot size (1/e 2 ) diameter of 12 ⁇ m.
  • various patterns of varying geometry including, but not limited to, squares, rectangles, ellipses, spirals, and/or combinations thereof, may be produced through programming of a tool path file used by laser system 10 and positioning system 30 to position silicon workpiece 12 along X and Y-axes during processing.
  • the beam positioning system 30 is preferably aligned to conventional typical saw cutting or other fiducials or a pattern on the wafer surface. If the wafers are already mechanically notched, alignment to the cut edges is preferred to overcome the saw tolerance and alignment errors.
  • Laser cutting destroys significantly less material (kerfs of less than 50 ⁇ m wide and preferably less than 25 ⁇ m wide) than does mechanical cutting (slicing lanes of about 300 ⁇ m and dicing paths of about 150 ⁇ m) so that devices on wafers can be manufactured much closer together, allowing many more devices to be produced on each wafer. Thus, the laser cutting process minimizes the pitch between rows and the pitch between devices.
  • Elimination of the mechanical cutting can also simplify manufacture of devices on workpieces 10. In particular, mechanical cutting can impart significant mechanical stress to devices such that they come off their carriers. To avoid losing rows, device manufacturers may employ strong adhesives or epoxies between the rows and the carrier.
  • An all laser process significantly reduces the mechanical strength requirements of the adhesive used for fixturing the rows onto a carrier.
  • Laser cutting therefore, permits the elimination of strong adhesives or epoxies used to affix the rows to the carrier and the harsh chemicals needed to remove them.
  • the adhesives can be selected for ease of debonding, such as the reduction of debond time and less exposure to potentially corrosive chemicals, and for amenability to UV laser processing, greatly reducing risk of damage to the devices, and thereby enhancing yield.
  • Laser row slicing reduces row bow because laser slicing does not exert as much mechanical stress as mechanical slicing. However, if row bow or other of the row defects are apparent, the rows can be laser diced (and re-sliced) to compensate for these defects without concern for the critical device to device alignment needed between rows for mechanical dicing.
  • the term (through)cutting may be used generically to include slicing (often associated with wafer row separation) or dicing (often associated with part singulation from wafer rows), and slicing and dicing may be used interchangeably in the context of this invention.
  • laser system 10 can process each row and/or each device independently. With respect to slanted rows, the laser spot can perform traverse cuts across the slanted rows at appropriate positions with respect to outer edges of the devices with stage and/or beam translations between each cut to effect a rectangular or curvilinear wave patterns as desired.
  • laser dicing can compensate for row fixturing defects and perhaps save entire rows of devices that would be ruined by mechanical dicing.
  • UV laser cutting throughput through silicon and like materials can be improved by dividing a long cut path into short segments.
  • these segments are preferably from about 10 ⁇ m to 1 mm, more preferably from about 100 ⁇ m to 1000 ⁇ m, and most preferably from about 200 ⁇ m to 500 ⁇ m.
  • the laser beam is scanned within a first short segment for a predetermined number of passes before being moved to and scanned within a second short for a predetermined number of passes.
  • the bite size, segment size, and segment overlap can be manipulated to minimize the amount and type of trench backfill.
  • FIG. 9 depicts a simplified representation of an exemplary segmented cutting profile 112a.
  • cutting profile 112a is shown, for convenience, having an path cutting direction from left to right and having distinct cutting segments 116k ⁇ -116r- (generally, cutting segments 116) formed from right to left.
  • cutting segments 116k ⁇ -116r 3 are depicted as parallel in FIG. 9 for convenience, cutting segments 116k ⁇ -116r 3 are actually collinear.
  • FIG. 9 depicts multiple segment sets 114a, each comprising an initial segment 116k and multiple gradually lengthening overlapping segments 116m-116r, preferably processed in alphabetical order.
  • each set 114a is processed to a selected intermediate depth or to a complete through cut before the next set is processed.
  • overlapping segments are shown for each set 114a, skilled persons will appreciate that a substantially greater number of overlapping segments 116 could be employed, particularly with smaller incremental length increases as needed to accommodate the thickness of the target material. Skilled persons will also appreciate that any or all of the segments 116 employed in cutting profile 112a could be processed in both directions instead of a single direction as shown in FIG. 9.
  • FIG. 10 depicts a simplified representation of an exemplary segmented cutting profile 112b that is somewhat similar to profile 112a.
  • profile 112b begins with the same segment set 114a that begins profile 112a.
  • segment sets 114b omit segments 116k and increasingly overlap the previously processed segment sets as much as 60% .
  • segment 116k ⁇ is cut with 30 passes and has a length of 200 ⁇ m.
  • segment 116m ⁇ is cut with 6 passes (1/5 of 30 passes) and has a length of 240 ⁇ m (200 ⁇ m plus 1/5 of the length of segment 116k ⁇ ).
  • segment 116m is cut with 6 passes and has a length of 280 ⁇ m (200 ⁇ m plus 2/5 of the length of segment 116k ⁇ ). This sequence is continued until segment set 114b is completed.
  • This example can exhibit dicing speeds of greater than or equal to 8.5 mm/minute.
  • Real-time monitoring can also be employed to reduce rescanning portions of the cut path where the cut is already completed.
  • polarization direction of the laser beam can be correlated with the cutting direction to further enhance throughput.
  • the MEMS device 120 is patterned using the method described above to create trenches 122a, 122b, 122c, 122d, and 122e (generically trenches 122) in silicon.
  • the directed laser system output pulses 32 can be directed to the work surface such that overlapped pulses create a pattern which expresses any complex curvilinear geometry.
  • This capability combined with the ultraviolet ablative patterning method may be used to produce complex curvilinear geometric patterns in silicon useful for efficient production of a variety of MEMS devices 120.
  • FIG. 12 is a representative illustration of ultraviolet ablative patterning of an AWG device 130.
  • the AWG 130 is patterned using the method described above to create curvilinear trenches 132, with portions 132a, 132b, 132c, 132d, and 132e in silicon, for example.
  • trench 132 is shown to be symmetric, skilled persons will appreciate that through computer control of the X and/or Y axes of the laser positioning system 30, the directed laser system output pulses 32 can be directed to the work surface such that overlapped pulses 32 create a pattern which expresses any complex curvilinear profile or geometry.
  • This capability combined with the ultraviolet ablative patterning method may be used to produce complex curvilinear geometric patterns in silicon useful for efficient production of a variety of AWG devices 120.
  • FIG. 13 is a representative illustration of a chuck assembly 140 on which silicon workpieces 12 are preferably placed for through processing using the ultraviolet ablative patterning method.
  • Chuck assembly 140 preferably includes a vacuum chuck base 142, a chuck top 144, and an optional retaming carrier 146.
  • Base 142 is preferably made from traditional metal material and is preferably bolted to an additional plate 148.
  • Plate 148 is adapted to be easily connected to and disengaged from at least one of the stages 36 or 38.
  • the engagement mechanism is preferably mechanical and may include opposing grooves and ridges and may include a locking mechanism. Skilled person will appreciate that numerous exact alignment and lock and key mechanisms are possible. Skilled persons will also appreciate that the base 142 may alternatively be adapted to be secured directly to the stages 36 or 38.
  • Chuck top 144 is preferably fabricated from a dielectric material, which has low reflectivity at the ultraviolet wavelength selected for the particular patterning application.
  • UV-transparent chuck top 144 is fabricated from ultraviolet-grade or excimer grade fused silica, MgF 2 , or CaF 2 .
  • UN-transparent chuck top 144 may alternatively or additionally be liquid- cooled to assist in mamtainmg the temperature stability of the silicon workpieces 12.
  • fused silica is an ultraviolet transparent material composed of amorphous silicon dioxide and is formed by chemical combination of silicon and oxygen.
  • a retaining carrier 146 may be placed over chuck top 144 for the purpose of supporting a silicon workpiece 12 and retaining it after ultraviolet ablative patterning.
  • Retaining carrier 146 is also preferably fabricated from an ultraviolet transparent material in order to prevent backside reflections from damaging workpieces 12 that are through processed.
  • Retaining carrier 146 is preferably machined to contain shallow cavities into which the processed silicon workpieces 12 settle after through processing operations.
  • chuck top 144 or retaining carrier 146 may be fabricated from an ultraviolet absorbing material, such as Al or Cu, in order that laser system 10 may use a tool path file of the pattern of shallow cavities to be drilled into the workpiece 12 to cut the corresponding pattern into the material of chuck top 144 or retaining carrier 146.
  • the cavities may, for example, correspond to intended drill holes or edge patterns and prevent backside damage to the workpiece 12 during through cut operations.
  • any debris from the process may settle into the cavities away from the backside of workpiece 12.
  • the pattern of the shallow cavities is processed to have dimensions slightly larger than those of the corresponding workpieces 12 after processing, thereby enabling processed workpieces 12 to settle into the cavities of the retaining carrier 146.
  • retaining carrier 146 is fabricated from an ultraviolet transparent material by alternative means, such as optical fabrication or etching, and is subsequently aligned and affixed on the chuck top 144.
  • alternative means such as optical fabrication or etching
  • chuck assembly 140 also have useful applications in UN via drilling in other materials such as poly amide.
  • purge gases such as nitrogen, argon, helium, and dry ah, may be usefully employed to assist in the removal of waste fumes from the workpiece 12.
  • purge gases can be delivered to the close vicinity of the work surface using delivery nozzles attached to laser system 10.
  • processed workpieces 12 may be cleaned using ultrasonic baths in liquids including but not limited to water, acetone, methanol, and ethanol. Those skilled in the art will recognize that cleaning of processed silicon workpieces 12 in hydrofluoric acid can be beneficial in removing unwanted oxide layers.
  • the description provided above has been largely directed toward processing silicon and GaAs, the methods described are also generally applicable to other semiconductors that may be used as the substrate 70 for workpieces 12, such as SiC, Si ⁇ , or indium phosphide.

Abstract

L'invention concerne un procédé permettant de former rapidement des motifs présentant des largeurs de lignes inférieures à 50 microns directement dans des semi-conducteurs, en particulier dans du silicium, par une ablation au laser ultraviolet. Ces motifs comprennent des ouvertures traversantes cylindriques présentant un facteur de forme très élevé, destinées aux connexions de circuits intégrés. L'invention concerne également un procédé de séparation des puces formées dans les plaquettes de semiconducteur ainsi qu'un procédé de découpage « microtab » permettant de séparer les pièces de microcircuit d'une plaquette mère de semi-conducteur. On dirige les impulsions (32) de sortie d'un laser Nd :Yag, Nd :YVO4 ou Nd :YLF de type déclenché, pompé par diodes, à fréquence triplée, sur la pièce (12) avec une précision grande vitesse, l'aide d'un positionneur de faisceau composite. Le système optique produit un point gaussien ou un profil de type « top hat » d'une taille d'au moins 10 microns. L'énergie d'impulsion utilisée pour le traitement ablatif haute vitesse du silicium avec un point focalisé de cette taille est supérieure à 200 νJ par impulsion pour une fréquence de récurrence (PRF) supérieure à 5kHz et de préférence supérieure à 15 kHz. La durée d'impulsion laser mesurée aux points situés à mi-distance de la largeur maximale est de préférence inférieure à 80 ns.
EP02707453A 2001-01-31 2002-01-10 Realisation des motifs de microstructures de semiconducteurspar ablation au laser ultraviolet Withdrawn EP1365880A4 (fr)

Applications Claiming Priority (5)

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US26555601P 2001-01-31 2001-01-31
US265556P 2001-01-31
US803382 2001-03-09
US09/803,382 US20020033558A1 (en) 2000-09-20 2001-03-09 UV laser cutting or shape modification of brittle, high melting temperature target materials such as ceramics or glasses
PCT/US2002/000867 WO2002060636A1 (fr) 2001-01-31 2002-01-10 Realisation des motifs de microstructures de semiconducteurspar ablation au laser ultraviolet

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EP1365880A4 EP1365880A4 (fr) 2008-04-16

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GB0317853D0 (en) 2003-09-03
CA2436736A1 (fr) 2002-08-08
EP1365880A4 (fr) 2008-04-16
CN1301178C (zh) 2007-02-21
CN1527754A (zh) 2004-09-08
GB2389811B (en) 2004-10-27
JP2004526575A (ja) 2004-09-02
WO2002060636A1 (fr) 2002-08-08
GB2389811A (en) 2003-12-24

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