Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01C—RESISTORS
  • H01C17/00—Apparatus or processes specially adapted for manufacturing resistors
  • H01C17/22—Apparatus or processes specially adapted for manufacturing resistors adapted for trimming
  • H01C17/24—Apparatus or processes specially adapted for manufacturing resistors adapted for trimming by removing or adding resistive material
  • H01C17/242—Apparatus or processes specially adapted for manufacturing resistors adapted for trimming by removing or adding resistive material by laser
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K3/00—Apparatus or processes for manufacturing printed circuits
  • H05K3/02—Apparatus or processes for manufacturing printed circuits in which the conductive material is applied to the surface of the insulating support and is thereafter removed from such areas of the surface which are not intended for current conducting or shielding
  • H05K3/08—Apparatus or processes for manufacturing printed circuits in which the conductive material is applied to the surface of the insulating support and is thereafter removed from such areas of the surface which are not intended for current conducting or shielding the conductive material being removed by electric discharge, e.g. by spark erosion
• • 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
  • B23K26/073—Shaping the laser spot

Definitions

• the present invention relates to laser trimming and, in particular, to laser trimming thick or thin film resistors with a uniform spot from a solid-state laser.
• FIG. 1 is an isometric view, of a work piece 10, such as a prior art thick-film resistor 10a, forming part of a hybrid integrated circuit device
• FIG. 2 is a cross- sectional side elevation view depicting thick-film,resistor 10a receiving a conventional laser output pulse 12.
• a conventional thick-film resistor 10a typically comprises a thick film layer 14 of a ruthanate or ruthinium oxide material extending between and deposited on portions of the top surfaces of metallic contacts 16. Layer 14 and metallic contacts 16 are supported upon a ceramic substrate 18, such as alumina.
• resistor 10a is largely a function of the resistivity of the resistor material and its geometry, including length 22, width 24, and height 26. Because they are difficult to screen to precise tolerances, thick- film resistors are intentionally screened to lower resistance than nomimal values and trimmed up to the desired values. Multiple resistors 10a having approximately the same resistance values are manufactured in relatively large batches and then subjected to trimming operations to remove incremental amounts of the resistor material until the resistance is increased to a desired value.
• one or more laser pulses 12 remove substantially the full height 26 of the resistor material within the spot dimensions 28 of laser output pulses 12, and overlapping spot dimensions 28 form a kerf 30.
• a simple or complex pattern can be trimmed through the resistor material of a resistor 10a to fine tune its resistance value.
• Laser pulses 12 are typically applied until resistor 10a meets a predetermined resistance value.
• FIG. 3 is an isometric view of a portion of a prior art resistor 10 showing for convenience two common pattern trim paths 32 and 34 (separated by a broken line) between metal contacts 16.
• "L-cut" path 32 depicts a typical laser-induced modification.
• a first removal strip 36 of resistor material is removed in a direction perpendicular to a line between the contacts to make a coarse adjustment to the resistance value.
• an adjoining second removal strip 38, perpendicular to the first removal strip 36 may be removed to make a finer adjustment to the resistance value.
• a "serpentine cut” path 34 depicts another common type or laser adjustment.
• resistor material is removed along removal strips 40 to increase the length of film path 42. Removal strips 40 are added until a desired resistance value is reached. Removal strips 36, 38, and 40 are typically the width of a single kerf 30 and represent the cumulative "nibbling" of a train of overlapping laser pulses 12 that remove nearly all of the resistor material within the prescribed patterns. Thus, when the trimming operation is completed, the kerfs 30 are "clean" with their bottoms being substantially free of resistor material such that the substrate 18 is completely exposed. Unfortunately, the formation of conventional clean kerfs 30 necessitates a slight laser impingement of the surface of substrate 18.
• U.S. Pat. Nos. 5,569,398, 5,685,995, and 5,808,272 of Sun and Swenson describe the use of nonconventional laser wavelengths, such as 1.3 ⁇ m, to trim films or devices to avoid damage to the silicon substrate and/ or reduce settling time during functional trimming.
• Microcracking is another challenge associated with using a solid-state Gaussian laser beam for trimming resistors. Microcracks, which often occur in the center of a kerf 30 on the substrate, may extend into the resistor film causing potential drift problems. Microcracks can also cause a shift associated with the temperature coefficient of resistance (TCR). Such microcracking is more pronounced in the newer 0402 and 0201 chip resistors that are fabricated on thinner substrates 18, with a typical height or thickness of about 100 to 200 ⁇ m, compared to those of traditional resistors. Microcracking in these thinner- substrate resistors can propagate and even result in catastrophic failure or physical breakage, particularly along the trim kerf 30, of the resistor during subsequent handling. Microcracking can also create "preferred" break lines that are more pronounced than the desirable break prescribed break lines in snapstrates. [0016] Improved resistor trimming techniques are, therefore, desirable.
• An object of the mvention is, therefore, to provide an improved system and/or method for solid-state laser trimming.
• Another object of the invention is to provide spot sizes of less than 20 ⁇ m to trim smaller chip resistors, such as 0402 and 0201 chips resistors.
• Some of the microcracking may be caused by the high intensity center of the Gaussian beam spot in much the same way that a Gaussian beam may be responsible for damaging the center of a blind via in a laser drilling operation (although the targets and substrates are different materials).
• the present invention preferably employs a uniform spot, such as an imaged shaped Gaussian spot or a clipped Gaussian spot, that is less than 20 ⁇ m in diameter and imparts uniform energy across the bottom of a kerf 30, thereby minimizing the amount and severity of microcracking.
• a uniform spot such as an imaged shaped Gaussian spot or a clipped Gaussian spot
• FIG. 1 is a fragmentary isometric view of a thick-film resistor.
• FIG. 2 is a cross-sectional side view of a thick-film resistor receiving laser output that removes the full thickness of resistor material.
• FIG. 3 is a fragmentary isometric view of a resistor showing two common prior art trim paths.
• FIG. 4 is an isometric view of a thick-film resistor with a surface ablation trim profile.
• FIG. 5 is a simplified side elevation and partly schematic view of an embodiment of a laser system employed for trimming films in accordance with the present invention.
• FIGS. 6A-6C is a sequence of simplified irradiance profiles of a laser beam as it changes through various system components of the laser system of FIG. 5.
• FIGS. 7A-7D are exemplary substantially uniform square or circular irradiance profiles.
• FIG. 8 is a graphical comparison of ideal fluence distributions at the aperture plane for imaged shaped output and clipped Gaussian output at several typical transmission levels under exemplary laser processing parameters.
• FIG. 9 is a graph of via taper ratio as a function of work surface location relative to the nominal image plane.
• FIG. 10 is a graph of via diameter as a function of work surface location relative to the nominal image plane.
• FIG. 11 is an electron micrograph of kerf showing microcracks formed in the substrate of a resistor trimmed by a Guassian beam.
• FIG. 12 is an electron micrograph of a kerf showing the absence of significant microcracks formed in the substrate of a resistor trimmed by a uniform spot.
• a preferred embodiment of a laser system 50 of the present invention includes Q-switched, diode-pumped (DP), solid-state (SS) UV laser 52 that preferably includes a solid-state lasant such as Nd:YAG, Nd:YLF, or Nd:YVO 4 .
• Laser 52 preferably provides harmonically generated UV laser pulses or output 54 at a wavelength such as 355 nm (frequency tripled Nd:YAG), 266 nm (frequency quadrupled Nd:YAG), or 213 nm (frequency quintupled Nd:YAG) with primarily a TEMoo spatial mode profile.
• YLF wavelengths include 349 nm and 262 nm.
• preferred YLF wavelengths include 349 nm and 262 nm.
• most lasers 52 do not emit perfect Gaussian output 54; however, for convenience, Gaussian is used herein liberally to describe the irradiance profile of laser output 54.
• Laser cavity arrangements, harmonic generation, and Q-switch operation are all well known to persons skilled in the art. Details of exemplary lasers 52 are described in International Publication No. WO 99/40591 of Sun and Swenson.
• a UV laser wavelength is preferred for trimming because it has an ablative, relatively nonthermal nature that reduces post trim drift.
• a UV laser wavelength also inherently provides a smaller spot size at the surface of workpiece 10 than provided by an IR or green laser wavelength employing the same depth of field.
• UV laser pulses 54 may be passed through a variety of well-known optics including beam expander and/or upcollimator lens components 56 and 58 that are positioned along-' " beam path 64. UV laser pulses 54 are then preferably directed through a shaping and/ or imaging system 70 to produce uniform pulses or output 72 that is then preferably directed by a beam positioning system 74 to target uniform output 72 through a scan lens 80 (The scan lens is also commonly referred to as a "second imaging,” focusing, cutting, or objective lens.) to a desired laser target position 82 at the image plane on a workpiece 10, such as thick film resistors 10a or thin film resistors. Uniform output 72 preferably comprises laser output that has been truncated (clipped), focused and clipped, shaped, or shaped and clipped.
• Imaging system 70 preferably employs an aperture mask 98 positioned between an optical element 90 and a collection or collimation lens 112 and at or near the focus of the beam waist created by optical element 90.
• Aperture mask 98 preferably blocks any undesirable side lobes in the beam to present a spot profile of a circular or other shape that is subsequently imaged onto the work surface.
• 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 used advantageously for resistor trimming.
• Mask 98 may comprise a material suitable for use at the wavelength of laser output 54. If laser output 54 is UV, then mask 98 may for example 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. The aperture of mask 98 may optionally be flared outwardly at its light exiting side.
• Optical element 90 may comprise focusing optics or beam shaping components such as aspheric optics, refractive binary optics, deflective binary optics, or diffractive optics. Some or all of these may be employed with or without the aperture mask 98.
• a beam shaping component comprises a diffractive optic element (DOE) that can perform complex beam shaping with high efficiency and accuracy .
• DOE diffractive optic element
• the beam shaping component not only transforms the Gaussian irradiance profile of FIG. 6A to the near-uniform irradiance profile of FIG. 6Bb, but it also focuses the shaped output 94 to a determinable or specified spot size.
• Both the shaped irradiance profile 94b and the prescribed spot size are designed to occur at a design distance Zo down stream of optical element 90.
• 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.
• FIGS. 6A-6C show a sequence of simplified irradiance profiles 92, 96, and 102 of a laser beam as it changes through various system components of laser system 50.
• FIGS. 6Ba-6Bc show simplified irradiance profiles 96a-96c of shaped output 94 (94a, 94b, and 94c, respectively) as a function of distance Z with respect to Zo' .
• Zo' is the distance where shaped output 94 has its flattest irradiance profile shown in irradiance profile 96b. In a preferred embodiment, Zo' is close to or equal to distance Zo.
• FIGS. 6A-6C show a sequence of simplified irradiance profiles 92, 96, and 102 of a laser beam as it changes through various system components of laser system 50.
• FIGS. 6Ba-6Bc show simplified irradiance profiles 96a-96c of shaped output 94 (94a, 94b, and 94c, respectively) as a function
• a preferred embodiment of shaped imaging system 70 includes one or more beam shaping components that convert collimated pulses 60 that have a raw Gaussian irradiance profile 92 into shaped (and focused) pulses or output 94b that have a near-uniform "top hat" profile 96b, or particularly a super-Gaussian irradiance profile, in proximity to an aperture mask 98 downstream of the beam shaping component.
• FIG. 6Ba shows an exemplary irradiance profile 94a where Z ⁇ Zo'
• FIG. 6Bc shows an exemplary irradiance profile 94c where Z > Zo' .
• lens 112 comprises imaging optics useful for inhibiting diffraction rings. Skilled persons will appreciate that a single imaging lens component or multiple lens components could be employed.
• FIGS. 7A-7D show exemplary substantially uniform irradiance profiles produced by a Gaussian beam propagating through a DOE as described in U.S. Pat. No. 5,864,430.
• FIGS. 7A-7C show square irradiance profiles, and FIG. 7D shows a cylindrical irradiance profile. The irradiance profile of FIG.
• beam shaping components 90 can be designed to supply a variety of other irradiance profiles that might be useful for specific applications, and these irradiance profiles typically change as a function of their distance from Zo' .
• a cylindrical irradiance profile such as shown in FIG. 7D is preferably employed for circular apertures 98; cuboidal irradiance profiles would be preferred for square apertures; and the properties of other beam shaping components 90 could be tailored to the shapes of other apertures. For example, for many straight forward via trimming applications, an inverted cuboidal irradiance profile with a square aperture in mask 98 could be employed.
• Beam positioning system 74 preferably employs a conventional positioner used for laser trimming systems. Such a positioning system 74 typically has one or more stages that move workpiece 10. The positioning system 74 can be used for moving laser spots of shaped output 118 in an overlapping manner to form kerfs 30 along trim paths 32 or 34. Preferred beam positioning systems can be found in ESI's Model 2300, Model 4370, or soon to be released Model 2370 Laser Trimming Systems commercially available from Electro Scientific Industries, Inc. of Portland, Oregon, Other positioning systems can be substituted and are well known to practitioners in the laser art.
• An example of a preferred laser system 50 that contains many of the above- described system components employs a UV laser (355 nm or 266 nm) in a Model 5200 laser system or others in its series manufactured by Electro Scientific Industries, Inc. in Portland, Oregon. Persons skilled in the art will appreciate, however, that any other laser type having a Gaussian beam intensity profile (before imaging or shaping as disclosed herein), other wavelengths such as IR, or other beam expansion factors can be employed.
• Laser system 50 is capable of producing laser system output 114 having preferred parameters of typical resistor trimming windows that may include: an ultraviolet wavelength, preferably between about 180-400 nm; average power densities greater than about 100 mW, and preferably greater than 300 mW; spot size diameters or spatial major axes of about 5 ⁇ m to greater than about 50 ⁇ m; a repetition rate of greater than about 1 kHz, preferably greater than about 5 kHz or even higher than 50 kHz; temporal pulse widths that are shorter than about 100 ns, and preferably from about 40-90 ns or shorter; a scan speed of about 1-200 mm/sec or faster, preferably about 10-100 mm/sec, and most preferably about 10-50 mm/sec; and a bite size of about 0.1-20 ⁇ m, preferably 0.1-10 ⁇ m, and most preferably 0.1-5 ⁇ m.
• the preferred parameters of laser system output 114 are selected in an attempt to circumvent thermal or other undesired damage to substrates 18. Skilled
• spot area of laser system output 114 is preferably circular or square, but other simple shapes such as ellipses and rectangles may be useful and even complex beam shapes are possible with the proper selection of optical elements 90 cooperating with a desirable aperture shape in mask 98.
• Preferred spot areas for laser trimming, more particularly for UV laser trimming are preferably smaller than about 40 ⁇ m in diameter, more preferably smaller than about 20 ⁇ m in diameter, and most preferably smaller than about 15 ⁇ m in diameter.
• the imaged shaped output 118 consequently facilitates formation of kerfs 30 with a very flat and uniform bottom 48 at or into ceramic substrate 18, and this flatness and uniformity are not possible with an unmodified Gaussian output 54. Moreover, the imaged shaped output 118 can also clean the resistor material from the bottom edges of the kerfs 30 more completely without risking undesirable damage to the underlying substrate 18 because the uniform shape of pulse 94 virtually eliminates the possibility of creating a hot spot at the bottom center of the kerf 30, so the amount and severity of microcracks are minimized. The trimming speed can also be increased with imaged shaped output 118 over that obtainable with an unmodified Gaussian output 54.
• Imaged shaped output 118 can be applied at greater laser power than can Gaussian because "hot spot" damage potential can be eliminated so the bite size, repetition rate, and beam movement speed can be favorably adjusted to trim faster.
• a clipped Gaussian spot can alternatively be employed advantageously over Gaussian output 54, substantially more energy would have to be sacrificed to obtain desirable uniformity than with an image shaped output 118.
• the imaged shaped output 118 also provides cleaner bottom edges and faster trimming speed than does clipped Gaussian output.
• FIG. 8 shows a comparison of ideal fluence profiles at the aperture plane for shaped output 94b and clipped Gaussian output at several exemplary transmission levels under typical laser processing parameters.
• Fluence levels on the workpiece 10 are equal to the aperture fluence levels multiplied by the imaging de-magnification factor squared.
• faiences at the aperture edge were about 1.05 J/cm 2 and 0.60 J/cm 2 or less for shaped output 94b and clipped Gaussian output, respectively.
• the fluences at the edge of the imaged spot were about 7.4 and 4.3 J/cm 2 for the imaged shaped output 118 and clipped Gaussian output, respectively.
• the rate at which typical resistor materials can be ablated typically differs between the center and edge fluence levels.
• processing of each kerf 30 can be completed in fewer pulses, with faster scanning speed, or with larger bite sizes (or smaller pulse overlaps) with the imaged shaped output 118, increasing the process throughput.
• An example of a strategy for trimming with imaged shaped output 118 in accordance with these considerations of present invention is described below.
• the fluence across the entire imaged spot can be maintained, for example, at 90% of the value at which unacceptable ceramic penetration or damage occurs, Fdama e.
• acceptable ceramic penetration into thick film resistors is typically less than 10 ⁇ m, and preferably less than 5 ⁇ m.
• the resistor material is then ablated at conditions which will not cause damage such as significant microcracking.
• the spot edge could be held at 90% of Fdama e, in which case the center would be at 180% of the damage threshold fluence, resulting in substantial damage. Maintaining the edges of the imaged spot at high fluence enables the resistor material to be cleared from the kerf edges with fewer laser pulses, since each pulse removes more material.
• the trimming throughput of imaged shaped output 118 can be much greater than that of the clipped Gaussian output.
• the imaged shaped output 118 can also clean the resistor material from the bottom edges of the kerfs 30 more completely without risking damage to the underlying ceramic substrate 18 because the uniform shape of pulse 94 virtually eliminates the possibility of creating a hot spot at the bottom center of the kerf 30
• the imaged shaped output 118 of the present invention also provides for a very precise laser spot geometry and permits better taper minimizing performance at higher throughput rates than that available with Gaussian or clipped Gaussian output, thus providing crisper edges than available with Gaussian output 54.
• the nominal image plane is the location where the kerfs 30 are most taper free, with the most sharply defined top edges.
• the 3 ⁇ error bar is shown for reference because bottom width measurements may be difficult to measure reliably.
• the average kerf top width increases steadily.
• the top width remains fairly constant out to 400 ⁇ m below the image plane.
• the average value decreases steadily from locations above to locations below the nominal image plane. Because the width of the kerf bottom is significantly more difficult to control than the size of the kerf top, the bottom width is shown for reference only. Statistical process control techniques that could be applied to laser system 50 are, therefore, applicable to the characteristics of the kerf tops.
• beam shaping components 90 can be selected to produce pulses having an inverted irradiance profile shown in FIG. 7C that is clipped outside dashed lines 130 to facilitate removal of resistor material along the outer edges of kerf 30 and thereby further improve taper.
• the present invention permits a taper ratio of greater than 80% at a maximum throughput without undesirable damage to ceramic substrate 18, and taper ratios of greater than 95 % (for low aspect ratio kerfs 30) are possible without undesirable damage to ceramic substrate 18. Better than 75% taper ratios are even possible for the smallest kerf widths, from about 5-18 ⁇ m width at the kerf top, of the deepest kerfs 30, with conventional optics.
• taper ratio is typically not a critical consideration in many trimming operations other than the extent to which it impacts kerf widths on small resistors 10a, the high taper ratios achievable with the present invention are further evidence of kerf bottom uniformity.
• the trimming techniques disclosed herein can be employed for both thick and thin film resistor processing applications as described in any of the references cited in the background of the invention, including partial depth trimming.
• thick film resistors particularly ruthenium oxide on ceramic including the 0402 and 0201 chip resistors with a ruthenium layer height or thickness of less than about 200 ⁇ m
• the preferred trimming criterion is to remove all of the ruthenium within the kerfs 30 with a minimal amount of penetration into the ceramic substrate 18.
• These desirable kerfs 30 are clean such that ceramic material is uniformly exposed and the bottom of the kerfs 30 are "white.
• Such cleaning often entails intentional penetration into the ceramic to a depth of about 0.1-5 ⁇ m and often at least 1 ⁇ m.
• the imaged shaped output 118 can provide these clean or white kerfs 30 without creating significant microcracking. UV is particularly preferred for processing resistor material over ceramic; however, other wavelengths may be employed.
• an IR wavelength may be a preferred wavelength for employing a uniform spot to trim materials, such as NiCr, SiCr, or TaN, from silicon substrates, especially for trimming active or electro-optic devices and in applications involving functional tr miming.
• a uniform spot to trim materials such as NiCr, SiCr, or TaN
• Skilled persons will appreciate that the uniform spot trimming techniques disclosed herein may be employed on single resistors, resistor arrays (including those on snapstrates), voltage regulators, capacitors, inductors, or any other device requiring a trimming operation.
• the uniform spot trimming techniques can be employed for surface ablation trimming or other applications where the imaged shaped output 118 does not penetrate the substrate 18, as well as the applications where substrate penetration is desirable.
• FIGS. 11 and 12 are electron micrographs showing the differences in microcracking between a resistor 10a trimmed with a UV Gaussian beam (FIG. 11) and a resistor 10a trimmed with a UV uniform (imaged shaped) beam (FIG. 12).
• a reisistor 10a was trimmed with a UV Gaussian output 54 having an average power of 0.6 W at a repetition rate of 14.29 kHz at a trim speed of 30 mm/sec with a bite size of 2.10 ⁇ m.
• the resulting kerf 30a exhibits numerous microcracks substantial microcracks 140, a substantially wide kerf edge 150a, and deep penetration into the ceramic substrate 18 at the center of kerf 30a.
• a resistor 10a was trimmed with UV imaged shaped output 118 having an average power of 2.86 W at a repetition rate of 8 kHz at a trim speed of 32 mm/sec with a bite size of 4 ⁇ m.
• the resulting kerf 30b exhibits no undesirable damage with few if any microcracks.
• the kerf edges 150b are relatively narrow and the substrate penetration is shallow and substantially uniform.

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