WO1999040591A1

WO1999040591A1 - Passive resistive component surface ablation trimming technique using q-switched, solid-state ultraviolet wavelength laser

Info

Publication number: WO1999040591A1
Application number: PCT/US1998/017891
Authority: WO
Inventors: Yunlong Sun; Edward J. Swenson
Original assignee: Electro Scientific Industries, Inc.
Priority date: 1998-02-06
Filing date: 1998-08-28
Publication date: 1999-08-12

Abstract

A method of laser trimming a passive resistive component changes its resistance value while leaving substantially unchanged coverage of the substrate area the component occupies to maintain its power dissipation capacity or preserve its high frequency response characteristics. In a preferred embodiment, the output of a 266 nm or 355 nm, Q-switched YAG laser or a 349 nm, Q-switched YLF laser is controlled so that no single output pulse completely removes thick-film resistor material depthwise at any given location. The laser output is moved in a line scan or raster scan fashion across an area of the resistor material to ablate a portion of its surface area to a depth that is less than the height of the resistor material. Varying the laser fluence, scan speed, bite size, repetition rate, and pitch makes possible the control of the amount of material, and particularly the depth of material, removed by the laser. The laser output is scanned across the exposed surface of the resistor until its resistance value reaches a desired value. Partly removing the resistor material allows resistance tuning with no damage to the substrate. Leaving substantially unchanged the substrate area that the resistor covers maintains its power dissipation capacity and preserves its frequency response characteristics.

Description

PASSIVE RESISTIVE COMPONENT SURFACE

ABLATION TRIMMING TECHNIQUE USING Q-SWITCHED, SOLID-STATE ULTRAVIOLET WAVELENGTH LASER

Technical Field

The present invention relates to methods of laser trimming passive electrical components and, in particular, to a surface ablation method carried out with a Q-switched, solid-state ultraviolet (UV) wavelength laser to adjust the resistance value of a passive resistive component while mamtaining its power dissipation capacity or preserving its high frequency response characteristics. Background of the Invention

Conventional laser systems are typically employed for processing targets such as electrically resistive or conductive films of passive electrical component structures, such as film resistors, inductors, or capacitors, in circuits formed on ceramic or other substrates. The use of laser processing to trim the resistance values of film resistors is the subject of this application and such laser processing may include passive, functional, or activated laser trimming techniques. Passive, functional, and activated laser trimming are described in detail in U.S. Patent No. 5,685,995, which is assigned to the assignee of this application. Fig. 1 A is an isometric view of a prior art thick-film resistor 10 forming part of a hybrid integrated circuit device, and Fig. IB is a cross-sectional side elevation view depicting thick-film resistor 10 receiving a conventional laser output pulse 12. With reference to Figs. 1A and IB (collectively Fig. 1), a conventional thick-film resistor 10 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. Modern ruthinium-based thick film pastes have been optimized to be stable after laser trimming with a 1.047 μm Nd.YLF laser or a 1.064 μm Nd : YAG laser .

With particular reference to Fig. 1A, the resistance value of resistor 10 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 than nomimal values and trimmed up to the desired values. Multiple resistors 10 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.

With particular reference to Fig. IB, one or more laser pulses 12 remove substantially all of the resistor material within the spot dimensions 28 of laser output pulses 12. A simple or complex pattern can be trimmed through the resistor material of a resistor 10 to fine tune its resistance value.

Fig. 2 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. In an L-cut 32, a first 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. Then an adjoining second strip 38, perpendicular to the first 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. In a serpentine cut 34, resistor material is removed along strips 40 to increase the length of path 42. Strips 40 are added until a desired resistance value is reached. Strips 36, 38, and 40 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, the substrate 18 underlying the resistor material is completely exposed when the trimming operation is completed.

As film resistors become smaller, smaller spot sizes are needed. With the 1.047 μm and 1.064 μm laser wavelengths, obtaining smaller spot sizes while employing conventional optics and maintaining the standard working distance ^" (needed to avoid ablation debris and to clear the probes) and adequate depth of field (ceramic, for example, is not flat) is an ever-increasing challenge. The desire for even more precise resistance values also drives the quest for tighter trim tolerances. The ability of a resistor to accommodate increasing power demand is also becoming a more significant design constraint. To achieve maximum power capacity, it is advantageous to maintain the surface area of a resistor. L-cuts 32 and serpentine cuts 34 reduce the surface area of a resistor and its area of contact with the substrate and thus reduce the ability of the resistor to dissipate heat to the air above or to the substrate below. For this reason, many thick-film power resistors are trimmed using abrasive trimmers to remove only the top surface to adjust the resistance value. The abrasive trimming technique is neither clean nor fast.

Similarly, the geometry of a resistor largely determines its response to high frequencies. Conventional L-cut, serpentine cut, and other cuts change the surface geometry of a resistor and modify its high frequency response characteristics.

Although stray capacitance and inductance are not significant problems at low frequencies, the industry keeps steadily increasing the operating frequencies, so there is an increasing desire to avoid changing the geometries of resistors when modifying them to meet specific tolerances. Summary of the Invention

An object of the present invention is, therefore, to provide a method of laser trimming a passive resistive component to change its resistance value while leaving substantially unchanged coverage of the substrate area the component occupies to maintain its power dissipation capacity or preserve its high frequency response characteristics.

In a preferred embodiment, the output of a 266 nm or 355 nm, Q-switched YAG laser or a 349 nm, Q-switched YLF laser is controlled so that no single output pulse completely removes thick-film resistor material depthwise at any given location. The laser output is moved in a line scan or raster scan fashion across an area of the resistor material to ablate a portion of its surface area to a depth that is less than the height of the resistor material. Unlike 1.047 μm or 1.064 μm laser trimming, which is a heat-melt-evaporation process that creates a large heat-affected area and consequently reduces trimming accuracy, UV laser output trimming transmits better the UV energy through the trimming-generated plasma plume over the resistor surface into metal-based resistive materials and couples better the UV energy to create a much smaller heat- affected zone. The result is a much sharper cut off at the transition between the removed and remaining volumes of resistor material in the lateral and depthwise directions after surface ablation of a resistor. Varying the laser fluence, scan speed, bite size, repetition rate, and pitch makes possible the control of the amount of material, and particularly the depth of material, removed by the laser, and thus increases the resistance of the resistor to a desired value. Partly removing the resistor material allows resistance tuning with no damage to the substrate. Leaving substantially unchanged the substrate area the resistor covers maintains its power dissipation capacity and preserves its frequency response characteristics.

An advantage of using a UV laser wavelength such as 355 nm allows the spot size of the laser output to be almost three times smaller (e.g., 7 μm for thick film targets) than a conventional 1.064 μm laser output spot for both conventional pattern trimming and the new surface trimming technique. This facilitates achieving more precise resistor values and enables trimming of smaller devices.

Additional objects and advantages of the invention will be apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings. Brief Description of the Drawings

Fig. 1A is a fragmentary isometric view of a thick-film resistor. Fig. IB is a cross-sectional side view of a thick-film resistor receiving laser output that removes the full thickness of resistor material. Fig. 2 is a fragmentary isometric view of a resistor showing two common prior art trim paths.

Fig. 3 is a partly schematic, simplified diagram of a laser system adapted for performing the surface ablation technique of the present invention.

Fig. 4 A is an isometric view of a thick-film resistor with a trim profile in accordance with one embodiment of the present invention.

Fig. 4B is a cross-sectional side view of the thick-film resistor shown in Fig. 4A. Fig. 5 is an isometric view of a thick-film resistor with a trim profile in accordance with a first alternative embodiment of the present invention.

Fig. 6 is an isometric view of a thick-film resistor with a trim profile in accordance with a second alternative embodiment of the present invention. Figs. 7 A and 7B are isometric views of resistor material in which multiple recesses and multiple thin grooves, respectively, are inscribed to trim the resistance value of the resistor material.

Detailed Description of Preferred Embodiments

Fig. 3 shows an embodiment of a simplified laser system 50 for generating preferred laser pulses that satisfy the energy distributions desirable for achieving surface ablation in accordance with the present invention. For convenience, laser system 50 is modeled herein only by way of example as a diode-pumped third harmonic Nd:YAG laser (355 nm). Laser system 50 includes a resonator 52 having a laser module 54 positioned between a highly reflective mirror 56 and an output coupler 58 along an optic axis 60. Laser module 54 is preferably a X-30 laser head module manufactured by Spectra Physics Lasers of Mountain View, California, that has been modified to have its resonator mirrors removed. Laser module 54 contains an Nd:YAG lasant rod that is pumped by two 20 W FCbαr™ diode lasers 62 supported by a T-40 laser power supply 64, all manufactured by Spectra Physics Lasers. Mirror 56 is preferably 100 percent reflective, and coupler 58 is about 99.98% reflective at 1064 nm, about 99% reflective at 532 nm, and about 90% transmissive at 355 nm to light propagating along optic axis 60. A pair of barium borate frequency converting crystals 66 and 68 in temperature controlled mounts (T=40°C) are positioned along optic axis 60 between laser module 54 and coupler 58. Crystals 66 and 68 cooperate to convert the 1064 nm output of the Nd:YAG rod to the third harmonic at 355 nm.

Other preferred lasers include a third harmonic Nd:YLF (349 nm) and a fourth harmonic Nd:YAG (266 nm). Skilled persons will appreciate that other suitable solid-state lasers emitting at wavelengths below 400 nm are commercially available and could be employed. Skilled persons will also appreciate that the UV wavelengths can be employed to produce practical ablation spot sizes that are significantly less than about 20 to 30 μm for thick film targets. These parameters are affected by target accessibility constraints such as the position of probes and surrounding electronic circuitry. A laser system of the Models 44, 4000, 4200, 4300, and 4410, manufactured by Electro Scientific Industries, Inc. in Portland, Oregon, is preferred and could be adapted by skilled persons to emit light energy at any of these wavelengths. Laser system output 80 can be manipulated by a variety of conventional optical elements including beam expander lens components 82 and 84 that are positioned along a beam path 86. Finally, laser system output 80 is passed through a focusing or imaging lens 88 before being applied to a laser target position 90 on a resistor 92 (Figs. 4 A and 4B). A beam positioning system 94 preferably operates in association with a laser controller 96 that controls the motion of an X-Y positioner to target and focus laser system output 80 to a desired laser target position 90 on resistor material 112 (Figs. 4 A and 4B). Beam positioning system 94 permits quick movement of output 80 along target positions 90 on the same or different resistors to effect unique trimming operations based on predetermining calculations or provided test data. A preferred beam positioning system 94 is a galvanometer-based beam positioner with 4" x 4" (100 mm x 100 mm) field coverage. A positioning command data base associated with controller 96 stores commands that direct the path of laser system output 80 so that it strikes the desired target positions 90. For intracavity laser beam modulation employing a quartz Q-switch 104 as shown in Fig. 3, laser controller 96 may process timing data that synchronizes the firing of laser system 50 to positioning stage motion as described in U.S. Patent No. 5,453,594 of Konecny for Radiation Beam Position and Emission Coordination System. Laser controller 96 determines the amplitude of preselected or variable RF oscillator signals delivered to an acoustic wave transducer of Q-switch 104. Laser controller 96 may be used to deliver laser output pulses having about 10 ns to 100 ns duration at about 1 to 5 kHz, preferably 30 ns at 5 kHz.

Figs. 4A and 4B (collectively Fig. 4) are respective isometric and side elevation views of a thick-film resistor 92 after it has been trimmed to a desired resistance value in accordance with the present invention. With reference to Fig. 4, resistor 92 includes a resistor material 112 layered upon a ceramic or glass substrate 114a and contacting electrodes 116. Resistor material 112 has a geometry that includes a length 122, widths 124a and 124b, and heights 126a and 126b that determine the volume and cross-sectional areas of resistor material 112. The ^" geometry of resistor material 112 determines its resistance value and greatly influences the power dissipation capacity of resistor 92 and its frequency response. Length 122, width 124b, and depth 126c dimensions define a trim profile such as surface ablation area 130 that can be removed by raster or line scanning multiple pulses of laser output 80 across resistor material 112. By varying the laser fluence, scan speed, bite size, repetition rate and pitch, it is possible to control the amount of resistor material 112, and particularly the depth 126c of resistor material 126c, removed by laser output 80 to ensure that no single spot receives a number of pulses that would remove the entire height 126a of resistor material 112 to expose substrate 114a. Skilled persons will appreciate that it may be necessary to scan laser output 80 beyond edges 132 to accomplish surface ablation area 130. Skilled persons will also appreciate that the dimensions of surface ablation area 130 are determined on-the-fly by the incremental nibbling of laser output pulses 80, such that each laser output pulse 80 removes a minute amount of the surface of resistor material 112 to slightly increase the resistance value of resistor 92. Laser output pulses 80 are applied until resistor 110 meets a predetermined resistance value.

Fig. 5 shows a resistor 140 after it has been trimmed with a first alternative trim profile such as a surface ablation area 142. For convenience, certain features of resistor 140 that correspond to features of resistor 92 in Fig. 4 have been designated with the same reference numerals. With reference to Fig. 5, surface ablation area 142 is also removed by the surface ablation technique, described in connection with Fig. 4, so that the entire height 126a of resistor material 112 within surface ablation area 142 is not removed. Surface ablation area 142 does not extend completely between edges 132 so laser output 80 can completely avoid exposure of substrate 114b.

Fig. 6 shows a resistor 150 after it has been trimmed with a second alternative trim profile such as a surface ablation area 152. Surface ablation area 152 is also removed by the surface ablation technique previously described. For convenience, certain features of resistor 150 that correspond to features 92 of Fig. 4 have been designated with the same reference numerals. With reference to Fig. 6, surface ablation area 152 extends over, but does not remove the full height 126a of resistor material above, electrode 116 so that it is free from exposure to laser ^"output 80.

Figs. 7A and 7B show isometric views of resistor material 112 in which two exemplary discontinuous trim profiles embodying the present invention are inscribed to trim the resistance value of resistor material 112. With reference to

Fig. 7A, a resistor 160 has an array of spaced-apart circular recesses 162 that do not extend depthwise into the full height of resistor material 112. Recesses 162 represent punch marks each of which is about the ablation spot size of the UV laser and which together form a composite surface ablation area 164. With reference to Fig. 7B, a resistor 170 has an array of mutually spaced apart, thin groove lines 172 extending in the direction of electrical current flow between electrodes 116. Adjacent groove lines 172 can be spaced very close to each other and together form a composite surface ablation area 174. Like recesses 162, groove lines 172 do not extend depthwise into the full height of resistor material 112. With reference to Figs. 4A, 4B, 5, 6, 7A, and 7B a laser output 80 that can be used to form surface ablation areas 132, 142, 152, 164, and 174 for high precision trimming of thick-film resistors includes individual laser output pulses at a power of about 5 mW, a repetition rate of about 1 kHz, a scan speed of about 9 mm/sec, and bite sizes of about 6 μm or 9 μm. More general laser output pulse parameters include power ranges of between about 10 mW and 50 mW, repetition rates of greater than about 300 Hz, scan speeds of between about 3 mm/sec and 300 mm/sec, and bite sizes selected in accordance with the accuracy, resolution, and throughput required. Skilled persons will appreciate that these output pulse parameters are interdependent and are dictated by the performance required. Unlike conventional laser trimming profiles, such as full height L-paths and serpentine paths, the surface ablation technique of the present invention does not reduce the amount of resistor material 92 in contact with substrate 114 (114a or 114b). Thus, the ability of resistors 92, 140, 150, 160, or 170 to dissipate heat to substrate 114 is not significantly affected. Similarly, the surface ablation technique of the present invention preserves the total surface areas of resistors 92, 140, 150, 160, or 170 and minimizes the creation of sources of unpredictable high frequency responses in resistors 92, 140, 150, 160, or 170. A consequence of using a UV laser is the creation of a relatively small heat-affected zone (as compared with that produced by 1.047 μm and 1.064 μm laser trimming) that results in higher accuracy trimming.

Skilled persons will appreciate that because the spot size of the UV laser output 80 is smaller than the spot size of conventional laser trimming output, and because the percentage of the depth of resistor material 112 removed by laser output 80 is also smaller, resistors 92, 140, 150, 160, and 170 portrayed in Figs. 4A, 4B, 5, 6, 7 A, and 7B can be trimmed to tolerances that are tighter than the tolerances possible for conventional full-height kerf trimming techniques.

It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments 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.

Claims

1. A surface ablation method of adjusting the resistance value of a film resistor while maintaining its power dissipation capacity or preserving its high frequency response characteristics, the film resistor formed on a substrate in accordance with electronic component film process technology, and the film resistor occupying an area of the substrate and having an exposed surface and a height, comprising: forming a pulsed laser output by optically pumping a Q-switched ultraviolet light-emitting solid-state laser, the laser output having a power density that is sufficient to ablate a portion of the exposed surface of the film resistor; and directing the laser output toward and scanning it across the exposed surface of the film resistor to selectively remove a depthwise portion of the exposed surface to a depth that is less than the height of the film resistor, the selective removal of the depthwise portion changing the resistance value of the resistor until it reaches a desired value but leaving substantially unchanged coverage of the substrate area the film resistor occupies to maintain its power dissipation capacity or preserve its high frequency response characteristics.

2. The method of claim 1 in which the laser output comprises a wavelength of 266 nm, 355 nm, or 349 nm.

3. The method of claim 1 in which the film resistor formed on the substrate has a resistance value that is less than the desired value, and further comprising scanning the laser output across the exposed surface of the resistor until its resistance value increases to the desired value.

4. The method of claim 3 in which the film process technology forms a thick-film resistor on a substrate made of ceramic or glass.

5. The method of claim 1 in which the film resistor forms part of a hybrid integrated circuit device.

6. The method of claim 1 in which the resistance value of the film resistor is adjusted in accordance with functional laser trimming techniques.

7. The method of claim 1 in which the pulsed laser output has on the exposed surface a spot size dimension of less than about 10 ╬╝m.

8. The method of claim 1 in which the optical pumping is implemented by operation of a laser diode.

9. The method of claim 1 in which the exposed surface has an area and the depthwise portion of target material removed includes an area that is less than or equal to the area of the exposed surface.

10. The method of claim 1 in which the depthwise portion of target material removed includes multiple recesses.

11. The method of claim 1 in which the depthwise portion of target material removed includes multiple spaced-apart grooves.

12. The method of claim 1 in which the pulsed laser output is generated at a repetition rate that is greater than about 300 Hz.

13. The method of claim 12 in which the laser output comprises a wavelength of 266 nm, 355 nm, or 349 nm.

14. The method of claim 12 in which the passive electrical component comprises a resistor and forms part of a hybrid integrated circuit device.

15. The method of claim 12 in which the depthwise portion of target material removed includes multiple recesses.

16. The method of claim 12 in which the depthwise portion of target material removed includes multiple spaced-apart grooves.

17. The method of claim 12 in which the film resistor formed on the substrate has a resistance value that is less than the desired value, and further comprising scanning the laser output across the exposed surface of the resistor until its resistance value increases to the desired value.

18. The method of claim 17 in which the film process technology forms a thick-film resistor on a substrate made of ceramic or glass.

19. The method of claim 12 in which the optical pumping is implemented by operation of a laser diode.

20. The method of claim 12 in which the exposed surface has an area and the depthwise portion of target material removed includes an area that is less than or equal to the area of the exposed surface.