1. Field of the Invention
This invention relates generally to the micro-machining of ceramic substrates.
2. Description of the Related Art
Integrated circuits designed to dissipate more than about a half watt of power are often mounted on ceramic substrates. These ceramic substrates are typically fabricated of bulk-crystalline alumina composed of 95 to 99 percent pure aluminum oxide (Al2O3), are opaque, and are typically supplied in a white, or off white, color. Other substrate materials can include aluminum nitride which is a better conductor of heat than aluminum oxide. However, aluminum nitride substrates are considerably more expensive than aluminum oxide substrates. The substrate material is supplied in thin sheets that are micro-machined to very exacting tolerances.
Conventional ceramic substrates are fabricated by drilling vias, i.e. holes, with a laser, printing and plating circuit board components on the substrate, and cutting the substrates utilizing a diamond saw into several different pieces to form individual circuit boards from the larger substrate blank material. Using a diamond saw, the processing speed is in the neighborhood of 18 inches per minute. A diamond saw cuts a kerf in the substrate having a width in excess of 25 microns.
Traditionally, carbon dioxide (CO2) lasers are used to produce machined features in alumina as small as 5 mil (0.005″). These lasers can have a power rating of from 500 to 1000 watts. CO2 lasers have the advantage of high productivity in the machining process. The laser operates by ablating the substrate material with a tightly focused and controlled beam of far-infrared (FIR) light. The laser is typically operated by on-off pulsing accompanied by manipulation of the substrate in order to create a break-line, a via, or other selected structural features.
When the laser is “on,” the substrate material is ablated, leaving a depression in the surface of the substrate, or a via extending completely through the substrate. When the laser is “off,” the substrate can be positioned for a subsequent “on” pulse, with the process being repeated in order to form the selected feature. For example, a break-line can be defined by a line of circular cavities or depressions formed in the substrate, which are created by a succession of “on” pulses of sufficient power and duration to melt and ablate the substrate material. As an example, the duration of the “on” pulse can be 50 millionths of a second, i.e. 50×10−6 second. The duration of the” off pulse can be 5 times the duration of the “on” pulse; in the example, 250 millionths of a second, i.e. 250×10−6 second.
The wavelength of the light produced by the CO2 laser is 10.6 micron (0.000417 inch). The CO2 laser has high power, but the light beam focus is too large, which creates relatively large vias and perforation lines. This beam size thus establishes a specific absolute minimum size of the features that can be machined using the CO2 laser, along with absolute limits to machined edge tolerances. For CO2 lasers in a production environment, this minimum feature size is 4-5 mil with edge tolerances of +/−1 mil. Using special techniques, features as small as 3 mil, with edge tolerances of +/−0.5 mil, are possible, but these techniques are not reliable enough to meet high-volume production requirements.
Mid-infrared lasers, such as alexandrite lasers, specialized gas lasers, and lead salt lasers typically emit light in wavelengths ranging from 2 to 5 micron. These lasers are typically inefficient, and have low-power outputs.
Near-infrared lasers, such as Yb:YAG lasers (ytterbium-doped yttrium aluminium garnet lasers) emit light in wavelengths of 1.7 to 1.8 microns. YAG lasers have wavelengths of 1.64 microns. These lasers can have very high power outputs, but typically suffer from beam structure, i.e. mode, instabilities.
The high beam quality and power of CO2 lasers have made them the principal laser used in the electronics ceramics processing industry. However, the electronics industry is now requiring smaller and more accurate feature sizes than can be produced using existing and foreseeable CO2 laser technology.
The only currently recognized solution for producing finer feature sizes is to use shorter wavelength lasers. While some shorter wavelength lasers are capable of greater precision, their throughput, or production rates, are far too low to be considered for any high-volume production requirements. Currently, the only types of laser that can produce enough power to be practical for mass-production of ceramic electronic components are those producing near-infrared (NIR) wavelengths, in the range of approximately 1.06 to 1.095 micron. Until very recently, however, use of these lasers was infeasible because of poor and unstable beam quality when operating at required power levels.
The most common example of an industrial laser that operates in the NIR range is a Nd:YAG laser (i.e. a neodymium-doped yttrium aluminium garnet laser) which emits light at a wavelength of 1.064 micron (0.000,042 inch). However, Nd:YAG lasers produce a variable and low-quality beam shape, precluding the satisfactory micro-machining of alumina at high-volume production rates.
Besides low throughput, another drawback to non-CO2 lasers is that alumina is highly reflective (greater than 90 percent) at ambient temperatures. Alumina is also translucent at shorter near-infrared wavelengths, so that the light at such wavelengths passes through the substrate and performs no manufacturing operations thereon. This greatly inhibits the coupling of the laser power to the material, and can interfere with the operation of the laser itself. Furthermore, as the temperature of alumina rises, it becomes relatively transmissive to the light beam—in excess of 90 percent—so that laser energy is not readily coupled with the material.
Conventional CO2 lasers do not exhibit such problems because alumina is a good absorber (near 100 percent) of laser energy at long, i.e. 10.6 micron, wavelengths, regardless of temperature. As a result of the limitations in NIR lasers, the electronics ceramics processing industry has been unable to realize the benefits of using lasers that emit in the near-infrared band.
Additionally, current methods present limitations that can preclude post-scribing operations on nearly finished assemblies. This leaves only three alternative methods to achieve singulation of such assemblies: the use of wet diamond-saws, the use of UV and/or green-blue lasers, or the use of ultra-fast lasers. Each of these methods is very precise and produces a cleanly machined structure, but these methods are also very slow and therefore expensive employ per unit produced. As a result, post-process singulation using saws or other low-damage methods is typically reserved for high-value components that must have an exceptionally low scrap rate to be economically viable.