Patent Application
20060086703
CO2 laser systems have been used extensively to cut ceramic substrates and other similar materials. Traditionally, a ceramic substrate has been scribed using a CO2 laser system along the perimeter of the sections of the ceramic substrate to be singulated; these sections corresponding to electronic circuits generally produced using thick film technologies. Prior art scribing processes have generally cut into the ceramic substrate about fifty per cent of the thickness of the ceramic substrate. After the ceramic substrate was scribed, the sections to be singulated were manually separated by breaking the ceramic substrate at the scribe lines. The cutting precision of these laser cutting systems was limited to electronic circuits which were large in comparison to the thickness of the ceramic substrate, as the scribe and break process had a typical tolerance of ±5 mils (0.127 mm). Also the edges of the singulated devices were rough due to the breaking process.
Low K dielectrics that are thick film screenable glass compounds are being used increasingly for thick film high frequency, low loss circuit applications. The low K dielectrics have been shown to be very brittle, and easy to crack by mechanical or thermal stress. Prior art laser systems have failed to cleanly cut dielectrics screened and fired onto ceramic substrate without generating micro-cracks, especially when the dielectric material is deposited on a metallic base material. These micro-cracks have been shown to lead to premature failure of electronic circuits, especially microwave circuits constructed using existing thick film technologies.
Thus what is needed is an apparatus for precisely separating electronic devices fabricated on substrates, such as ceramic substrates, as is increasingly needed for microwave hybrid circuits. What is further needed is an apparatus for singulating substrates upon which low K dielectrics are screened and fired upon the substrate. The apparatus for singulating substrates using low K dielectrics must prevent both mechanically and thermally generated micro-cracks from being formed in the low K dielectrics during the laser singulation process. What is also needed is an apparatus for precisely generating cutouts in electronic devices fabricated on substrates, especially when the cutouts include a low K dielectric.
While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail one or more specific embodiments, with the understanding that the present disclosure is to be considered as exemplary of the principles of the invention and not intended to limit the invention to the specific embodiments shown and described. In the description, like reference numerals are used to describe the same, similar or corresponding parts in the several views of the drawings.
In addition to the corner holes 104, two additional holes 108 are provided which enable the removal of micro-block 102 from substrate holder 310. Micro-block 102 is customized for each electronic device to be singulated from a ceramic substrate. Typically three to four vacuum holes 110 are provided beneath each electronic device to be singulated. Vacuum holes 110 are provided within micro-block 102 to hold the electronic devices in place as the laser singulation process. Depending upon the shape of the electronic device to be singulated, channels are laser machined into the micro-block 102 as will be described later. These channels may be orthogonal channels 112, e.g. channels that are machined orthogonal to each other and corresponding to the edges of a square or rectangular electronic device, or non-orthogonal channels 114, corresponding to non-orthogonal edges of the electronic device. The channels in accordance with the present invention, whether they are orthogonal channels 112 or non-orthogonal channels 114 are machined using a laser cutting system 300, to be described in
Within the channels described above are also machined slag removal holes 116 that are spaced regularly about the perimeter of the electronic device. The slag removal holes provide three functions, i.e. to provide removal of slag (metallic, dielectric or ceramic debris), to provide stress relief where corners are encountered, and prior to the ceramic substrate being cut to provide a vacuum to hold the electronic device in place.
Metallic rods 118, e.g. aluminum rods, are inserted into holes bored through micro-block 102. The metallic rods 118 are located within the boundaries of the perimeter of the electronic device and are used to strengthen micro-block 102 should the laser beam eventually cut through the thickness of micro-block 102. The holes into which the metallic rods 118 are inserted are preferably positioned one-third of the thickness of micro-block 102 below the upper surface, or one-third of the thickness of micro-block 102 above the lower surface of the micro-block 102 in order to maximize the integrity of the micro-block 102 and minimize interaction with the laser beam. Multiple metallic rods 118 may be inserted at the two levels orthogonal to each other, as shown in
Metallic rods 118 are added to maximize the integrity of the micro-block 102, however, it will be appreciated that there may be instances when no metallic rods are utilized. When utilized, the metallic rods may only be utilized in the upper holes, the lower holes, or in a combination of both upper and lower holes.
In many instances, a small cutout is required in the ceramic substrate, such as cutout 120, that may generally have a rectilinear geometry, such as square or rectangular, and is often on an order of magnitude of the thickness of the ceramic substrate (0.040″ or 1 mm). Such cutout 120 is often generated to provide relief for an integrated circuit that is affixed to a mother board (not shown), which is then wire-bonded to pads provided on the electronic device fabricated in accordance with the present invention. When a small cutout, such as cutout 120, is required, it is important to provide stress relief at each corner of the cutout 120. For purposes of example a small cutout is one which has a length and width which is defined as generally equal to or less than five times the thickness of the ceramic substrate. Put another way, a cutout that would be susceptible to being easily dislodged by the high pressure air stream 306, and in the process either causing micro-cracks in the dielectric or should the cutout be broken loose would interfere with the laser cutting system movement and alignment. It will be appreciated that this definition of a small cutout is provided for example only, and the cutout length and width to ceramic thickness ratio may be larger or smaller when defining a small cutout. As shown, stress relief at the corners of the small cutout is provided by preferably positioning a slag removal hole at each corner of the cutout 120. The vacuum provided by the slag removal holes on the corners of cutout 120 helps to prevent the small cutout from prematurely breaking away during cutting and being dislodged by the high pressure air stream 306 and as a result potentially corrupt the laser beam 308 alignment. It will be appreciated that while only a single cutout 120 was described above for an electronic device, there are instances where more than one cutout 120 is needed within the same electronic device. The laser cutting system 300 in accordance with the present invention handles multiple small cutouts when singulating an electronic device.
An important aspect of the present invention is depositing of a laser beam dispersing material 202 within the channel machined into the upper surface of the micro-block 102. After laser machining the channel, laser beam dispersing material, e.g. aluminum foil, gold flakes, or the like, is placed into the machined channel. The laser beam dispersing material 202 minimizes further cutting of the channel within micro-block 102 by the laser beam. Over time, however, some deterioration of the channel will occur through heating of the plastic material due to the unfocused laser beam repeatedly sweeping through the channel. Without the laser beam dispersing material 202, the useful life of the micro-block would be very short, i.e. only a limited number of ceramic substrates would be able to be singulated before the micro-block 102 would have to be changed. With the inclusion of the laser beam dispersing material 202, hundreds of ceramic substrates can be singulated during the useful life of the micro-block 102.
The laser cutting system 300 utilizes a laser 302 capable of cutting through a ceramic substrate 310, a dielectric, and in some instances metallization that is screen and fired onto the ceramic substrate 310 or the dielectric as will be described in further detail in
Compressed air is provided to the laser 302 and is delivered as an high pressure air stream 306 which clears the slag as the laser beam 308 cuts through the ceramic substrate 310, insulator, and metallization in accordance with the present invention. The compressed air pressure utilized in accordance with the present invention can be from 30 psi to 80 psi. The actual compressed air pressure that is utilized within the given range of compressed air pressures specified is dependent upon such factors as the electronic device to be singulated, e.g. the ceramic substrate composition and thickness, as well as the particular dielectric being used and the thickness of the dielectric and metallization layers.
A substrate holder 312 is attached to a mounting plate 318. The mounting plate 318 is attached to a base plate 324 that is mounted on an X-Y table 326. The positioning of the X-Y table 326 is precisely controlled by a laser system controller, such as a computer (not shown) in a manner well known to one or ordinary skill in the art. The computer controls linear stepper motors (not shown) within the X-Y table 326. Under the control of the computer the X-Y table 326 is moved at a rate of speed suitable to singulate an electronic device 410 from the ceramic substrate 310. The computer also controls the machining of the micro-block 102 as will be described below.
Attached to the substrate holder 312 is a rectangular vacuum nozzle 320 that is held in place relative to the substrate holder 312 by a clamp 322 attached to the base plate 324. The vacuum nozzle 320 is connected through a flexible hose to a Torit® dust collection system manufactured by the Donaldson Company, Inc. of Minneapolis, Minn. Air intakes 328 provide air to help sweep out the slag which includes ceramic, metallization and dielectric debris generated during singulation through the vacuum manifold generated between the bottom of the micro-block 102 and the mounting plate 318. The substrate holder 312 accepts the micro-block 102 machined to correspond to the pattern of the electronic circuits to be singulated. The ceramic substrate 310 rests on the top surfaces of the micro-block 102 and the substrate holder 312. The substrate holder 312 includes vacuum holes 316 that will be described in further detail below. The vacuum holes 316 are used to initially retain the ceramic substrate 310 as the laser cutting process proceeds, and once the singulation has been completed are used to retain the excess ceramic substrate, otherwise known by one of ordinary skill in the art as the “wings”. As mention above, the space between the bottom of the micro-block 102 and the top of the mounting plate 318 provides a vacuum manifold that couples the vacuum generated by the dust collecting system to the device vacuum holes 110 and slag removal vacuum holes 116. The device vacuum holes 110 hold the electronic device 410 from being displaced by the high-pressure air stream 306 as the electronic device 410 is being singulated from the ceramic substrate 310.
Also shown in
The entire surface of the ceramic substrate 310 and electronic device 410 is coated with a poly vinyl alcohol solution, used in the prior art as a solder mask, and is now used in the present invention as a slag mask 508. Slag mask 508 captures laser slag that is not removed by the vacuum system or by the high pressure air stream 306 applied to the surface of the ceramic substrate while the laser beam 308 is cutting. The poly vinyl alcohol solution is preferably Photomask Coating 2060, sold as a protective polymer coating designed to increase photomask life, is manufactured by Transene Co. located in Rowley, Mass. It will be appreciated that other similar poly vinyl alcohol solutions from other manufacturers that are often used as solder masks may be utilized as well. It has been found that the slag re-deposited on the surface of the electronic device in the vicinity of the laser cut is almost impossible to remove, and interferes with further processing of the electronic device, e.g. wire bonding. The poly vinyl alcohol solution used to form the slag mask 508 is easy to deposit on the ceramic substrate 310 ready to be singulated, is readily cut by the laser beam 308, is robust enough to catch and hold the slag not blown away by the high pressure air stream 306 or vacuumed through the slag vacuum holes 116, and washes away readily with water as will be described below.
In the preferred embodiment of the present invention, the slag mask 508 is applied using a brush, although it will be appreciated that other methods of application can be utilized as well, such as a spray system. Since the poly vinyl alcohol solution utilized is clear, it has been found that the addition of a vegetable coloring, or similar water-soluble dye is desirable. Once the electronic devices have been singulated from the ceramic substrate, the slag mask 508 can be completely washed free from the surface of the electronic device 410 as a result of the added coloring. Any colored residue remaining is an indication that washing is incomplete.
As shown in
The ceramic substrate 310 is preferably a 96% Al2O3 (alumina) ceramic. Alumina ceramic is essentially a composite of fine-grained poly-crystals held together at their grain boundaries. The existence of this multitude of randomly oriented grain boundaries has the desirable effect of interfering with the propagation of cracks within the ceramic. Thus cracking of the ceramic substrate edges during singulation is generally not a problem encountered neither in the laser cutting system 300 of the present invention, nor in most prior art laser cutting systems. The conductor 604 and ground plane 702 metallization in the electronic device 310 in accordance with the present invention is preferably gold. Such metallization system is generally very difficult to cut, even with a CO2 laser system unless the laser beam power provided is sufficient to overpower the reflectance of the metallization and cut through the metallization. The dielectric 704 is a low K dielectric having a dielectric constant less than 5. A suitable low K dielectric is KQ CL-90-7858 dielectric (a glass dielectric) available from Heraeus Cermalloy (West Conshohocken, Pa.) which has a dielectric constant of 3.95. However, the dielectric 704 may be another dielectric and, particularly, may be another low K glass dielectric with suitable electrical properties. The dielectric 702 is inherently weaker in tension than in compression. The dielectric 702 has been found to generally cut cleanly when the laser beam 308 cuts through the dielectric 702, but as the laser beam 308 encounters any metallization layer below the dielectric 702, the laser beam 308 heats the metallization, expanding it laterally, thus placing the already cut dielectric 702 into tension. Such tension will result in micro cracking unless the heating of the base metallization while singulating the ceramic is properly controlled. In this regard, the channel which has been laser machined in micro-block 102, and which includes slag vacuum holes 116 reduce the micro-cracking potential by enabling air to be swept by the edges of the electronic device thereby controlling the heating of the base metallization, as the electronic device is being singulated.
Movement of the electronic device, as described above, can also initiate micro cracking, as it is being singulated. As described above, the movement of the micro-block 102 is constrained by the device vacuum holes, and to a lesser extent by the slag vacuum holes in accordance with the present invention.
Once the micro-block design pattern has been completed, a pre-machined micro-block blank is obtained and the four leveling screws are inserted. The micro-block with leveling screws inserted is placed into the laser cutting system 300 and leveled with the top surface of the substrate holder. The pre-machined micro-block blank is a plastic block having the width, length and thickness dimensions of the final micro-block, as well as having the four corner holes for the leveling screws and two removal holes pre-machined, The four corner holes are tapped to accept the leveling screws.
The pre-machined micro-block blank is machined in the laser cutting system 300 exactly as it will be used when the laser cutting system 300 is singulating the electronic devices from the ceramic substrates. This insures the depth and width of the perimeter channels and placement of the slag holes, etc. are in accordance with the micro-block design pattern, and further correspond to the precise laser beam positions utilized to singulate the electronic devices from the ceramic substrate. A perimeter channels are cut to a depth preferably one-third the thickness of the micro-block and to a width preferably three times the laser beam width as described in
Ceramic substrates that have been fabricated with the thick film electronic device are first coated with the slag protection material referred to above as the slag mask, at step 804. The ceramic substrate with the slag mask is baked preferably at 85° C. for approximately 30 minutes at step 806, or according to the manufacturer's specification. Completed ceramic substrates are then placed in the laser holding fixture as described above, at step 810. The laser cutting system 300 is then used to singulate the electronic devices, at step 812. The singulated electronic devices are next removed from the laser holding fixture. Since it is impossible to singulate the electronic devices without leaving some slag clinging to the bottom surface of the electronic device, this residual slag is easily removed by using a raw ceramic substrate as a scraper and scraping the edges on the bottom surface of the electronic device, at step 816. The singulated electronic devices are next washed in a warm water bath in which nitrogen is bubbled through the warm water, at step 818. The turbulence generated by the nitrogen assists in removing the slag mask and captured slag from the top surface of the electronic device. After a visual check to insure all slag mask has been removed, and all slag has been removed from the bottom edges of the electronic device, the cleaned electronic circuits are dried in an oven preferably at 85° C. for approximately 15 minutes. The dried electronic circuits are then packaged for shipping, at step 820.
Unlike prior art laser cutting systems, the laser cutting system 300 in accordance with certain embodiments of the present invention cuts completely through the ceramic substrate 310 as well as through any dielectric and metallization that may be in the path of the laser cutting beam 308. The micro-block 102 plays an important part in singulating the electronic device 410, as the vacuum holes 110 in micro-block 102 are key to holding the electronic device in place. By maintaining the position of the electronic device 410 as it is singulated from the ceramic substrate 310, and by properly maintaining the parameters of the laser cutting beam 308, micro-cracking of the dielectric is avoided.
In summary what has been described above is a laser cutting system 300 that includes a laser 302 generating a laser cutting beam 308 for singulating an electronic device 410 from a ceramic substrate 310. A micro-block 102 is laser machined to include a channel that may have orthogonal segments 112 and non-orthogonal segments 114 corresponding to an outline of the electronic device 410 to be singulated. Laser machined within the channel 112, 114 are also slag removal vacuum ports 116. The slag removal vacuum ports 116 are used to remove slag and when a small cutout 120 is required, hold the small cutout 120 during singulation. The support block 102 also includes device vacuum ports 110 for holding the electronic device 410 in position during and after being singulated. Laser beam dispersing material 202 is placed in the channel 112, 114 to retard cutting of the support block 102 by the laser cutting beam 308 during singulation. The laser cutting system 300 can also singulate the electronic device 410 cleanly without creating micro cracks in dielectric materials 704 when used in conjunction with a metallization 602, 604 that is used in fabricating the electronic device 410 on the ceramic substrate 310 being singulated. The electronic device 410 may comprise one or more cutouts 120 that when singulated and dislodged can become a problem to the alignment of the laser cutting system 300. The micro-block 102 is further laser machined to include slag removal vacuum ports 116 at the corner of the cutouts 120 to prevent the cutout from being dislodged during singulation. The device vacuum ports 110 hold an electronic device that includes at least first metallization pattern deposited on the ceramic substrate and a dielectric pattern deposited on at least a portion of said first metallization pattern. The dielectric pattern may also a second metallization pattern deposited on at least a portion of the dielectric pattern. The device vacuum ports 110 hold the electronic device 410 as the electronic device 410 is being singulated, and in combination the channel 112, 114 and the slag vacuum ports 116 are used to prevent micro-cracking of the dielectric as the dielectric is cut by the laser cutting system 300.
While the present invention has been described above as being applicable for singulating electronic devices from various substrates, such as ceramic substrates, and in particular for singulating electronic devices fabricated using low K dielectrics having a dielectric constant less than 5, it should be appreciated that the present invention can be utilized to singulate electronic devices using higher K dielectrics as well. When using higher K dielectrics in the fabrication of the electronic devices, consideration must be taken that the higher K dielectric is environmentally stable, and remains environmentally stable once cut with the laser cutting system in accordance with the present invention.
While the invention has been described in conjunction with specific embodiments, it is evident that many alternatives, modifications, permutations and variations will become apparent to those of ordinary skill in the art in light of the foregoing description. Accordingly, it is intended that the present invention embrace all such alternatives, modifications and variations as fall within the scope of the appended claims.
