applications sEMiconDUctor procEssinG
material processing: ‘As an example, the holes can need to be as small as 8µm in diameter,’ he adds.
already offering advantages over mechanical processing, laser dicing of silicon wafers has been further enhanced by Hamamatsu’s stealth dicing technique
➤
QFN-packaged chips, again replacing sawing techniques. As is the case for the silicon chips themselves, the QFN packages are produced in large moulded blocks before being separated afterwards by a dicing process. QFN stands for ‘quad-flat, no leads’ – a flat resin block with leadless contacts on four sides of its base. These contacts are soldered directly onto a PCB, removing the need to drill holes through it first. The complete QFN package, Schmidt
explains, consists of both a mouldable resin and layer of a conductive element such as copper. In contrast to the all-laser dicing of the wafers, QFN dicing uses a laser and saw hybrid process: ‘With a disc or ultra-short pulse laser, the QFN can be cut at the copper site for electrical isolation,’ he explains. ‘The final separation of the IC packages is done afterwards with a standard mechanical sawing system. The advantage of such a process is that as well as increasing the speed at which the mechanical sawing system cuts, the package quality will also increase, because of a reduction of copper smearing, solder bridging, [and other undesirable effects of the mechanical saw].’ In the future, Schmidt says, integrated
circuits will move into the third dimension, with many layers of silicon wafer stacked on top of each other. Where these wafers need to interconnect, so-called through-silicon vias (TSVs) will be required, and Rofin is already developing some of the laser processing techniques to produce these structures. ‘TSV applications represent an interesting field,’ he says. ‘It’s 3D technology for integrated circuits, where lasers are used to drill the holes through the silicon wafers.’ The challenge lies in the fact that the holes must be drilled to a precision rarely required in
12 ElEctro optics l MARCH 2011
touching precision Midaz Lasers (London, UK) has developed a DPSS laser emitting at a UV wavelength (355nm) with an unusually high pulse frequency of more than 1MHz. Dennis Camilleri, business development manager at the company, explains that the laser has proven useful in producing the kind of touch screen found in modern smartphones. These devices, he explains, rely on structured layers of transparent circuitry, which is etched into a layer of indium tin oxide (ITO). ‘When removing ITO from glass for touch panel circuitry, the quicker we can go while still controlling the UV power, the more throughput we can achieve. The UV megahertz laser has been tested for that very purpose – selectively removing ITO from both the top and bottom surfaces of a touch panel display.’ The unusually high repetition rate of the company’s laser, he adds, allows users to keep the throughput of their production lines high, maximising profits. ‘No other laser can go to 1MHz repetition speeds, and our laser does it at a relatively high average power of 10 to 13W, with 10 to 15ns pulses.’ The high repetition rate is achieved
through a patented multi-band design with a folded cavity, q-switch control, and
careful thermal management. ‘It’s a unique and innovative design of DPSS laser,’ says Camilleri, adding that in addition to high pulse rate and average power, the laser also maintains good beam quality with an M2
value
of 1.1 to 1.2. ‘What tends to happen with DPSS lasers is that users can’t hold those three parameters together. Going up to very high pulse rates of 500kHz or above while having a decent 10 to 30W output in the UV usually means that beam quality will suffer.’
in contrast to the all-laser
dicing of the wafers, QFn dicing uses a laser and saw hybrid process
The pulsed power, explains Camilleri,
allows the operators to keep the process under control, with each pulse delivering no more energy than is required to ablate the ITO layer without affecting the substrate beneath. CW lasers, he adds, would not permit this control. As well as being readily absorbed by ITO, the use of the 355nm wavelength enables tighter focusing of the beam at the work surface than is possible with IR wavelengths. The application of Midaz’s megahertz UV
laser is perhaps an indication that if a new capability emerges (such as a high pulse frequency at high power), the semiconductor industry is the most likely place for it to be paired with a suitable application. l
a QFn-packed chip. the separation of these chips requires cutting both the polymer resin and the conductive metal substrate
www.electrooptics.com
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