applications sEMiconDUctor procEssinG
non-dicing applications of laser processing in the semiconductor fab. In LED fabrication, he says, tailor-made lasers are required due to the great variation in the substrates used to obtain different colours of light. The marking of ID numbers directly onto the LED wafers is a common application,’ he says, adding that sapphire coatings in particular can provide a challenge to the laser user. Elsewhere among Rofin’s semiconductor customer base the trend for minimising waste silicon continues, with Schmidt identifying a trend: ‘Wafers are becoming thinner and thinner, and some of them are getting so thin that they can no longer be processed properly unless they are mounted on some other special carrier, such as a glass for example. In these cases, lasers are used to delaminate the product wafer from this so-called transport wafer.’
Despite these other uses, dicing remains the most well-established application of lasers in the semiconductor and LED supply chain. Conventional wafer dicing, Schmidt explains, can consist of either a scribe-and-break or through-thickness cut. In scribing processes the laser cuts through approximately 30 per cent of the thickness of the wafer material, relying on a subsequent mechanical process to break the wafer along the scribed lines. ‘Diamond blades or UV laser scribers are typical LED device separation methods,’ says
and we can process up to 400µm thickness with a single pass.’ As well as advantages in terms of the process and its results, Schmidt highlights the long lifetime and minimal servicing requirements of the picosecond fibre lasers employed in this application.
the technique has already
been established in the asian semiconductor market
Schmidt. ‘However, the pressure to lower the LED manufacturing cost has boosted the demand for alternative dicing technologies to increase the throughput and lower the manufacturing cost of an LED chip dicing line.’ To meet this demand, Rofin has developed
a process with its picosecond fibre laser to scribe the sapphire substrates used in the LED industry, enabling high processing quality at high speeds, without the need for an expensive UV laser. ‘Such a technique is especially suitable for manufacturing high-brightness LEDs, because of minimal brightness loss [as a result of edge scattering losses] and minimal thermal effects,’ says Schmidt. ‘For example, on a four-inch wafer with a thickness of 140µm, such a process can deliver at least 50 per cent more throughput than other methods,
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cutting from the inside out Laser scribing and through-thickness cutting offer great advantages over sawing, but there is still room for improvement in terms of minimising material loss and damage to the edge of the chips. Hamamatsu developed its ‘stealth dicing’ technique in order to further improve the processes of laser dicing. The lasers used in the process vary from implementation to implementation; the only key criterion is that the wafer must be transparent to the wavelength used, i.e. near-IR for silicon. The beam is focused to a point within the wafer, and not on its surface, which heats and stresses the material, cutting it from the inside out. Palmer explains that the process leaves the wafer stable enough to be moved, but ready to be separated: ‘The laser causes small perforations within the wafer,’ he says. ‘Once the machine has diced the wafer with our process, the wafer remains intact, but a subsequent expansion of the film or tape to which the wafer is stuck breaks open the chips and leaves them separated.’ As well as producing cuts of just 1µm in width, the process leaves exceptionally clean edges, even relative to conventional laser scribing or cutting.
Palmer explains that Hamamatsu’s technique is also able to deal with layered wafers: ‘Currently, instead of just processing a single silicon wafer, semiconductor companies are using wafers that are bonded together – silicon-silicon or silicon-glass for example. They may need to cut at different depths within the wafer, and this is possible with the stealth technique by focusing the laser down into the part of the wafer the user wishes to dice. Multiple layer dicing is possible.’ The technique has already been established in the Asian semiconductor market, and is incorporated into specialised chip-dicing machines produced by several companies.
a neat package Silicon chips are not sold as tiny slivers of silicon, but rather as polymer-encased packages with some sort of external interface for connecting them to a PCB. Rofin’s Schmidt highlights the way lasers are used to dice
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