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Polished 20 Microns Silicon Wafers that Bend and Transmit Light -- Denso Corporation

Polished 20 microns silicon wafers that bend and transmit light

The technology that makes silicon wafers thin

The Denso Corporation is grappling with a new technology that makes silicon LSI wafers thin, as part of the effort to miniaturize electronics for wearable devices. Denso has succeeded in reducing a 0.6-mm wafer down to only 50 microns at the practical level and to 20 microns at the research level. The latter is almost as thin as a sheet of paper (1 micron equals 1/1000 mm). This thin wafer can be bent by hand, and moreover can transmit light. What are the merits of such thin wafers?

The difficulty of handling wafers during the installation of electronic circuits

The integrated circuit, called IC or LSI, is used in personal computers and mobile telephones. Though the integrated circuit is a silicon chip measuring a mere 5 mm to 10 mm per side and is only a silver-colored particle, it is packed with up to several million transistors. Usually, the chip is enclosed inside a larger package or casing, and a lead frame connected to the casing is soldered to the substrate. Recent technology, however, allows the chip to connect directly to the substrate without the use of a casing, in turn allowing miniaturization of the device. Incidentally, an integrated circuit is usually made by forming parts and their wiring into the surface of a silicon wafer with a diameter of 6 inches and a thickness of 0.6 mm, like a photograph. When a circuit is being manufactured, the wafer flows through each process at a wafer thickness of 0.6 mm, which is too thick for bending, and the circuit is made some microns under the surface of the wafer. Therefore, the part used as a circuit is only in the pellicle, a sort of skin on the wafer surface, and the rest of the silicon is there only to maintain the integrity of the wafer. So, if we can shave the back side of the wafer and make it almost as thin as paper, we will be able to miniaturize electronic devices and manufacture thin cards, such as telephone card and tickets, with silicon chips implanted in them.

But manufacturing such circuit-embedded wafers is not easy. The part of the wafer containing the circuit is very delicate, and even a small injury would destroy the circuit. And, because the wafer manufacturing process includes a number of different treatments, including heat treatment, which generate stress inside it. When the wafer is very thin, such stresses gradually bend the wafer, eventually causing fractures and cracks. Of course, cracked or fractured electronic circuits cannot be used.

We have therefore developed the technology to make a thin wafer with a surface that protects the circuit and corrects against bending, thus preventing fractures and cracks.

Thinning the wafers

There are two processes to make thin wafers. The first is to shave the back side of the wafer with a grindstone, and the second is chemical mechanical polishing (CMP), which gives the shaved plane a mirror finish.

Many problems arise when the back side of the wafer is shaved: how to fix the wafer so that the whole of its back can be shaved, how to protect the circuit from scratches made by particles in the grinding liquid (which could also destroy the wafer in which the circuit is made) and how to correct for bending resulting from internal stress.

In order to solve these problems, we utilize a pressure-sensitive adhesive sheet and a special jig made of stainless steel called a guide ring. The guide ring is a stainless steel ring, with a hole larger than the wafer. The ring is stuck to the pressure-sensitive adhesive sheet, and the wafer is stuck in the center of the hole. Hence, the face on which the electronic circuits of the wafer are to be made are adhering to the sheet. This process holds the wafer steady so that it can be shaved and prevents foreign matter from getting near the wafer surface. It also protects the electronic circuit under the wafer surface. However, if adhesion is too weak, the wafer moves while it is being shaved and/or comes off by bending and losing contact with the adhesive. On the other hand, if adhesion is too strong, the wafer cannot be removed from the adhesive without being damaged. Therefore, proper adhesion is necessary.

With the wafer adhering in its place, the guide ring is set in the grinding instrument, the guide ring and the grindstone rotate against each other, and the back side of the wafer is shaved under suitable pressure from the stone. At this time, the wafer is fixed to the sheet and its shape is stabilized by this backing. Therefore, the wafer never bends no matter how thin it becomes. When the wafer reaches the target thickness, its face is polished to a mirror finish. A polishing fabric and a polishing liquid are used in this process. The guide ring and polishing fabric rotate against each other, and the wafer is polished under suitable pressure. The polishing liquid contains a constituent that melts silicon and grains of micron-sized quartz glass. Because quartz glass is harder than silicon, the back side of the wafer is melted little by little and is shaved, and it eventually becomes a mirror-finish surface. After this undergoing this polishing, the wafer is divided into tips with a dicing saw like a circular saw, and these are installed to the substrate.

It is supposed that in the future, IC cards will be used as credit cards and prepaid cards for prevention of forgery, and that miniaturization of personal computers will lead to tiny computers being mounted on the body. The technology to make thin silicon wafers is applicable to all these fields.

Fig. Caption

Fig. 1

If an LSI silicon wafer is shaved to a thickness of 50 microns, it can be bent freely.

Fig. 2

A silicon single crystal wafer that can transmit white light (thickness: 20 microns).

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