Power production

As we head for the next upturn in the semiconductor industry, it becomes obvious that this positive move goes hand in hand with the introduction of new products, new production techniques and new materials.

Market In the front-end SiGe market, highK and lowK materials are some of the latest buzz words, however, paper-thin chips and stacked chips describe the next main direction of the pre-assembly and backend area. In particular, smart labels, contactless smart card, and hybrid applications for stacked chips are the new and upcoming applications for paper-thin chips. Pre-assembly describes the manufacturing process located between the front-end and the backend section: a thick wafer is transferred onto thin chips which are finally mounted onto substrates in the backend. In the past, several methods for thin wafer production1 have been proposed, however all of them are generally based on the introduction of non-established and non-matured wafer thinning, dicing and handling methods. Considering the challenges of the new products and processes in the front-end, unproved processes in pre-assembly for paper thin chips will increase the risk even more for the million US$ capital investment spent for front-end equipment.

New method For more than 20 years Nitto Denko, a manufacturer of taping, wafer mounting, de-taping machines and the corresponding tapes, has grown up as the quasi standard for the pre-assembly line of today. As described in detail2 the Nitto patented "Never handle a thin wafer without tape" method has allowed processing of thin and relatively strong warped wafers to be mounted onto a dicing tape frame. Today, this method (Fig. 1) safely performs the production of 50µm thin wafers/chips on mass production level. This is finally achieved by the unique combination of a specially designed backgrinding tape and the new generation of wafer mounter with an integrated protection tape peeling function. The protection tape covers the wafer frontside during the backside thinning process. However, in Nitto's patented pre-assembly concept (peeling-after-mounting) the protection tape is not peeled-off after thinning but will still protect and support the thin wafer until beeing mounted on the dicing tape frame. The backgrinding protection tape is then removed. The fundamental methodological advantage to be mentioned is that the Nitto's mounting technology (see Fig. 2) for the first time practically opens the conventional pre-assembnly process window. All the well established thinning and dicing techniques can be genereally used now down to real paper thin wafers and chips. The new Mounter-Peeler system is a hybrid-system: a wafer mounter with additional functions of a protective tape peeler, UV-illumination system, optical character reader (OCR) and bar code printer (BCP), or barcode reader (BCR). The fully equipped PM-8500 covering less than 2m3 floorspace achieves a throughput in range of 45 w/h. In principle, applying the same established internal process sequence (loading wafer, alignment, UV-illumination, mounting onto dicing frame) like the previously released model, the MH-8500, the new PM-8500 handling concept is designed to accomplish the needs of extremely warped thin wafers. The PM-8500 handles a warpage of up to 15.000 µm which can easily happen for instance at thin 6 and 8 inch wafers by some metal layer deposition or remaining thinning stress. The new Nitto backgrinding tape dedicated to the PM-8500, called BT-RF, is designed for the ultra-thin wafer production, where "BT" stands for Backgrinding Tape and "RF" for Real Flat. It theoretically allows a thickness of a few µm, finally limited by the wafer diameter. In the research and application laboratory at Nitto, tests have been performed measuring the wafer slag at different 8 inch wafer thicknesses (Fig 3). The tests are carried out with a convential backgrinding tape in comparison to the BT-RF tape. At about 100µm thin wafers – the limit of stand-alone de-taper systems – the effect is is already obvious: 4.5 vs. 7mm. The thinner the wafer, the larger the slag is. This is shown at a wafer thicknesses of below 80 µm which gives clear evidence of this non linear ratio between thickness and slag: 5,5. vs. 12 mm. Applying the BT-RF tape to a 50 µm thin wafer, the slag of 25 mm with a conventional tape on the wafer is reduced down to 7-8 mm which can easily be handled by the PM-8500 wafer mounter giving a large safety margin left for additional wafer stress induced by the production process. The strong slag of wafer at 50 µm is shown in Fig 4. On the left side the conventional backgrinding tape is not able to give rigid support to the wafer, whereas the new BT-RF tape is able to make the wafer as flat as much thicker wafers normally are. This comparison makes it obvious that the patented combination of Nitto "Peeling-after-Mounting" machine concept with the wafer support tape BT-RF offers a suitable solution for paper thin chip production. The PM-8500, in combination with the BT-RF backgrinding tape allows at a given thinning/dicing equipment and process, to significantly enlarge the fab manufacturing window and helps the IC manufacturer to enter the thin wafer market easily.

For further information contact Eva Mittner tel: +49 8945 111 288 email: Eva.Mittner@macrotron-pt.com