Wafers that have passed a wafer test after a front-end process goes through a back-end process, which starts with Back Grinding. Back grinding is a step of grinding the back of a wafer thinly. This isn’t just simply about reducing the thickness of a wafer; this connects the front-end process and the back-end process to solve problems that occur between the two processes. The thinner semiconductor chips are, the higher the integration can be through higher chip stacking. However, that leads to the lower performance of the product at the same time, thus there is a contradiction that the performance should be improved through back grinding. Therefore, the grinding method that determines the thickness of a wafer is one of the variables which reduces the cost per semiconductor chip and dictates the product quality.
1. Purpose of Back Grinding
Figure 1. The process that changes the form of wafer manufacturing and
semiconductor manufacturing
In the journey where a wafer is reborn as a semiconductor, the external form continues to change. First, during the wafer manufacturing process, the edge and surface of the wafer are polished. In this process, both sides of a wafer are ground in general. After that, the wafer goes through the front-end process, and then the back grinding, which grinds the back-side only. This is to remove chemically contaminated parts during the front-end process and reduce the thickness of the chip. This is useful for making very thin chips mounted on IC cards or mobile devices. In addition, it has the advantage of reducing resistance, contributing to decreasing power consumption. At the same time, by increasing the thermal conductivity, it can quickly dissipate the heat generated during the normal operation to the back side. Instead, however, as the wafer is thinner, it can be easily broken or warped by external forces, making handling it very difficult.
2. Detailed Processes of Back Grinding
Figure 2. Three detailed processes of back grinding
Back grinding is divided into three detailed processes. 1) Tape lamination is conducted to attach tape to a wafer. 2) The back side of a wafer is ground. Then, before the sawing process which separates a chip from a wafer, 3) wafer mounting is carried out to place the wafer on the tape. As the third process, wafer mounting is a preparatory step for separating chips (chip saw), it can be included in the sawing process. As the chips are becoming thinner and thinner these days, the order of processes can change sometimes, and each process is subdivided more and more as well.
3. Tape Lamination, a Process for Protecting a Wafer
Figure 3. Tape lamination and wafer structure from the front side
The first step of back grinding is tape lamination. This is a type of coating, which is a process to attach adhesive tape to the front of a wafer. When conducting back grinding, the silicon compound spreads in all directions, and the wafer can be broken or warped by the grinding force. Especially, as the area of the wafer is larger, it becomes more vulnerable to this phenomenon. For this reason, before the back grinding, a blue thin tape for ultraviolet (UV) should be attached to protect the wafer.
When laminating, increasing the adhesion is necessary so that there are no gaps or bubbles between the wafer and the tape. However, after the back grinding, the tape on the wafer should be detached by irradiating ultraviolet to the surface of the tape to decrease the adhesive strength. After peeling, the residue of the tape shouldn’t remain on the surface of the wafer. Although a tape for non-UV is used sometimes, it has a weak adhesive strength, making it vulnerable to air bubbles. It has a lot of disadvantages, but it is cheap. Also, tapes for bumps which are about twice the thickness of tapes for UV, are applied as well, and it is expected that these will be used more often in the future.
4. Inversely Proportional Relation between Wafer Thickness and Chip Packaging
Figure 4. Multi Chip Package (MCP) structure
The thickness of a back-ground wafer is reduced from 800-700㎛ to 80-70㎛ in general. Wafers thinned to about a tenth are stacked in four to six layers. Recently, through two grindings, a wafer can be made even thinner to about 20㎛ and it can be stacked up to 16 to 32 layers. This multi-layer structure of semiconductors is called a Multi Chip Package (MCP). In this case, even if it is a structure made with multiple layers, the total height of the finished package should be no more than 1.4mm of thickness, which is the reason why wafers should be ground even thinner. The thinner the wafer is, the more defects occur, making it difficult to proceed to the next process. As a result, advanced technologies are required.
5. Changes in Back Grinding Method
Figure 5. Diversification of grinding method by the thickness of wafers
Back grinding has been developed by overcoming limitations of processing technology by cutting wafers as thin as possible. A typical wafer with a thickness of 50 μm or more has 3 steps. First, Rough Grinding. Second, Fine Grinding. Through two grindings, the wafers are cut and polished. In this process, we insert slurry and deionized water between the pad and the wafer just like the Chemical Mechanical Polishing (CMP). This abrasive work reduces the friction between wafers and pads, making the surface shine. When the wafer is thick, super fine grinding can be performed, but the thinner the wafer is, the more necessary the grinding is to be carried out.
If a wafer becomes even thinner, external defects occur during the sawing process. For this reason, if the thickness of a wafer is 50㎛ or less, the process order can be changed. In this case, a Dicing before Grinding (DBC) method is used, where sawing for a wafer is performed to half the level, before the first grinding. In the order of dicing, grinding and dicing, chips are separated safely from wafers. Using a sturdy glass plate to protect wafers from breaking is one of the special grinding methods.
As with the growing demand for increased integration due to the miniaturization of electric devices, the back grinding technique should also continue to develop by overcoming its limitations. In other words, it is necessary not only to address the current defects in wafers, but also to prepare for new defects that would occur in the next process. To solve these issues, it is necessary to change the process order or to introduce a chemical etching technique applied to the front-end process to comprehensively develop a new processing method. Many attempts for various changes have been made in the grinding method to solve the problems that might occur in the wafer with the larger area. In addition to this, research on recycling silicon residues after grinding wafers has been being conducted as well.
ByJong-moon Jin
Teacher at Chungbuk Semiconductor High School
