Method for Stacked Three Dimensional Device Manufacture

1. Field of the Invention

The present invention generally relates to semiconductor chip manufacture, and more particularly to methods of making stacks of chips by lamination of wafers.

2. Background Description

Since the development of integrated circuit technology, computers and computer storage devices have been made from wafers of semiconductor material comprising a plurality of integrated circuits. After a wafer is made, the circuits are typically separated from each other by dicing the wafer into small chips, each containing one or more circuits. Thereafter, the individual chips are bonded to carriers of various types, and interconnected by wires to form modules. Such "two-dimensional" modules of chips fail to optimize the number of circuits that may be packaged in a given space and introduce undesirable signal delays, capacitance, and inductance as signals travel between chips.

Recently, three-dimensional arrays of chips have emerged as an important packaging approach. A typical multichip electronic module comprises multiple integrated circuit (IC) chips adhesively secured together as a monolithic structure. A metallization pattern is often provided directly on one (or more) side surface(s) of the module for IC chip interconnections and for electrical connection of IC chips to circuitry external to the module. The metallization pattern can include both individual contacts and bussed contacts. Multichip modules comprising stacks of IC chips are referred to herein as "stacks".

The current stack fabrication process suffers from problems that negatively effect overall stack manufacturing yield and efficiency. As a result, stack manufacturing costs remain high and profit margins remain low. Several of these problems involve: (1) IC chips becoming unusable in the stacks due to edge chipping during wafer dicing; (2) difficult side-surface channel via fabrication; (3) side-surface polyimide edge bead thickness; and (4) variable T-connect quality.

In addition, the current stack fabrication process requires much tighter dicing tolerance than is currently required for IC chip dicing associated with single IC chip, plastic encapsulation type packaging. The dicing tolerance for plastic packaging is approximately, for example, .+-.20 .mu.m, while the stack process requires a dicing tolerance of approximately, for example, .+-.5 .mu.m.

When IC chips of inadequate tolerance are stacked and laminated, the varying IC chip sizes result in IC chips shifting within the stack. Such shifting causes misalignment of, for example, the transfer metal leads of the IC chip in the stack. Accordingly, the side-surface of the stack requires polishing to expose all of the transfer metal leads, thereby reducing the total number of times that the stack side-surface can be reworked. Furthermore, the misalignment of the IC chips results in wider side-surface wiring being required to "capture" all the side-surface connections. Side-surface wiring density is therefore reduced.

As yet another problem, the varying IC chip sizes force the stack/lamination fixture used for assembly of the stack to be large enough to accommodate IC chips of varying sizes up to the maximum specification limit. This increases the likelihood of IC chip shifting. Moreover, the forces on the stack during lamination are concentrated on the largest chips in the stack because they are in direct contact with the sides of the lamination fixture. Therefore, these large chips tend to become damaged during lamination. This further compromises stack yield and requires increased side-surface polishing.

It is an object of this invention to provide a new and improved method of making three-dimensional arrays of chips.

It is another object of this invention to provide a method in which several three-dimensional arrays can be produced simultaneously.

The method of the present invention involves several steps beginning with forming chips on several wafers. As the wafers are processed an oxide ring is formed between each chip. The wafers are then stacked, matching notches on the periphery of each wafer, and then laminated. After laminating the stacks of wafers, stacks of chips are separated from the laminated wafer structure by etching, dicing, combinations thereof (i.e., dicing followed by etching), or by other process.

The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of the preferred embodiments of the invention with reference to the drawings, in which:

FIG. 1 is a cross sectional view of a wafer at the beginning of the fabrication process;

FIG. 2 is a cross sectional view of the wafer after backside polish;

FIG. 3 is a cross sectional view of the wafer after a selective etch of silicon;

FIG. 4 is a cross sectional view of the wafer after a selective etch of nitride;

FIG. 5 is a cross sectional view of the wafer after application of adhesive (FIG. 5 being rotated 180.degree. from the orientation shown in FIG. 4);

FIG. 6 is a cross sectional view of a laminated wafer structure; and

FIG. 7 is a top view of a wafer showing division of the wafer into device regions.

Referring now to the drawings, and more particularly to FIG. 1, there is shown a cross-section of a portion of a wafer 1, having active device layers 2 and cube transfer layers 3. In addition, oxide 4 fills trenches 5 which are lined with nitride 6 in the silicon wafer 1. A top view of the prepared wafer is shown in FIG. 7. As can be seen in FIG. 7, the oxide filled trench 5 surrounds each chip 8 in the wafer 1.

In the practice of this invention, a plurality of wafers 1 are created, each of which includes a plurality of chips 8 surrounded by oxide filled trenches 5. Upon creation of these wafers 1, a backside polish is performed to expose the nitride liner 6. As can be seen in FIG. 2, the silicon wafer is polished to the nitride liner 6. Then, as shown in FIG. 3, a selective etch of the silicon wafer 1 exposes the nitride liner 6 further. Next, as shown in FIG. 4, the exposed nitride liner is removed by selective etch. The removal of the nitride liner creates a raised oxide ring 9 surrounding each chip 8.

FIG. 5 shows a die by die application of epoxy or other adhesive 11. To apply the adhesive, the wafer is rotated and the adhesive 11 is applied to each chip 8. On application, the adhesive forms a meniscus. The volume of the adhesive 11 is controlled so that the adhesive will not flow over the raised oxide ring once devices are stacked.

FIG. 6 shows the last step, where wafers are stacked and laminated. The chips 8 are matched up using notches on the edges of the wafer. These notches 12 are shown in FIG. 7. In FIG. 7, notches 12 are shown at 120.degree. angles around the periphery of the wafer 1. The specific location of these notches is not important. However, the notches do need to be placed in the same location on each wafer so that chip areas are matched during lamination.

In order to properly match the wafers which will be stacked, notches are preferably formed in a first wafer. After the notches are formed, the wafer is placed in a stepper which puts down a pattern and sets an alignment mark relative to the notches. In all subsequent wafers processed for stacking, alignment marks are made according to the settings calculated by the first wafer processed. The ability to line up the device regions accurately is important in reducing the number of chips which fail and increasing sidewall wire density. Accordingly the circuits on each wafer should be aligned to the alignment mark.

After lamination, the integrated circuit chip stacks are separated. In one embodiment, the wafer may be dip etched in hydrogen fluoride (HF) or solutions of HF (including HF diluted with water and buffered HF having NH.sub.4 F) to separate the individual stacks. In a second embodiment dicing could be performed or dicing followed by a dip etch. FIG. 7 shows separation of the stacks of chips 8 occurs along the trenches 5. Because the wafers have been aligned and laminated, separation produces stacks of laminated chips from the wafer. Hence, several stacks are formed simultaneously.

A dress up polish of the sides of the stacks to be face wired is then performed to remove any minor chip defects to chip misalignment that the process may introduce.

An unlimited number of wafers may be stacked in this process. The number of wafers stacked is determined by the amount of working chips needed in a stack. Accounting for yield loss during processing will raise the number of stacks which are usable after processing. Following wafer processing, a certain number of chips on the wafer will likely fail. In addition, some chips may go bad during the stacking process. Both types of yield loss should be accounted for when stacking wafers. For example, if stacks of ten working integrated circuits are required, stacking only ten wafers to form stacks of chips may result in less than 50% of the resulting stacks being usable for the desired application, based on a typical yield 70% per wafer. Due to the random distribution of defective chips on a wafer, an amount is stacked to provide for redundancy to take possible bad chips into account. During wiring, chips are tested and accounted for so that the stack still may be used, despite one or more defective chips.

While the invention has been described in terms of its preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.