Thinner Dice: the Next Step for Miniaturization

John Baliga, Associate Editor -- Semiconductor International, 10/1/1999

The continuing effort to put more functionality onto a circuit board keeps pushing everyone in the IC manufacturing chain to miniaturize, making feature sizes and interconnect pitches smaller at every level. In the packaging area, chip scale packaging is the technology on the rise, and in some cases, dice are stacked in those packages. The next big step most likely will be thinning dice to fit into thinner packages, or to stack more in a package.

A good amount of thinning is done now, primarily by grinding wafers before singulation. Grinding has its limits, though, since it leaves scratches and stress points in the wafer. The typical method for 'healing' those problems is to wet etch ~50 µm of material from the backside after grinding. This usually involves taping the wafer before the etch and detaping after. Though taping has been automated, it still can be the bottleneck step in thinning a wafer.

Disco Corp. (Tokyo), in partnership with Toshiba and LINTEC, has developed a dicing before grinding (DBG) method to grind wafers and avoid etching. The 'dicing' is actually scoring to a depth of ~50 µm to separate the die. The wafer is then taped, and grinding is performed until the scores are reached from the back side. According to Disco, the resulting dice are stronger than those that have been diced after chemical thinning, and etching dice after the DBG method yields the strongest dice.

The increased use of flip-chip poses an extra challenge for wafer thinning. It is very difficult, if at all possible, to grind a wafer after it has been bumped. Wafers can be thinned before bumping, but only to a certain thickness. Estimates of the limit range from 250 µm to 500 µm. Thinning further requires a non-contact holding method, so it can be done after bumping.

So far, two companies have stepped forward with methods of etching the backsides of wafers without taping or contacting the wafer. Both methods use a stream of inert gas, like nitrogen, to levitate the wafer during etching. But the similarities end there.

SEZ's spin wet processing station has been used extensively in Europe to thin dice for smart card applications. The wafer rests face down on a stream of inert gas, and the chuck spins. When the chemicals are introduced, the gas stream protects the front and edge of the wafer. The wafer 'chuck' is moved vertically from one chamber to another for the different steps in the process.

The company provides the option to leave a polished finish on the back of the wafer, or a rough finish. The rough finish allows for greater adhesion of encapsulant or die attach materials, as well as reduced contact resistance. This rough finish does not contribute to crack formation or stress.

Tru-Si Technologies (Sunnyvale, Calif.) uses atmospheric downstream plasma (ADP) technology to perform the etching. The company uses a Bernoulli effect holder to levitate wafers over the plasma. The gas passes radially over the wafer, using the pressure differential to hold the wafer up. In addition to protecting the front and sides of the wafer, the holding gas also performs the bulk of the wafer cooling, since liquid chemicals are not cycled over the wafer.

ADP has an advantage over 'direct' plasma processes in that ADP is purely chemical. The short mean free path at atmosphere causes ions to neutralize, and reduces the kinetic energy of all species, before they reach the wafer. There is no ion impact damage, and the etching is isotropic.

With wet etching, of course, there are no energetic ions or vacuum pressures to worry about, and methods for handling the waste chemicals are known. However, bulk material etching with wet chemicals can consume a great deal of chemicals, especially if the need for bulk material etching rises rapidly. Since SEZ's method is very similar to well-known spin and spray processes, methods for minimizing chemical use through inlet design and recycling should be well-refined.

Although fluorine-based chemicals are used in the ADP process, Tru-Si claims the waste effluent contains very little PFCs, since the reactant gases are completely decomposed. The company claims only a simple water-based scrubber is required.

Tru-Si also is introducing the idea of through-silicon contacts. The first step in wafer processing is a deep etch, possibly 100 µm or more deep. The holes are coated with oxide, then with aluminum. After wafer processing, the backside is etched to expose the aluminum, which is used as a through contact. The company has demonstrated this method, using an etch recipe that leaves an oxide 'collar' protruding from the silicon, keeping it insulated from the aluminum.

More choices exist now for thinning wafers. IC makers soon may use them in production quantities, and in a variety of ways.

For more information:
www.disco.co.jp
www.sez.com
www.trusi.com