Semiconductor devices and structures thereof are typically produced on a wafer or other bulk semiconductor substrate, which may be referred to herein as a “device wafer.” The array is then singulated into individual semiconductor devices, which may also be characterized as “dice” that are incorporated into a package for practical mechanical and electrical interfacing with higher level packaging, for example, for interconnection with a printed wiring board. Device packaging may be formed on or around the die while it is still part of the wafer. This practice, referred to in the art as wafer-level packaging, reduces overall packaging costs and enables reduction of device size, which may result in faster operation and reduced power demands in comparison to conventionally packaged devices.
Thinning device wafer substrates is commonly performed in semiconductor device manufacture because thinning enables more devices to be stacked in a given height, and helps dissipate heat. However, thinned wafer substrates are fragile and, thus, relatively more difficult to handle than unthinned wafer substrates of the initial wafer thickness without damage to the substrate or to the integrated circuit components thereon. To alleviate some of the difficulties, device wafer substrates are attached to larger and more robust carrier wafers. After processing, the device wafer substrates may be removed from the carrier wafers.
Conventional carrier materials include silicon (e.g., a blank device wafer), soda lime glass, borosilicate glass, sapphire, various metals, and ceramics. The carrier wafers are substantially round and sized to match a size and shape of the device wafer, so that the bonded assembly can be handled in conventional processing tools. Polymeric adhesives used for temporary wafer bonding are conventionally applied by spin coating or spray coating from solution or laminating as dry-film tapes. Spin- and spray-applied adhesives are increasingly preferred because they form coatings with higher thickness uniformity than tapes can provide. Higher thickness uniformity translates into greater control over cross-wafer thickness uniformity after thinning. The polymeric adhesives also exhibit high bonding strength to the device wafer and the carrier wafer.
The polymeric adhesive may be spin-applied onto the device wafer, the carrier wafer, or both. The coated wafer, conventionally the carrier wafer to conserve the thermal budget of the device wafer, is baked to remove all of the coating solvent from the polymeric adhesive. The device wafer and carrier wafer are then placed in contact in a heated mechanical press for bonding through the polymeric adhesive. Sufficient temperature and pressure are applied to cause the adhesive to flow and fill into the device wafer structural features and achieve intimate contact with substantially all mutually adjacent areas of the device wafer and carrier wafer surfaces.
Removal of the device wafer from the carrier wafer after processing is conventionally performed by chemical means (e.g., with a solvent), photodecomposition, thermomechanical means, or thermodecomposition. Each of these methods has drawbacks in production environments. For example, chemical debonding by dissolving the polymeric adhesive is a slow process because the solvent must diffuse over large distances through the polymeric adhesive to effect release. That is, the solvent typically must diffuse from the edge of the bonded substrates, or from a perforation in the carrier wafer, into the local region of the adhesive. In either case, the minimum distance from an exposed surface to a bonded area required for solvent diffusion and penetration is typically at least 3-5 mm and can be much greater, even with perforations to increase solvent contact with the adhesive. Treatment times of several hours, even at elevated temperatures (e.g., greater than 60° C.), are usually utilized for debonding, meaning wafer throughput is low.
Photodecomposition is, likewise, a slow process because the entire bonded substrate cannot generally be exposed at one time. Instead, an exposing light source, such as a laser having a beam cross-section of only a few millimeters, is focused on a small area at a time to deliver sufficient energy to decompose the adhesive bond line. The beam is then scanned (or rastered) across the substrate in a serial fashion to debond the entire surface, which leads to long debonding times and low wafer throughput.
Though thermomechanical debonding can be performed typically in a few minutes, it has other limitations that reduce device yield. Back side processes for temporarily bonded device wafers often involve working temperatures higher than 200° C. or even 300° C. If polymeric adhesives either decompose or soften excessively at or near the working temperature, debonding may occur prematurely. Adhesives are normally selected to soften sufficiently at about 20° C. to about 50° C. above the working temperature of the device wafer. The high temperatures required for debonding such adhesives imposes significant stresses on the bonded wafer pair as a result of thermal expansion. At the same time, the high mechanical force utilized to move the device wafer from the carrier wafer by a sliding, lifting, or twisting motion creates additional stress that can cause the device wafer to break or produce damage within the microscopic integrated circuitry of individual devices of the device wafer, which leads to device failure and yield loss.
Thermodecomposition debonding also tends to cause wafer breakage. Gases are produced when the polymeric adhesive is decomposed, and these gases can become trapped between the device wafer and the carrier wafer before the bulk of the adhesive has been removed. The accumulation of trapped gases can cause the thin device wafer to blister and crack, or even rupture. Another problem with thermodecomposition debonding is that polymer decomposition is often accompanied by the formation of intractable, carbonized residues that cannot be removed from the device wafer by conventional cleaning procedures.