1. Field of the Invention
The present invention generally relates to semiconductor device fabrication and more particularly to a process of forming ultra thin wafers having an edge support ring.
2. Description of Related Art
Vertical power devices including power MOSFETs and insulated gate bipolar transistors (IGBTs) are being fabricated on ultra thin wafers on the order of less than 4 mils. The fabrication of these power devices on ultra thin wafers provides for devices with reduced electrical and thermal resistances. The use of ultra thin wafers further ensures that the power devices meet stringent total package thickness requirements. Furthermore, ultra thin wafers—including ultra thin float zone wafers—are replacing expensive wafers with an epitaxially formed silicon layer for device junctions and buffer zones.
For ultra thin wafers with thickness smaller than 4 mils, wafer bowing and warping, and/or breakage and fracture occur frequently during handling processes. The prior art generally provides for two wafer thinning and handling approaches. In a first approach, a temporary support substrate such as a handle wafer, a glass substrate, a thick tape, a polymer substrate, and a polymer based composite substrate is applied. Disadvantageously, this first approach requires complex operations when applying and removing the temporary support structure. Some removal processes may involve risk of wafer breakage and fracture. Additionally, adhesives and other polymeric materials used to secure the wafer to the temporary support structure may outgas in the vacuum chamber during metallization and adversely affect the quality of the ohmic contact.
In a second approach, a support ring is formed at a wafer edge to facilitate handling and processing of the ultra thin wafer. This second approach advantageously eliminates the introduction of extraneous materials such as adhesives into the processing of the ultra thin wafer. As disclosed in U.S. Pat. No. 6,162,702 entitled “Self-supported Ultra Thin Silicon Wafer Process”, and illustrated in FIGS. 1 and 2, a silicon wafer 2 has an ultra thin central portion 4 that is supported by a circumferential rim 3 of thicker silicon. Removing a volume of silicon from the silicon wafer 2 by a controlled mechanical or physical means such as grinding, milling, drilling or laser forms the ultra thin central portion 4. Alternatively, a mask may be formed on the rim of the silicon wafer 2 and the silicon wafer 2 may be etched to form the ultra thin central portion 4.
With reference to FIGS. 3, 4 and 5, a grinding wheel 30 having teeth 32 is shown for use in grinding a backside 34 of a wafer 31 and forming a thick edge support ring 36 therearound. The grinding wheel 30 has an axis of rotation “G” while the wafer 34 has an axis of rotation “W”. Grinding of the wafer backside 34 yields an ultra thin central portion 38 having a thickness less than 4 mils surrounded by the edge support ring 36. The edge support ring is roughly the same thickness as the original wafer. The edge support ring is at least 2 mm wide. The wider the ring, the stronger it is, but also the less space is available to form dies. The edge support ring 36 facilitates the handling of the wafer 31.
Following the back grinding process, it is conventional to spin etch the ultra thin central portion 38 of the wafer 31 before the back metallization process. The spin etch process includes a chemical etch of silicon and oxides followed by cleaning with de-ionized water. The spin etch process increases the mechanical strength of the ultra thin central portion 38 and ensures good ohmic contact between the metal subsequently deposited and the highly doped silicon substrate of the wafer 31. Following the spin etch process, no drying process is necessary before loading the wafer 31 into the vacuum chamber for back metallization.
It has been discovered that the structure of the edge support ring 36 disadvantageously interferes with the spin etch process. As seen in FIG. 6 and indicated by the arrows, rotation of the wafer 31 about the axis of rotation “W” causes the chemical etch and de-ionized water to flow outwardly and encounter a wall 60 of the edge support ring 36 formed at a right angle or nearly a right angle to the plane of the ultra thin central portion 38. The wall 60 prevents the entirety of the chemical etch and de-ionized water from being spun from the wafer 31 during the spin etch process. As such, additional cleaning and/or drying steps are required before the back metallization process can be effectively performed. Alternatively, the wafer 31 may be baked at a high temperature to remove the residual chemical etch and de-ionized water. In either case unacceptable complexity or delays in the fabrication process may be introduced by the structure of the edge support ring 36 of the prior art that additionally may adversely affect the quality of the back metal contact.
There remains a need in the art for a process of forming ultra thin wafers having an edge support ring that overcomes the limitations of the prior art. There is also a need in the art for a process of forming ultra thin wafers having an edge support ring that provides for an edge support ring compatible for use with conventional spin etch processes before back metal deposition. There is a further need for an edge support ring having an angled inner wall.