Methods of improving the surface flatness of thin glass sheets and silicon wafers

The manufacturing of high quality sheet glass has allowed for many technologies to advance to astonishing frontiers. With dimensions reaching ∼3 × 3 m², sheet glass is pushing the envelope for producing massive size flat panel displays that can be hung on walls like paintings. Many other applications utilize sheet glass, such as the hard disk drive industry for making platters, the x-ray telescope industry for making high precision optics, and the semiconductor industry for making masks and substrates.

The exceptional optical qualities of sheet glass give them a leading advantage in many technologies; however, one main impediment that remains with manufacturing larger sheets is their surface waviness. The sheets have large warps, on the order of hundreds of microns, that present many challenges in all the industries utilizing such sheets, especially in the liquid crystal display and precision optics industries. The thinner the sheets, the larger their waviness, thus placing a limit on the minimum thickness that can be used in such applications before surface distortions become unacceptable.

A novel method of shaping sheet glass is presented. This method reduces the surface waviness of a glass sheet and changes its shape while it is in its hot state and without contacting its surface. A sheet of glass is inserted between two parallel porous mandrels such that it is at a predefined distance from the two. A thin layer of pressurized gas flows through each mandrel and out against the glass surfaces. The resulting viscous flow against the heated soft glass sheet changes its surface topography. By using flat mandrels and controlled pressurized gas at temperatures close to 600°C, the outcome is a flat sheet of glass with its original immaculate optical qualities.

The flow in porous mandrels and the resulting pressure distribution along the surfaces of a glass sheet inserted between two porous mandrels is modeled. The design and manufacturing of an apparatus used to reduce the surface waviness of glass sheets at elevated temperatures is described. The apparatus designed addresses individual sheets; however, guidelines on how to incorporate this method of shaping glass in a continuous glass sheet manufacturing facility are provided. A method of rigidly assembling stacks of glass and silicon sheets with precision for x-ray telescope mirrors and gratings is also presented. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)