The internal transmittance of one ion exchanged surface glass layer was measured by placing the HEBS-glass plate 0.086" thick with one ion exchanged surface in the sample beam of the U2000 spectrophotometer and placing a base glass plate 0.090" thick in the reference beam. The internal transmittance from 350nm to 500nm of the ion exchanged glass layer is listed in Table 3.

Also listed in table 3 are the corresponding transmittance values of the base glass plate 0.090" thick, the HEBS-glass plate 0.090" thick having two ion exchanged surface glass layers (i.e. 2 IEed surfaces) and the HEBS-glass plate 0.086" thick having one ion exchanged surface glass layers (i.e. 1 IEed surface).

The residual sensitivity of HEBS glass to I-line at 365nm is not detectable for optical lithographic exposure of less than one million I-line stepper exposures.

The HEBS-glass mask can in general be employed in I-line as well as G-line optical lithographic exposure systems. However, it is recommended that the optical exposure tool for DOE fabrication whenever possible to use G-line stepper and/or contact aligners having exposure wavelengths longer than about 400nm.

An exchange of H⁺ and/or H₃O⁺ ions for alkali metal ions takes place concurrently with the exchange of Ag⁺ ions for alkali-metal ions when HEBS-glass is ion exchanged in an acidic aqueous solution containing silver ions. As a result, H⁺ and/or H₃O⁺ ions entered into the silicate glass network and silanol groups SiOH formed in the glass network. The formation of the silanol groups in a silicate glass is referred to as hydration of glass. HEBS-glass was hydrated, and a moving boundary type concentration profile formed. When water species are among the diffusion species in glass, the diffusion of water species (i.e., H⁺ and/or H₃O⁺) and Ag⁺ ions through a hydrated layer is accompanied by an instantaneous and irreversible immobilization of the diffusion species at the boundary surface. The moving boundary type diffusion profile is due to the fact that the diffusion coefficient of H⁺ , H₃O⁺ and Ag⁺ in the hydrated layer is many order of magnitude larger than that in the anhydrous base glass.

An essential feature of diffusion accompanied by an instantaneous and irreversible immobilization of the diffusion species is that a sharp boundary surface moves through the medium, separating a region in which all of the sites are occupied from one in which none are occupied. In front of the advancing boundary the concentration of freely diffusing species is zero whereas behind it immobilization is complete.

Fig. 1 is a qualitative representation of the result of silver ion exchange of HEBS-glass in an acidic aqueous solution containing soluble ionic silver. There exists a leached surface glass layer, x₁ in thickness, wherein essentially all of the alkali ions are leached out instead of being exchanged by Ag⁺ ions. The leached surface glass layer is essentially fused silica in composition and contains no mobile ions such as sodium, potassium and lithium ions. The standard HEBS-glass photomask blanks have a leached surface glass layer of about 0.5µm, i.e., x₁ = 0.5µm and has an e-beam sensitized glass layer of 3µm, i.e., x₂-x₁=3µm. HEBS-glass photomask blanks having an e-beam sensitized glass layer (x₂-x₁) of 2µm, 4µm, 5µm and other thickness' are available on special order.

By controlling the operation parameters of the solution ion exchange reactions, the thickness of the e-beam sensitized glass layer can be controlled precisely.

Coloring Speck of Silver

Upon e-beam exposure, coloring specks of silver are formed in the HEBS-glass. Since there are no chemical or physical development steps, the silver specks are of atomic dimensions and the image has no graininess. The recorded image has a continuous tone even when observed at the highest magnification under microscope. eg. 1500x. On the contrary, at this high magnification the image in a conventional high resolution photographic emulsion plate is intrinsically halftone, because isolated grains of photographic emulsion plates resembling dispersed halftone dots exist at gray levels of low optical density values.

Sub 0.25µm Resolution

Since there is no graininess, HEBS-glass is capable of very high resolution. Sub 0.25µm features were written in HEBS-glass.

Vertical resist profile exists at the boundary of sub micron resist features which were printed using HEBS-glass masks