Migration of metal ions in a solid body, such as a glass, is a well recognized phenomenon. The alkali metal ions, Li.sup.+, Na.sup.+ and K.sup.+, are generally the most mobile ions. The sodium ion (Na.sup.+), being the most mobile of the alkali metal ions in silicate glasses, is of particular concern here.
Migration of ions, particularly Na.sup.+, has considerable commercial significance. From a beneficial standpoint, the exchange of alkali metal ions at, and within, a glass surface is a well recognized mechanism for glass strengthening. On the other hand, the sodium ion is the most feared contaminant in the integrated circuit industry. It has a serious effect on electrical properties and behavior of materials.
An area of particular concern is liquid crystal display devices, whether passive or active. These devices customarily consist of thin, parallel, spaced glass panels with an intermediate, liquid crystal layer. A quite unrelated area of concern is chemical and pharmaceutical glass ware where high purity is often critical.
Initially, soda lime glass panels were used in producing passive liquid crystal display devices. When such panels were employed, degradation of the liquid crystal occurred at normal operating temperatures. Sodium ions at the glass surface exchanged for hydrogen ions, thus contaminating the liquid crystal.
To avoid this problem, a silica film was applied to the surface of the glass panel. This film acted as a barrier layer to stop sodium ion migration from the glass, and thus prevented exposure of the liquid crystal layer to the sodium ions.
Subsequently, panels were produced from a glass that contained no more than about 0.1% by weight Na.sub.2 O. Availability of these glass panels made it unnecessary to employ a barrier layer film in passive matrices. However, maintaining such low levels of alkali in a glass imposes severe limits on the physical properties of the glass.
Active matrix liquid crystal displays (AMLCDs) have recently become increasingly popular. These displays utilize either amorphous silicon or polysilicon thin film transistors. The use of a barrier layer is optional, although recommended, in conjunction with the former.
However, producing a polysilicon thin film transistor involves processing temperatures that approach the strain point of the glass. At these temperatures, sodium, even at the low level in the glass, creates a contamination problem. As a result, it is necessary to provide a barrier layer on the glass panel to prevent sodium ion migration that results in device instability.
Pending application Ser. No. 07/853,587 was filed Mar. 18, 1992, and refiled Oct. 5, 1993 as Ser. No. 08/132,554. Both were filed in the name of F. P. Fehlner and assigned to the assignee of the present application. These applications were based on the discovery that an inert, refractory oxide film can perform dual functions. Thus, such a film can function as both a barrier layer film against sodium migration, and as a parting agent to prevent glass adhesion during a compaction process. Thereafter, the barrier layer remains on the glass panel becoming part of the finished display device.
Heretofore, it had been assumed that sodium ion migration occurred only from a glass toward a barrier layer film having a smaller sodium ion concentration. The present invention is predicated on our discovery that this assumption is not correct. Rather, we have found that sodium ion migration can occur in either direction depending on certain factors. Our present invention is based on our determination of these factors, and, consequently, a mechanism for controlling the direction of ion migration.