Glass etchants

The TFTs are made on transparent glass substrates, onto which gate electrodes are patterned. Typically, the gate electrode is made of chromium. This substrate is introduced in a PECVD reactor, in which silane and ammonia are used for plasma deposition of SiN as the gate material. After subsequent deposition of the a-Si H active layer and the heavily doped n-type a-Si H for the contacts, the devices are taken out of the reactor. Cr contacts are evaporated on top of the structure. The transistor channel is then defined by etching away the top metal and n-type a-Si H. Special care must be taken in that the etchant used for the n-type a-Si H also etches the intrinsic a-Si H. Finally the top passivation SiN, is deposited in a separate run. This passivation layer is needed to protect the TFT during additional processing steps. 

A transparent electrode substrate was prepared by coating indium tin oxide (ITO) on a glass substrate and washing the substrate. ITO was then patterned using a photoresist resin and an etchant to specified patterns and the substrate washed. A hole injection layer was formed by coating a selected experimental agent dissolved in toluene to a thickness of about 50 nm and baking at 110°C for 1 hour. 

A second practical aspect is the up-scaling of the etching equipment, which is not available as an off-the-shelf product from equipment manufacturers. The task is to etch glass sheets of a size of about 1 m2 in diluted HCI with excellent homogeneity over the whole substrate area. This requires precise control of both etch time and etchant concentration, while additionally the requirements of a clean process have to be fulfilled to avoid layer imperfections or pinholes in the silicon layers. With a prototype in-line etcher, we have achieved excellent uniformity of the etching step on substrates up to 30 x 30 cm2 [48]. This process can be easily up-scaled to production size. 

Usually, dry etching of polyimide is performed by RIE with O2, CF, or their mixtures as an etchant gas, utilizing positive photoresist (2), metals such as aluminum (3), spin-on-glass (4), or SiN (5) as an etching mask. However, in the former case, it is difficult to define a fine via-hole as small as 2 pm or less because the resist thickness must be two or more times that of the polyimide as a result of the equal etching rates between photoresist and polyimide. In the latter case, though the fine pattern can be obtained the additional pattern transformation from the photoresist to the masking layer is necessary. 

Dissolution experiments were carried out in a 500 ml glass beaker containing 400 ml of the etchant solution. A rectangular copper coupon (2.3 cm x 2.3 cm x 0.2 cm, 99.99 % pure) was used as the sample. The copper coupon was first washed with dilute HCl to remove any native oxide from the surface, dried in an mr stream and weighed. It was then immersed in the solution for a predetermined time interval. The solution was stirred using a mechanical stirrer at 1000 rpm, unless specified otherwise, to minimize mass transfer effects. The coupon was removed, washed repeatedly with D1 water, dried in an mr stream and reweighed. The weight loss was... 

The bifluorides syn. hydrogendifluorides, are crystalline solids containing the HF2 species. Examples include sodium bifluoride and potassium bifluoride. They are crystalline solids soluble in water or water-alcohol solutions and extremely corrosive to metals, glass, and other siliceous materials. They are used as etchants. 

Fig. 4.19 Phase behavior of P(F)S49-b-PLA192 (, on silanized ITO-coated glass, a Cross-section revealing that the top part of the film formed cylinders orientated perpendicular to the substrate which were voided during PLA etch. In contrast, cylinders aligned parallel to the substrate were adopted at the bottom part of the film which were not voided because of lacking etchant infiltration. Compared to Fig. 4.15 this highlights the advantages of a multicontinuous structure like the DG. b Top-view onto lying cylinders forming a terrace-like structure...

As implied from the Materials section, the more corrosion-resistant materials will require more aggressive etchants than easy-to-corrode materials. Thus, ceramics and glasses are usually etched in etchants containing the toxic and hazardous etchant hydrofluoric acid (HF), certain polymers such as polyimide are etched in... 

Acetic acid Ammonium bifluoride Ammonium fluoborate Fluoboric acid Nitric acid etchant, semiconductor prod. glass Hydrofluoric acid etching... 

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