Chromium etchants

Optical Exposure. Multicomponent LB films were prepared from solutions of novolac/PAC varying in concentration from 5-50 wt% PAC, and transferred at 2.5 -10 dyn/cm. The films were composed of 15 - 20 monolayers, with an average film thickness of 30 nm, as measured by ellipsometry. Exposures were performed with a Canon FP-141 4 1 stepper (primarily g-line exposure) at an exposure setting of 5.2 and with a fine line test reticle that contains line/space patterns from 20 to 1 pm (40 to 2 pm pitch). They then were then developed in 0.1 - 0.2 M KOH, depending on the PAC content The wafers received a 20 min 120°C post development bake to improve adhesion to the Cr. Finally, the Cr was etched in Cyantek CR-14 chromium etchant, and the resist and Cr images were examined by SEM. 

Non-chromium etchants have been used successfully by some manufacturers in order to reduce the toxicides of waste streams (USEPA 1986). 

Gate Piranha clean Gate evaporation Photo Etch Strip 30 minutes in fresh, hot solution 5nm Cr/lOOnm An (e-beam or thermal) Gate mask KI, rinse, chromium etchant, rinse Soak in stripper 10 min, rinse in water soak in fresh stripper 10 min, rinse in water, dry... 

See, for example, Chromium etchants applications notes, Cyanetek Corp., Fremont, CA [cited in H.J. Levinson, Principles of Lithography, 2nd ed., p. 261, SPIE Press, Bellingham, WA (2005)]. B.J. Curtis, H.R. Brunner, andM. Ebnoether, Plasma processing of thin chromium films for photo masks, J. Electrochem. Soc. 130(11), 2242 2249 (1983). 

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. 

Chromium from the etchant and copper from the boards are typical contaminants in spent etchant solution and rinse water. 

T. C. Chandler, MEMC etch— A chromium trioxide-free etchant for delineating dislocations and slip in silicon, J. Electrochem. Soc. 137, 944, 1990. 

After photolithographic pattern definition (which is covered in more detail in the next section), gold can be etched in a 30% solution of KI in DI water, then rinsed. The chromium adhesion layer can be etched using any of several etching cocktails [62] [63] or pre-mixed commercial reagents (e.g. Transene Chromium Mask Etchant), then rinsed and stripped. 

Chromium is readily etched by aqueous solution of ceric salt, prepared from ceric nitrate, percholoric acid, and water. Chrome masks are now being fabricated with this type of etchant solution. ... 

A second, more sophisticated type of devices was fabricated in the following way. First a temporary gold layer, which closed the nonconducting gap between the two chromium electrodes, was evaporated. Subsequently a voided double-gyroid-structured template was prepared on this continuously metal-coated substrate and refilled with PPy. Then the redundant template was dissolved with xylene. Finally the superficial gold layer was etched with a wet gold etchant, which reestablished the gap between the chromium electrodes. 

Waste disposal. Spent or by-produd etchant is usuahy sent off-site for copper reclamation. There is usually a fee for this service depending on copper content and distance to the reclamation fadhty. The solutions must be free of unreacted oxidizer (see previous chlorate system discussion). Etchant can contain traces of zinc, chromium, and arsenic from the foil treatments. 

Etching Spent etchant Waste rinse water Ammonia, chromium, copper, iron, acids... 

Salts Ammoniimi Fluoride (etchants) Boron trichloride Chromium trioxide Cupric nitrate Ferrous sulfate Tetraethyleortho silicate Trimethyl borate Timgsten hexafluoride... 

Various etchants have been recommended for the remaining metals of engineering practice, but it is doubtful if sufficient work has been reported to differentiate between them or to assess their effect on the durability of the bonds formed with different adhesives. Strong, durable bonds are uncertain with copper because of the ease with which a weak, friable oxide is formed. Even when coated with an adhesive, oxygen can diffuse to the interface and eventually cause failure. Brass has an oxide film almost entirely of zinc oxide and, as with zinc galvanized iron, it can hydrate or form salts with the tackifiers added to some contact adhesives. Cadmium is met with as a plating if a strong, durable adhesive bond is essential, it should be replaced by chromium, the surface of which can be treated as stainless steel. 

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