Amorphous? Again??
by John F. Wager
In the beginning, there was amorphous hydrogenated silicon. That is the gospel; at least with respect to the materials aspects of flat-panel-display backplane technology. More recently, there are amorphous-oxide semiconductors. The article I have contributed to this issue’s special coverage of materials, “Oxide TFT: A Progress Report,” is essentially a report card on the above-mentioned materials. They form the basis of oxide thin-film-transistor (TFT) or indium gallium zinc oxide (IGZO) technology. Oxide-TFT or IGZO technology is vital to what is going on with backplanes today. The other materials- related article in this issue, “Amorphous-Metal Thin Films Enable UHD Display Backplanes,” by Sean Muir, Jim Meyer, and John Brewer from Amorphyx, Inc., proposes that the future is the amorphous-metal non-linear resistor (AMNR). What gives with this obsession with amorphous?
It comes down to scalability. Amorphous thin films are homogenous in two dimensions. Amorphous thin films do not possess grain boundaries. This is important – very important! Impurities and defects tend to originate from and/or segregate to grain boundaries. Grain boundaries introduce bulk and surface morphological inhomogeneities to a thin film. Thus, compared to an amorphous thin film, a polycrystalline thin film with grain boundaries is harder to manufacture and integrate into a product, tends to be less stable and reliable, and is much more challenging to scale to the large-area meter-sized dimensions required for high-volume flat-panel-display commercial applications. Bottom line: amorphous thin films scale.
OK. But why an amorphous metal? Answering this question requires a bit of explaining. The starting point is to employ a thin-film diode (TFD) as a flat-panel-display backplane switch instead of using a TFT. However, Amorphyx does not want to use a conventional TFD, i.e., a metal-insulator-metal (MIM) device in which the insulator possesses a large density of traps so that electronic conduction through the insulator occurs by trap-assisted thermal emission. (This is often referred to as Poole–Frenkel emission.) Poole–Frenkel-based TFDs do not have the manufacturability, performance, or reliability required for next-generation commercial flat-panel displays (relying on a material filled with traps is usually a dicey proposition). Instead, the Amorphyx approach is to use a new type of MIM device in which electronic conduction through the insulator occurs by quantum-mechanical tunneling, often referred to as Fowler–Nordheim tunneling. Fowler–Nordheim TFDs appear to be much better suited to meet the commercial challenges associated with future flat-panel-display products. In order to fabricate a Fowler–Nordheim TFD, Amorphyx employs an amorphous-metal bottom contact because it has an atomically smooth surface. Thus, an amorphous metal is required for its atomically smooth surface, allowing for control of the uniformity of the electric field across the TFD insulator, thereby facilitating the realization of a TFD based on Fowler–Nordheim tunneling.
That is why an amorphous metal is used. However, there is a bit more to this story. Even with the availability of higher-performance Fowler–Nordheim MIM diodes, a conventional single-TFD switch backplane architecture is unlikely to be able to compete with alternative state-of-the-art TFT-based flat-panel-display backplane technologies. Instead, a dual-select diode strategy is required. Basically, a dual-select diode pixel consists of two identical TFDs that are biased across two select lines. Essentially, an AMNR is an elegant realization of a dual-select diode pixel. That is the basic idea. A more complete introduction to AMNR technology is presented in the article in this issue by Muir, Meyer, and Brewer.
What are the commercial prospects of AMNR backplane technology? Here is my take.
Advantages:
• A simple 2–3 mask process using only three thin-film layers.
• Temperature-independent operation expected for Fowler–Nordheim-based TFDs.
• Light insensitivity anticipated due to absence of a semiconductor.
Challenges:
• Insulator fabrication via plasma-enhanced chemical vapor deposition, or is atomic layer deposition required?
• Performance/Stability/Reliability/Scalability?
The Amorphyx vision is “Simple” – for a change. Whether or not the company’s undoubtedly simple backplane approach can satisfy the extraordinarily demanding requirements associated with flat-panel-display commercialization will require (i) validation of the technology via prototype fabrication and testing and (ii) feasibility scaling to ensure that expectations associated with amorphous thin-film scaling are indeed warranted for this new case of an amorphous metal.
So, regardless of whether you speak about the past (amorphous hydrogenated silicon), the present (amorphous oxide semiconductors), or the Amorphyx-envisioned future (amorphous-metal non-linear resistors) of flat-panel-display backplane technology, I suspect that “amorphous” will be a useful adjective.