The Wave Front: Cutting-Edge Applications Using Spectroscopic Ellipsometry

The Wave Front: Cutting-Edge Applications Using Spectroscopic Ellipsometry
◆Perovskites
In the 2000s, research and development in thin-film solar led to higher efficiency solar panels at a lower cost, primarily driven by the performance of CdTe and CIGS panels. Between 2009 and 2012, the price of crystalline silicon panels dropped by almost 3x, eliminating the cost-benefit advantage of thin-film solar, leading to a collapse of the thin-film solar market and reduced research in this area. Lower-cost silicon panels have been great news for consumers, but the general excitement in research and development of solar diminished as funding slowed. Fast forward a few years, and thin-film research for solar has again gained momentum, but now driven by perovskites.
Perovskite is the general term referring to organic-inorganic hybrid materials that have an octahedral crystal structure (shown above) with the formula ABX3 where A is an organic cation; B is generally lead, cadmium, or tin; and X is iodine, chlorine, or bromine. The hybrid material acts as a highly efficient absorber for use in solar cells and can be produced at a significantly lower cost than bulk crystalline silicon.
Research into perovskites has substantially increased over the last decade. Scientists have investigated different material hybrids, searching for the best combination of efficiency and stability. Researchers caution that our understanding of perovskites for solar cells is still in the early stages of development with many challenges ahead. One often-discussed challenge involves the lifespan of perovskites. In the early days, degradation caused by air and moisture was so rapid that some of the perovskite cells lasted only seconds in the open atmosphere. Since then, lifespans have grown to weeks, then months, and now years, but there’s still a long road ahead to achieve the 25-year lifespans of crystalline silicon. While challenges remain, researchers are optimistic about the outlook for perovskite solar cells.
◆Lithography + Computer Chips
Electronic devices are continuously updated with noticeable performance improvements in every new generation of computer, laptop, smartphone, and gaming system. The technology driving these improvements includes faster processor chips, memory chips with larger capacity, and higher-resolution cameras and displays. In each case the circuitry in these chips and displays is smaller than the previous generation, allowing billions of transistors to occupy a small area of silicon and to be used for smaller, higher-resolution pixels for ultra-high-definition cameras and displays.
How is this accomplished? Optical lithography is a method of patterning circuitry or pixels directly onto a semiconductor or glass surface (a lithograph). The circuit pattern is first laid out on a glass plate called a photomask. The photomask contains areas that transmit light and others that reflect or absorb light. Ultraviolet light shines through the mask, and the circuit pattern focuses on the surface. The circuit pattern must be captured by the surface, which is accomplished by a layer called a photoresist. A photoresist acts like photographic film but is actually a thin film applied to the surface. Areas of the photoresist exposed to light from the mask become harder to etch (i.e., resist etching) than areas not exposed. To see the circuit pattern, the resist must be etched using wet chemicals, somewhat similar to photographic film processing.
Ellipsometry is used to determine the thickness and absorption properties of photoresist films for semiconductors, phase-shift layers on photomasks, and the thickness and optical properties of the red, green, and blue subpixel films in displays. Ellipsometry helps make sure these properties are uniform over the entire surface of the wafer or glass panel, which is always challenging since they keep increasing in size!
◆Optical Coatings + Lenses
Have you ever wondered why camera lenses often have a purple or green color? How are colored sunglasses made? Your optometrist recommends anti-glare coatings for your eyeglasses. How’s it done? With optical coatings! These coatings are vacuum deposited with their thickness and optical properties monitored with spectroscopic ellipsometry.
Historically, a single layer of magnesium fluoride (MgF2) was the first antireflective optical coating, which is still used today. MgF2 is often the purple color you see on camera and binocular lenses. Transmission through the lens is improved, since the MgF2 coating has a refractive index between air and the glass lens, which
serves to reduce reflections at the air-glass interface. Coatings can be made better by adding a second layer, causing even lower reflectance over a wider spectral range.
With enough interfaces, it’s possible to make mirrors with reflectance values extremely close to 100%! This is accomplished using many layers with different refractive indices. Reflections occur at each interface in the stack, so reflections from many interfaces combine to create extremely high reflectance. However, the stack must be carefully designed for maximum reflectance at a particular wavelength or band of wavelengths. These multilayer film stacks can be complicated to design, and the layer stacks are even more difficult to produce, which is where ellipsometry can help to monitor the coating process in real time.
The optical properties can vary with deposition conditions, so ellipsometry is used to determine if the film thickness is correct and if the optical properties such as refractive index and absorption are correct. These values are used to further refine the stack design and performance. Additionally, by combining this with lithography and displays, the world of AR/VR becomes possible!
◆Displays
Pure electrical insulators are highly transparent, but highly conductive metals are opaque. So what material can be used that is both transparent and conductive? We must find such a material to make our displays and touchscreens work.
Indium tin oxide (ITO) is both transparent and conductive. ITO optical properties vary a lot with deposition conditions and annealing, so monitoring the quality of the ITO and its thickness is important. Ellipsometry is used for monitoring ITO film thickness and transparency at visible wavelengths while also being sensitive to absorptions in the ultraviolet, but it is very important for monitoring absorption in the infrared, which corresponds to the film’s electrical conductivity. Improvements in display speed are critical for fast action in movies and sports. Imagine a baseball, golf ball, or hockey puck leaving a comet tail behind on the screen because the display cannot refresh fast enough. Display pixel speed has been significantly enhanced using crystallized silicon films on the rear panel of the display. Ellipsometry is used to monitor the thickness and crystallinity of these deposited polysilicon films.
LCD displays used to be small, monochrome in color, and used in calculators, digital watches, etc. Modern displays are full color, extremely fast, and very large. The coated films must be uniform over the entire panel size, and film uniformity has been a limiting factor in the size of displays for decades. With each new generation of larger displays, the film uniformity must be maintained. An ellipsometer can fly over the large panels as they move on a production line to monitor film quality.

來源出處JAW_NEWSLETTER_2023_Interactive