LEDmanufacturing technology
prevents use of non-flat substrates or sufficiently thick photoresists, and requires very precise positioning and alignment of the wafer with respect to the mask. While there have been many demonstrations and research studies, up until now the limited DOF has prevented application of this method in industrial fabrication, especially for high-resolution structures. The PHABLE technology promises to lift this restriction.
In order to explain the principle behind this new technology, we show the intensity distribution and self- image planes formed behind a linear grating in Figure 1a. According to the conventional method, the photoresist- coated substrate is precisely positioned at one of these self-image planes to record the pattern which has a DOF smaller than p2
the wavelength. In the PHABLE method the wafer is not kept stationary at a self-image plane. Instead, it is moved toward the wafer by a full Talbot period (p2 an integral or average image (Figure 1b).
/2λ) to record
The resultant image is shown in Figure 1c. This image is also periodic along the lateral direction but, interestingly, is not sensitive to the starting distance of the wafer from the mask. Therefore the image has effectively no DOF limitation. A further advantage is that the printed pattern has half the period of the grating in the mask, therefore a resolution gain is achieved with respect to the mask.
To ensure a reliable and reproducible lithographic process, the contrast of the aerial image has to be high enough so that the non-linear response of a photoresist converts the image into a binary pattern. An inspection of the calculated image in Figure 1c reveals a peak-to-valley intensity ratio of about three – this is ample contrast for photoresist exposure. In general, simulations show that images obtained with this method have high contrast, which is supported by the experimental results presented below.
Any pattern you like This principle illustrated in Figure 1 is applicable to both one-dimensional patterns, such as lines and spaces, and two-dimensional patterns, such as hexagonal or square lattices. Examples of patterns printed using this method are shown in Figure 2. A hexagonal pattern of holes with 500 nm period printed in photoresist is seen in Figure 2a, and top-down and cross-section images of a hexagonal array of holes with 600 nm period etched in a silicon wafer are shown in Figures 2b and 2c, respectively. Exposures were performed with a PHABLE tool using collimated UV light and a standard photoresist. In each case the wafer was displaced over one Talbot period during exposure to print large-area patterns over 2-inch wafers.
Evaluation of the printed structures showed that good uniformity and reproducibility were obtained despite an uneven gap and large resist thickness, proving that the pattern is indeed insensitive to the distance between the mask and the wafer. The large gap between the mask and the wafer ensures a practically unlimited lifetime for the masks.
Since PHABLE is a mask-based photolithography method, printing a different pattern simply requires a change of mask. What’s more, many different patterns can be simultaneously printed on a single chip or a wafer in much the same way as different circuits are printed on silicon wafers. The limiting resolution of the printed features depends on the wavelength of the light used, with the smallest period being close to half the wavelength.
/2λ, where p is the pattern period and λ is
PHABLE is ideally suited for patterning LED wafers because of its non-contact nature and ability to print over large topographical features and on non-flat surfaces. Photonic nanostructures can be created on LED surfaces after epitaxial deposition steps or on sapphire substrates before the device layers are grown. Relatively thick standard photoresists can be used, such as those with a thickness of 0.5-1.0 µm. This enables etching into semiconductor layers without the added complexity or cost of hard masks, such as SiO2
. Photonic crystal
patterns with various different periods, orientations or symmetries can be incorporated on individual chips to effectively tailor and control the distribution of light emission. The high reproducibility and uniformity of the lithographically produced patterns can improve yield and reduce the costs associated with binning products with large performance variations.
Other emerging technologies also stand to benefit from this innovative photonic patterning technology. For example, lithographic patterning for nanowire-based LEDs and photovoltaic devices can be accomplished with PHABLE. Heteroepitaxy on patterned silicon substrates and epitaxial lateral overgrowth for Blu-ray laser production are other potential applications. Wire-grid polarizers needed in both LCD displays and projectors are other areas where this technology can make a strong impact.
The PHABLE technology enables low-cost fabrication of photonic patterns. The time-tested approach of a mask- based UV exposure and its associated infrastructure will ensure a smooth adoption of this approach. In particular, there is no requirement to invent or develop new materials. Standard photoresists with optimized resolution and etch properties are available from multiple vendors. The infrastructure for mask fabrication is also already in place. This means that the HB-LED and other industries can rely on the usual, well-established sources for the required consumables and a low-cost process for realizing their photonic nanostructures.
We are now offering samples and wafer batch processing services to companies and researchers developing nanostructure-based products, who are interested in taking advantage of this breakthrough technology. We are also currently offering laboratory lithography tools for 2- inch to 4-inch wafers that are suitable for product development. High-volume production tools with throughput in excess of 100 wafer-per-hour will be made available to manufacturers in the near future. Many future photonic devices will shine even brighter with the introduction of our proprietary technology.
Further reading
Jonathan J. Wierer et al, Nature
Photonics, (2009) K Bergenek et al, IEEE J. of Quantum Electronics, (2009) F. Rahman, Optics and Photonics News, (2009)
November / December 2010
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