News Article
Gallium Nitride LEDs: Bridging Substrate Size Evolution
Gallium nitride based technology is generating new and exciting applications that will dramatically change how we view the world and Unaxis is participating at the advancing edge.
Gallium nitride based technology is generating new and exciting applications that will dramatically change how we view the world and Unaxis is participating at the advancing edge. GaN’s wide electronic bandgap (Eg) opens opportunities for novel electronic and optoelectronic devices that include light emitting diodes (LEDs), laser diodes (LDs), and high frequency, high voltage power, and high temperature sensors. The relatively recent emergence of this material due to advances in its epitaxial growth means that many of the applications are only on the edge of market penetration. Only LEDs, with the ability to provide blue, green, ultra-violet, and white light emission have achieved strong commercial success. GaN LEDs have experienced remarkable double digit growth (~17.6% CAGR) and $3.2 billion in 2004 sales from uses in illumination, displays, automobiles, backlighting, and signals. Unaxis recognizes the impact of these applications and provides a tool set to address this rapidly expanding market.
Though LED manufacturing costs are spread among many important factors, such as packaging and efficiency, the epitaxial material itself is perhaps one of the most significant contributors. To produce the necessary film quality, the epitaxial films of interest, GaN, InGaN, and AlGaN, are presently grown on relatively small substrates. Currently substrates are either sapphire or silicon carbide (SiC) for lattice matching and are a de facto industry standard of 50 mm or 2 inches in diameter. However, as metal organic chemical vapor deposition (MOCVD) growth technology progresses, defects are reduced and substrate size is increasing. This substrate size increase will reduce costs in much the same way as the mainstream silicon industry; due to the advantageous scaling and processing fewer wafers.
This evolution in GaN substrate size undoubtedly presents some interesting production requirements.. Of particular relevance is how processing tools will address the overlap period during the increase in substrate size from 2 inch to 3 inch to 4 inch diameter. Borrowing from the mainstream silicon history where the predicament of substrate size changes are well documented, the tool sets used to address these transition periods are referred to as “bridge” tools. Hence, the following discussion in this article will focus on the Unaxis solution for GaN bridge tools for etching processes.
In a typical GaN LED etching steps are usually on the frontside and are for mesa or an isolation street. Other etching steps such as defining masking materials, shaping, or etching vias in SiC substrates are also potentially required. A typical LED with various features is shown in Figure 1. Since wet etching of GaN is uniquely difficult, dry etching using either RIE (reactive ion etching) or ICP (inductively coupled plasma) with chlorine based chemistry is standard. As described, presently most GaN manufacturers are using 2” substrates and will likely transition to large substrates over in the near future. Since 2” single wafer processing is prohibitively expensive the accepted approach is to run processes on batches of several wafers. Batch handling can be addressed with Unaxis tool sets in various configurations to accommodate specific customer needs.
The primary goal of a bridge tool is to facilitate as efficiently and with as much flexibility as possible the requirements to run in batch or single wafer modes. But this is not the only important requirement. Due to the high cost of epitaxial substrates, there is often a request that pieces or wafer fragments can be handled; an unusual condition when contrasted to the mainstream silicon industry. Another demanding restriction is a result of the relatively small substrate size (2 inch diameter). Little or no physical exclusion of the wafer is often preferred to maximize the process-able surface area. Additionally, process flows often require photoresist as the masking material for the dry etching. Using photoresist masks and providing high GaN etching rates is particularly challenging when batch processing is requested. In addition to these requirements is the need to satisfy ever increasing throughputs with enhanced etching rates. By offering both RIE and ICP configurations the enhanced etching rates can be achieved. Process parameters must be sufficiently aggressive to etch the GaN yet mild enough to permit resist removal following the etching. Fortunately, this difficult set of process requirements can be met with the Unaxis SHUTTLEINE and VERSALINE platforms.
SHUTTLELINE
The SHUTTLELINE is an economical approach using a shuttle that can transport into the process module individual clamped and cooled wafers up to 8 inches or a shuttle that can hold an unclamped batch of 2 inch and 3 inch wafers. By satisfying the many requirements described in the previous paragraph, this platform has found acceptance among many GaN LED fabrication facilities. This platform utilizes a manual handling system with a locklock due to corrosive gas use. Figure 2 shows a shuttle loaded with either seven 2 inch substrates, four 3 inch substrates or a single 4 inch substrate. With this arrangement it is convenient to include fragments or substrates. Processed area is provided in the figure as a reference point for single load throughput. The entire substrate area is provided since these shuttle held substrates are unclamped and the entire wafer surface is processed. Process conditions are adjusted to masking materials and take into account the clampless arrangement. The flexibility of this platform is recognized by its transparent capability to also process other diameter substrates by simply changing the shuttle size A more automated system that can be adapted to higher volume is the VERSALINE.
VERSALINE
This platform has the batch and single wafer processing capability of the SHUTTLELINE but can also be automated to include cassette to cassette transfer of substrates or carriers. With the increased automation it is straightforward to run a variety of configurations for varying capacity, wafer sizes, and process demands. The VERSALINE, designed with flexibility and modularity from the start, can accommodate configurations ranging from a single carrier, to a stack of four carriers, to twelve carriers to cassette-to-cassette of single wafers. Table 1 shows the volume of wafers that can be placed in the loadlock with the various configurations. An unprecedented 84 2inch substrates using seven substrates on each of 12 carriers with can be run without operator involvement. Just as easily a cassette of twenty-five 4 inch substrates can also be run.
Table 1: Wafer volume per system loading.
Wafer Size Single Batch
Carrier 4 Carrier
Configuration 12 Carrier
Configuration Cassette-to-Cassette
2 inch 7 28 84 NA
3 inch 4 16 48 25
4 inch 1 4 12 25
Cost effective upgrades that can quickly be made in the field allow the customer to efficiently match their tool set with capacity needs.
Process Considerations
Similar to hardware flexibility, processes must be adapted to the range of masks, GaN material quality, and desired profiles. Thus, it is difficult to establish a one-size fits all process. Although for cost considerations or tool matching, RIE is sometimes preferred, the well known ICP configuration with control over the physical and chemical components is often superior. In this arrangement, there are process conditions that will enhance the etching rate yet moderate the substrate temperature without clamping. Excellent cooling is obtained with individual substrate clamping and etching rates can be further increased without concern for burning photoresist.
Though LED manufacturing costs are spread among many important factors, such as packaging and efficiency, the epitaxial material itself is perhaps one of the most significant contributors. To produce the necessary film quality, the epitaxial films of interest, GaN, InGaN, and AlGaN, are presently grown on relatively small substrates. Currently substrates are either sapphire or silicon carbide (SiC) for lattice matching and are a de facto industry standard of 50 mm or 2 inches in diameter. However, as metal organic chemical vapor deposition (MOCVD) growth technology progresses, defects are reduced and substrate size is increasing. This substrate size increase will reduce costs in much the same way as the mainstream silicon industry; due to the advantageous scaling and processing fewer wafers.
This evolution in GaN substrate size undoubtedly presents some interesting production requirements.. Of particular relevance is how processing tools will address the overlap period during the increase in substrate size from 2 inch to 3 inch to 4 inch diameter. Borrowing from the mainstream silicon history where the predicament of substrate size changes are well documented, the tool sets used to address these transition periods are referred to as “bridge” tools. Hence, the following discussion in this article will focus on the Unaxis solution for GaN bridge tools for etching processes.
In a typical GaN LED etching steps are usually on the frontside and are for mesa or an isolation street. Other etching steps such as defining masking materials, shaping, or etching vias in SiC substrates are also potentially required. A typical LED with various features is shown in Figure 1. Since wet etching of GaN is uniquely difficult, dry etching using either RIE (reactive ion etching) or ICP (inductively coupled plasma) with chlorine based chemistry is standard. As described, presently most GaN manufacturers are using 2” substrates and will likely transition to large substrates over in the near future. Since 2” single wafer processing is prohibitively expensive the accepted approach is to run processes on batches of several wafers. Batch handling can be addressed with Unaxis tool sets in various configurations to accommodate specific customer needs.
The primary goal of a bridge tool is to facilitate as efficiently and with as much flexibility as possible the requirements to run in batch or single wafer modes. But this is not the only important requirement. Due to the high cost of epitaxial substrates, there is often a request that pieces or wafer fragments can be handled; an unusual condition when contrasted to the mainstream silicon industry. Another demanding restriction is a result of the relatively small substrate size (2 inch diameter). Little or no physical exclusion of the wafer is often preferred to maximize the process-able surface area. Additionally, process flows often require photoresist as the masking material for the dry etching. Using photoresist masks and providing high GaN etching rates is particularly challenging when batch processing is requested. In addition to these requirements is the need to satisfy ever increasing throughputs with enhanced etching rates. By offering both RIE and ICP configurations the enhanced etching rates can be achieved. Process parameters must be sufficiently aggressive to etch the GaN yet mild enough to permit resist removal following the etching. Fortunately, this difficult set of process requirements can be met with the Unaxis SHUTTLEINE and VERSALINE platforms.
SHUTTLELINE
The SHUTTLELINE is an economical approach using a shuttle that can transport into the process module individual clamped and cooled wafers up to 8 inches or a shuttle that can hold an unclamped batch of 2 inch and 3 inch wafers. By satisfying the many requirements described in the previous paragraph, this platform has found acceptance among many GaN LED fabrication facilities. This platform utilizes a manual handling system with a locklock due to corrosive gas use. Figure 2 shows a shuttle loaded with either seven 2 inch substrates, four 3 inch substrates or a single 4 inch substrate. With this arrangement it is convenient to include fragments or substrates. Processed area is provided in the figure as a reference point for single load throughput. The entire substrate area is provided since these shuttle held substrates are unclamped and the entire wafer surface is processed. Process conditions are adjusted to masking materials and take into account the clampless arrangement. The flexibility of this platform is recognized by its transparent capability to also process other diameter substrates by simply changing the shuttle size A more automated system that can be adapted to higher volume is the VERSALINE.
VERSALINE
This platform has the batch and single wafer processing capability of the SHUTTLELINE but can also be automated to include cassette to cassette transfer of substrates or carriers. With the increased automation it is straightforward to run a variety of configurations for varying capacity, wafer sizes, and process demands. The VERSALINE, designed with flexibility and modularity from the start, can accommodate configurations ranging from a single carrier, to a stack of four carriers, to twelve carriers to cassette-to-cassette of single wafers. Table 1 shows the volume of wafers that can be placed in the loadlock with the various configurations. An unprecedented 84 2inch substrates using seven substrates on each of 12 carriers with can be run without operator involvement. Just as easily a cassette of twenty-five 4 inch substrates can also be run.
Table 1: Wafer volume per system loading.
Wafer Size Single Batch
Carrier 4 Carrier
Configuration 12 Carrier
Configuration Cassette-to-Cassette
2 inch 7 28 84 NA
3 inch 4 16 48 25
4 inch 1 4 12 25
Cost effective upgrades that can quickly be made in the field allow the customer to efficiently match their tool set with capacity needs.
Process Considerations
Similar to hardware flexibility, processes must be adapted to the range of masks, GaN material quality, and desired profiles. Thus, it is difficult to establish a one-size fits all process. Although for cost considerations or tool matching, RIE is sometimes preferred, the well known ICP configuration with control over the physical and chemical components is often superior. In this arrangement, there are process conditions that will enhance the etching rate yet moderate the substrate temperature without clamping. Excellent cooling is obtained with individual substrate clamping and etching rates can be further increased without concern for burning photoresist.
