Jianzhong Jiao discusses the progress made in the SEMI organization on LED manufacturing standards, including a Wafer Task Force draft document, that should ultimately lead to more efficient and less expensive components for SSL.
With the ever broadening availability of LED products for general lighting and other applications, many new standards have been, or are in the process of being developed. Primarily these standards are LED product based standards and in general they can be categorized into three areas – testing, performance, and safety. The standards can also be grouped into component level and system level (final product) standards. Standards are the measure of the industry's maturity and are used as tools to interface between product producers and product end users. The next step for the industry is developing LED process related standards that should ultimately yield lower-cost LEDs for solid-state lighting (SSL).
In early 2011, the Semiconductor Equipment and Materials International (SEMI) organization formed the High-Brightness (HB) LED Committee. This committee was formed to create specifications, guidelines, and practices for LED manufacturing at the process level with the intent to improve communication between LED makers and suppliers of HB materials, carriers, and automation systems. This approach of developing LED process standards, according to SEMI, should enhance the manufacturing efficiency, capability, and shorten the time-to-market, as well as help reduce overall manufacturing cost in the LED industry.
After the HB LED Committee was formed, several task force (TF) proposals were initiated and discussed. So far, the committee has approved the establishment of three TFs – Wafer, Equipment Automation, and Impurities and Defects.
Wafer Task Force
The Wafer TF has drafted a document for specifying sapphire wafers that are intended for use in manufacturing HB-LED devices. This document is designed to serve as the link from wafers to LED package production. Currently sapphire wafers are widely used in producing HB-LED devices, and these LEDs are used in multiple applications including SSL. The committee believes that improving manufacturing efficiency and enabling cost reductions are critical elements in continuing to advance the industry. Furthermore, SEMI recently conducted surveys that help to identify key parameters and dimensions critical to enabling manufacturing automation across multiple manufacturing steps.
The drafted scope of the new SEMI standard is to cover the single-crystal, high-purity, single-side-polished, C-axis sapphire wafers used in HB-LED manufacturing. It includes 100- and 150-mm diameter wafers that have either flat or notched fiducials (alignment marks). The standard specifies the requirements of wafer dimensions, surface characteristics, physical properties, and methods of measurements suitable for determining the characteristics in the specifications.
There is some debate whether or not this type of standard is even necessary. Some LEDs makers are vertical integrators who make wafers specifically for their own LED package products. SEMI members and Wafer TF participants believe the standardization of sapphire wafers is an effective approach to reducing inconsistency, and in turn, beneficial to the LED industry.
Specification scope
What should be included in the specifications, however, may not be as easily agreed upon by all TF members. There are some elements of the specifications such as the epitaxial growth method that can be strongly dependent on specific LEDs and a company's technological advantages. Other parameters such as a wafer's flatness, scratches, etc., could be dependent on the mutual purchase agreement between a wafer maker and an LED maker based on commercial values. After deliberation by the Wafer TF members, it is likely that the standard will include agreed upon specifications as a requirement. Other characteristics that the TF cannot reach consensus on as firm requirements will be listed as recommendations.
So far, two thicknesses of 150-mm diameter and one thickness of 100-mm diameter wafers are included in the standard. For the 150-mm wafer, the standard will specify two fiducial configurations: flat and notched. For the agreed upon properties such as the dimensions and geometries, the standard provides target values and tolerance ranges. These specified values are based on today's practices and are widely accepted by both wafer and LED makers.
To promote consistent interfaces between wafer producers and users, the standard provides ordering information as part of the requirements. In a purchase or contract agreement, a list of specifications is to be used in a table format. Furthermore, the requirements for each specified item are also provided in the standard. These requirements include general characteristics of the crystals, dimensions, permissible variations, and inspection characteristics.
Recommendations
The standard also provides the method of measurements as a suggestion. It does not preclude other methods agreed upon between suppliers and customers. Many of the methods of measurements recommended in this document are cited to other SEMI standards that are used in the semiconductor industry. For example, the wafer orientation measurement references the SEMI MF26 Standard. For some methods such as measuring flat fiducials, there is no existing SEMI standard and the Wafer TF is working to establish a specific recommendation. If the agreement can't be reached within the TF, these methods will most likely be referred to as the agreement between supplier and customer.
Similar to the Wafer TF, the members in the Equipment Automation TF are working to develop the agreed upon specifications. The LED industry wants to make the process of manufacturing LEDs more consistent or standardized.
How much can these standards benefit the entire SSL industry? We won't know overnight nor will it be easy to quantify the benefits. SSL is an industry that produces hardware devices that directly interface with users in their day to day lives. Some products require user interaction, as is the case with commercial building lighting which forces the user to take action when they enter into a hotel or office. As an industry, it is crucial that any standardized approach that could eventually benefit users, whether to improve energy savings, safety, quality of light, or reduce cost, be given its due diligence.
