In semiconductor manufacturing, measures are needed to improve productivity from the viewpoint of entire plants with advances in semiconductor manufacturing technology innovation in order to respond to intensifying price competition and develop higher performance. In order to improve productivity, tangible aspects that need to be addressed include the miniaturization of circuit line width and spacing and increasing wafer sizes. In addition, intangible aspects the need to be addressed include the need to reduce operational wastes related to manufacturing equipment and transport in plants overall. In this section, we introduce an automated material handling system (AMHS) that supports 450mm wafers and system configuration in order to further reduce wastes in transport.
Developments in Semiconductor Wafers and Carriers
The diameter of wafers are becoming larger and larger (Photo 1). From 1990, 200mm wafers were manufactured, and it was common to use unsealed shipping containers, or cassettes (Table 1). Accordingly, high level of cleanliness was necessary throughout plants. In addition, because transport consisted of holding the base or side of containers for automatic transport of cassettes, the design had to minimize the occurrence of dust from transporters.
When wafer size increased to 300mm at the beginning of the 2000s, a container called FOUP (front opening unified pod) was developed. A high level of cleanliness was maintained within the FOUP, and upper surfaces with flanges allowed automatic transport with an overhead hoist transfer vehicle (OHT) became standard.
As a result, cleanliness needed to be maintained at only certain areas, such as in manufacturing equipment, increasing the level of freedom in facility planning for AMHS. This also led to cost reductions in plant construction costs for air conditioning that maintain cleanliness. In addition, several IC chip manufacturers are moving forward with plans to commence mass production of next-generation 450mm wafers from around 2015.
By increasing wafer size from 300mm to 450mm, the plant area is increased by 2.25 times and productivity per wafer is greatly improved. However, if wafer size is increased from 300mm to 450mm, the container weight for 25 wafers also increases from about 10kg to 25kg. Taking this and ceiling transport with OHT into consideration, there are even more stringent demands for AMHS reliability in terms of safety improvements and maintaining wafer cleanliness in order to improve yield of the increased wafer size.
Layout of Semiconductor Plants
Because the OHT travel rail can extended up to a total length of 10km with up to several hundred cars in large plants, optimal transport management systems that take into consideration congestion avoidance, set travel lines and passing line, and prompt handling of transport request instruction from high-level systems are required.
Current plant layouts can be roughly classified into two types: Inter & Intra (separated interbay and intrabay) transport systems (Diagram 1) and the unified (direct between equipment) transport system (Diagram 2).
Under an Inter & Intra system, goods in progress must go through a stocker to supply FOUPs to be transported from one manufacturing device to the next manufacturing device. Because goods in process are always stored in stockers, this layout method is suitable for plants that mass produce few model types such as memory chips. However, the number of tranport times is high as it goes from manufacturing equipment to stocker A, stocker B, and then to the next manufacturing equipment.
On the other hand, under a unified system, while there are stockers for temporary storage, direct transport between manufacturing equipment is possible without going through stockers. For example, for the custom IC chips produced by contracted manufacturing companies (foundries), because speed is a must for product shipment of low volumes of multiple models, reducing goods in process is necessary to achieve more efficient production. Under a unified system, goods in progress is transported directly from manufacturing equipment to manufacturing equipment, and the higher the direct transport rate, the lower the number of transfers. However, both AMHS and high-level process management are essential to ensure proper timing for supplying goods from FOUPs to manufacturing equipment.
In order to achieve this, it is also necessary to break away from the current situation for AMHS in which transport is conducted based on instructions from high-level systems. We are also working on autonomous transports that incorporates some production information such as where things should be moved to next, as well as what kind of lots should be created.
From DAIFUKU NEWS No.200 (January 2012)