What is a Dummy Wafer?

1. Definition and function of Dummy Wafer

Dummy Wafer is a wafer specially used to fill machine equipment during wafer manufacturing. It is usually not used in actual production and will not be sold directly as a finished product. Its main function is to meet the specific requirements or constraints of equipment operation, ensure the stability of equipment process performance, optimize resource utilization and reduce production risks. The design and use of Dummy Wafer is an important part of wafer fab production management. In different process stages of wafer manufacturing, due to the characteristics and process requirements of the equipment, Dummy Wafer plays an irreplaceable role in ensuring the normal operation of equipment, optimizing resource allocation, and reducing the loss of good wafers.

2. Specific use of Dummy Wafer

Dummy Wafer has a variety of usage scenarios, which can be mainly divided into the following aspects according to its use:

Filling equipment capacity: Some equipment (such as furnace tubes and etcher) has certain requirements for the number of wafers during operation. For example, the heat treatment process of furnace tube equipment requires a certain number of wafers to form a stable airflow, temperature field and chemical reaction environment. If only a small number of production wafers are placed, the performance of the equipment may be unstable, which will eventually affect the process quality. Therefore, Dummy Wafer is used to fill the equipment to reach the required number. Comparing the wafer equipment to an oven, if only one piece of bread is placed in the oven, the heat may not be evenly distributed, but if it is full of bread, it can be evenly heated. Similarly, Dummy Wafer plays the role of "making up the number of people" to ensure that the equipment operates at the optimal load.

Protecting production wafers: In some high-risk processes, such as ion implantation, etching, and chemical vapor deposition (CVD), there may be process instability or more particles generated during equipment debugging or initial process stages. If production wafers (PW) are used directly, irreversible yield loss may occur. Dummy Wafer plays a tentative role in such processes to prevent production wafers from being directly exposed to potential risks. Dummy Wafer is like a pathfinder, first confirming that the road ahead is safe, and then letting the production wafer "pass with confidence".

Evenly distribute the process load: When some equipment is performing process processing, the wafers in the process carrier (such as a furnace tube or etching chamber) need to be evenly distributed. For example, in physical vapor deposition (PVD), if the number or placement of wafers is asymmetrical, the deposition rate and thickness uniformity may be affected. The addition of Dummy Wafer can balance the layout of wafers in the equipment and ensure the stability and uniformity of the entire process.

Reduce equipment idle costs: In wafer manufacturing, the startup and shutdown of equipment consume a lot of time and resources. If there are no production wafers to be processed, long-term idleness of equipment may lead to waste of resources and degradation of equipment performance. By processing Dummy Wafer, the equipment can be kept active while preparing for subsequent production.

Perform equipment verification and process debugging: Dummy Wafer is often used as a verification carrier for equipment. For example, after equipment maintenance and cleaning, Dummy Wafer is needed to test whether the equipment status has returned to normal. If an abnormality is detected, the equipment parameters can be adjusted or cleaned again without directly wasting production wafers. The equipment is like a car, and Dummy Wafer is like a test tire. After ensuring that the car performs normally, the "expensive" production tires are replaced.

3. Material selection and usage characteristics of Dummy Wafer

The material and specifications of Dummy Wafer are usually determined by process requirements and equipment characteristics. The following are the main features of Dummy Wafer:

Material: Most Dummy Wafers use the same substrate as the production wafer (such as monocrystalline silicon or polycrystalline silicon) to ensure consistent performance in the equipment. For some special process scenarios (such as high-temperature processes), the material of Dummy Wafer may be more durable, but the cost is relatively low.

Surface treatment: The surface of Dummy Wafer usually does not require the same fine treatment as the production wafer. They can be polished wafers or test-grade wafers. Some Dummy Wafers may be reused until they are severely damaged or contaminated by particles, and then they will be replaced.

Service life: Dummy Wafer is usually recycled, but its service life depends on the specific process and equipment requirements. For example, in a high-temperature, high-corrosion environment, its life will be shorter.

Cost considerations: The cost of Dummy Wafer is usually much lower than that of production wafers, but it still needs to be properly managed and maintained to avoid unnecessary waste.

IV. Management and optimization of Dummy Wafers

During the wafer manufacturing process, the management of Dummy Wafers is a task that requires high attention. The following are relevant management and optimization measures:

Usage tracking: Establish a record of the use of Dummy Wafers, track their number of uses, usage processes and losses. Optimize the replacement cycle of Dummy Wafers through data analysis.

Reducing pollution: The repeated use of Dummy Wafers may cause particle or chemical pollution, so they need to be cleaned or replaced regularly to ensure the cleanliness of the internal environment of the equipment.

Process adaptation: Different processes have different requirements for Dummy Wafers, and suitable Dummy Wafers need to be selected according to the process characteristics. For example, in thin film deposition, the surface finish of Dummy Wafers may directly affect the quality of the film.

Inventory management: Since Dummy Wafers will not be used directly in production, their inventory management needs to consider the balance between cost control and production needs.

Disposal: Scrapped Dummy Wafers should be properly handled according to environmental protection requirements, such as recycling silicon materials or using them as low-level test wafers.

5. Actual case analysis of Dummy Wafer

Furnace tube filling: In the oxidation process, the furnace tube equipment has strict requirements on the number of wafers. Too few wafers may cause uneven temperature field and affect the thickness of the oxide layer. The addition of Dummy Wafer not only fills the quantity requirement, but also plays a role in protecting the production wafers.

Etching equipment debugging: After process switching or maintenance, the etching equipment needs to test the etching rate and uniformity. If the production wafer is used directly, serious losses may occur. After verifying the etching process through Dummy Wafer, the safety and stability of production can be ensured.

In a all, Dummy Wafer is an indispensable part of the wafer manufacturing process, and its role runs through equipment maintenance, process debugging, resource optimization and other aspects. In actual operation, the reasonable use of Dummy Wafer can not only reduce production costs, but also improve equipment utilization and process stability.