Series on Semiconductor Processing and Integrated Circuits, Part 5: Development Methods
Below is our fifth article in a series intended as an overview for those who are not technical specialists in the semiconductor processing field. We will briefly describe the development methods in semiconductor processing.
Once a semiconductor wafer has completed the alignment and exposure step, the device or circuit pattern is developed in the resist. Development techniques are designed to leave an exact copy of the mask or reticle pattern in the resist layer. Below we will discuss the various development methods, as well as the develop inspect that occurs after developing and baking.
There are several methods used to develop resist films, and selection of a method is dependent on the resist polarity, the feature size, defect density, the thickness of a layer to be etched, and productivity. The oldest development method is immersion. Simply put, the wafers are immersed in a tank of developer solution and then transferred to a rinse tank. There are numerous problems associated with this method however. For example, the surface tension of the liquids could prevent the chemicals from penetrating into small openings. The immersion tanks and wafers can easily become contaminated. Also, developer chemicals can become diluted through use on numerous wafers.
A preferred chemical development method is spray development. For this process, fewer chemicals are required, and process improvements include better image definition. Better definition is achieved by the mechanical action of the spray pressure in defining resist edges and the removal of partially polymerized pieces of resist. This process is also cleaner then the immersion system since fresh chemicals are used with each wafer. The spray process can be done either in single or batch systems. In a single wafer configuration, the wafer is clamped on a vacuum chuck and rotated while first the developer and then the rinse are sprayed onto the surface. The rotational speed is increased immediately after the rinse cycle to allow for the wafer to dry. This method has been standard for negative resists, but for temperature sensitive positive resists it is less effective. The issue is with this method is the phenomenon called adiabatic cooling, which causes the rapid cooling of a fluid dispensed through an orifice under pressure. To account for this phenomenon, spray developers used for positive resists have a heated wafer chuck or a heated spray nozzle to control the develop temperature. Batch development is done either by single-boat or multiple-boat. Batch development systems aren’t as uniform as direct-spray developing.
Puddle development is a process used to obtain the benefits of spray development for positive resists. Puddle development differs from spray development in how the developer chemical is applied to the wafer. First, enough developer is applied to the wafer to cover the surface. The puddle sits on the wafer, which is usually on a heated chuck, causing the majority of the development to take place. After development occurs the wafer is sprayed with more developer, rinsed, dried, and moved along to the next step.
To replace liquid chemical developers, the plasma etch process is used. In this process ions, energized by a plasma field, chemically dissolve exposed layer surfaces. Dry resist development requires a photoresist chemistry that leaves either the exposed or unexposed portions of the resist layer readily removable by plasma-energized oxygen. This process is difficult to integrate into automated lines, and the chemicals are more expensive to purchase, store, control, and remove.
After the developing and baking process, a develop inspect is performed. The goal of this inspection is to identify the wafers that will most likely not pass the final masking inspection. While the number of wafers that pass this first inspection is not factored into the overall yield formula, it is important for two reasons. First, wafers can be identified that have issues with the quality of the pattern. These wafers can be reworked. Wafers that have problems from other steps that prevent them from continuing are identified and discarded. During the develop inspect, the wafers are inspected for deviations in the pattern dimensions, misaligned patters, surface problems such as contamination or holes, and patterns that are distorted in shape. The first inspection is done with the naked eye either in a normal light room, or with a high-intensity ultraviolet light. Thickness irregularities, gross developing problems, scratches and contamination issues can often be detected. Wafers that pass this stage are then inspected with a microscope. However, with processes getting more numerous and more sophisticated, automatic inspection is the inspection system of choice for off-line and on-line inspections. Automatic inspections offer larger amounts of data that allow the process engineers more control.
We hope that you found this review of semiconductor processing development methods helpful. Please feel free to comment below and let the bloggers at Glew Engineering know if there is a specific topic you’d like us to blog about in the future.
Van Zant, P. (2000). Microchip fabrication, a practical guide to semiconductor processing. (4th ed.). New York, NY: McGraw-Hill.