Oxide Semiconductor Transistor Fabrication Reducing Photolithography Steps
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Solution Overview
Problem
The existing methods for manufacturing active matrix light-emitting display devices require a large number of photolithography steps, which increases complexity and costs, and often compromise the electric characteristics of thin film transistors, while techniques to simplify these steps, such as using multi-tone masks, still require additional processing steps and can decrease yield.
Innovation Solution
A method that omits the step of processing a semiconductor layer into an island shape by using a high-resistance oxide semiconductor, reducing the number of photolithography steps by forming transistors with a semiconductor layer that can be regarded as an insulator, allowing multiple transistors to operate without interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional photolithography steps are used to manufacture thin film transistors, then manufacturing precision can be maintained, but the number of steps increases significantly and production cost increases
Solution Approach 1:
The patent combines multiple photolithography steps into a single step by using a multi-tone mask that defines multiple patterns simultaneously. This merging approach reduces the number of separate photolithography operations while maintaining the precision needed for transistor fabrication, directly addressing the contradiction between manufacturing precision and productivity.
Solution Approach 2:
The invention changes the parameters of the photomask by using multi-tone (gray-scale) masks with varying light transmission characteristics. This allows different regions of the mask to expose the photoresist to different amounts of light, creating multiple pattern depths and shapes in a single exposure step, thereby reducing the total number of photolithography steps required.
2Productivity
If the number of photolithography steps is reduced, then productivity increases and cost decreases, but manufacturing precision may be compromised
Solution Approach 1:
Multiple patterning operations are merged into a single photolithography step using a multi-tone mask, which maintains manufacturing precision by defining multiple critical patterns simultaneously with controlled light exposure, avoiding the precision loss that might occur with simpler reduction methods.
Solution Approach 2:
The multi-tone mask utilizes varying light transmission parameters to create different exposure doses across the mask area, enabling precise control over the developed pattern shapes and dimensions. This parameter variation allows complex transistor structures to be formed with high precision in fewer steps.
3Device complexity
If multi-tone masks are used to reduce photolithography steps, then the number of masks decreases, but additional processing steps such as ashing are required
Solution Approach 1:
The patent extracts and eliminates the additional ashing step by using a positive-working photoresist system where the multi-tone mask directly creates the desired pattern differences during development. This removes the need for the separate ashing process that would otherwise be required to remove excess resist, thereby reducing total processing steps while using fewer masks.
4Device complexity
If conventional techniques are used to simplify photolithography steps, then some steps can be reduced, but yield decreases due to various problems
Solution Approach 1:
The multi-tone mask system is designed to be self-sufficient, where the mask itself contains all the information needed to create multiple patterns without requiring additional corrective steps like ashing or lift-off. This self-service approach eliminates process variations and potential failure points, maintaining high yield while simplifying the overall photolithography process.
Data Source
AI summary
It is an object of one embodiment of the present invention to manufacture a light-emitting display device by simplifying a manufacturing process of a transistor, without an increase in the number of steps as well as the number of photomasks as compared to those in the conventional case. A step for processing a semiconductor layer into an island shape is omitted by using a high-resistance oxide semiconductor which is intrinsic or substantially intrinsic for the semiconductor layer, used to form transistors. Formation of an opening in the semiconductor layer or an insulating layer formed over the semiconductor layer and etching of an unnecessary portion of the semiconductor layer are performed at the same time; thus, the number of photolithography steps is reduced.