Variable Frequency Microwave Curing of Polyimide Films
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Solution Overview
Problem
Conventional thermal curing methods for thermoplastic polymers, such as polyimides, require high temperatures and long processing times, making them unsuitable for advanced semiconductor applications where low thermal budgets restrict the use of these materials due to high residual stress and limited thermal expansion coefficients.
Innovation Solution
A method involving the deposition of thermoplastic films in soluble form onto substrates, followed by soft baking and curing using Variable Frequency Microwave (VFM) at temperatures no higher than 100°C below the glass transition temperature, allowing for rapid densification and improved crystallinity without exceeding the thermal budget of semiconductor processes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional thermal curing methods are used to cure polyimide films, then complete curing and high mechanical strength are achieved, but processing time is excessively long (4-6 hours) and high temperatures (350°C) damage underlying circuit elements
Solution Approach 1:
The patent replaces conventional thermal conduction heating with microwave radiation heating to cure polyimide films. This substitution enables rapid curing (20-120 minutes vs. 4-6 hours) while maintaining complete curing and high mechanical strength, as the microwave energy directly excites molecular dipoles in the polymer chains, providing volumetric heating throughout the film thickness simultaneously.
Solution Approach 2:
The patent changes the heating parameter from conventional thermal conduction to microwave radiation, and adjusts the curing temperature to be no higher than 100°C below the glass transition temperature of the thermoplastic. This parameter change enables complete curing within 20-120 minutes while protecting underlying circuit elements from thermal damage.
2Manufacturing precision
If high cure temperature (350°C) is applied to achieve complete curing, then polymer chain orientation and low CTE are achieved, but the thermal budget is exceeded and circuit elements are damaged
Solution Approach 1:
The patent replaces conventional thermal conduction heating with microwave radiation heating to achieve polymer chain orientation without exceeding the thermal budget. The microwave energy directly excites molecular dipoles, enabling complete curing and proper orientation at temperatures no higher than 100°C below the glass transition temperature, thus protecting circuit elements.
Solution Approach 2:
The patent utilizes the glass transition temperature as a reference point to determine the optimal curing temperature range. By curing at temperatures no higher than 100°C below Tg, the polymer chains achieve proper orientation and crystallinity without requiring the conventional high cure temperature of 350°C, thus resolving the contradiction between orientation quality and thermal budget constraints.
3Reliability
If slow temperature ramp rates and extended hold times are used to allow for outgassing and chain orientation, then low residual stress and low CTE are achieved, but productivity is severely reduced
Solution Approach 1:
The patent replaces conventional thermal conduction heating with microwave radiation heating, which provides volumetric heating throughout the film thickness simultaneously. This enables rapid outgassing of reaction by-products and solvent, and achieves complete chain orientation within 20-120 minutes, thereby maintaining low residual stress and low CTE while dramatically improving productivity and throughput.
Solution Approach 2:
The patent employs continuous microwave irradiation during the curing process, maintaining constant energy input to the polymer matrix. This continuous useful action enables simultaneous outgassing, chain orientation, and crosslinking throughout the film volume, achieving complete curing and low residual stress in 20-120 minutes compared to the extended hold times required by conventional methods.
4Productivity
If conventional oven curing or rapid hotplate curing is used, then curing speed is improved, but cure temperature remains at 310-350°C which is beyond the allowable maximum temperature for many semiconductor applications
Solution Approach 1:
The patent replaces conventional thermal conduction heating (oven or hotplate) with microwave radiation heating. This substitution enables rapid curing (20-120 minutes) while maintaining cure temperature no higher than 100°C below the glass transition temperature, thus achieving both high productivity and compliance with semiconductor thermal budget constraints.
Solution Approach 2:
The patent fundamentally changes the heating mechanism from thermal conduction to microwave radiation, and sets the curing temperature parameter to be no higher than 100°C below Tg. This parameter change enables rapid curing speed while keeping the temperature within the allowable maximum for semiconductor applications, resolving the contradiction between productivity and temperature constraints.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enables the creation of highly oriented, low-coefficient-of-thermal-expansion polyimide films at significantly lower temperatures, reducing residual stress and enabling the use of thermoplastic films in advanced semiconductor applications without damaging underlying circuit elements, thus enhancing processing efficiency and reducing production costs.
Implementation Method 1
curing the film by VFM for 20 to 120 minutes at a temperature no higher than 100° C. below the glass transition temperature, Tg, of said thermoplastic
Implementation Method 2
providing a method for densifying thermoplastic films; providing a low-temperature process for making a thermoplastic film with improved crystallinity
Data Source
AI summary
A method for densifying thermoplastics, particularly polyimides, for use in conjunction with electronic circuits while producing improved physical properties and a high degree of crystallinity, involves variable frequency microwave (VFM) processing at temperatures typically 100° C. below the glass transition temperature or lower, for times of about 50 to 100 minutes. It is particularly applicable to polymers based on BPDA-PPD, but may also be generally applied to other intentionally designed polyimide structures with the same features. The invention enables the creation of layered structures involving integrated circuits with small feature sizes and overcoatings of polymers with high Tg and other desirable properties.