Electrostatic Chuck Feed-Forward Cooling for RF Heat Load Changes
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Solution Overview
Problem
Conventional plasma reactors face challenges in maintaining uniform wafer temperature and etch rate distribution under high RF heat loads, due to inefficiencies in cooling systems that lead to temperature drift and non-uniformities, which degrade process control and etch rate uniformity.
Innovation Solution
A plasma reactor design incorporating a thermal model and a two-phase refrigeration loop with an evaporator inside the electrostatic chuck, using a thermally conductive gas for enhanced heat transfer and a feedback control system to manage backside gas pressure and coolant flow, ensuring heat transfer through latent heat of vaporization for uniform temperature control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If conventional cooling systems with refrigeration loops are used to control wafer temperature, then cooling capability is provided, but temperature drift and non-uniformity occur under high RF heat loads
Solution Approach 1:
The patent employs a two-phase refrigeration system where the refrigerant undergoes phase transitions (liquid to vapor and back) to absorb and reject heat. The evaporator causes refrigerant to vaporize, absorbing heat from the electrostatic chuck, while the condenser condenses the vapor back to liquid, rejecting heat externally. This phase transition mechanism provides efficient heat transfer and maintains temperature uniformity under high RF heat loads.
Solution Approach 2:
The patent implements a feedback control system using temperature sensors to monitor wafer temperature and adjust refrigerant flow accordingly. The control system receives temperature signals from sensors positioned near the wafer and electrostatic chuck, and dynamically adjusts the refrigeration system to maintain desired temperature uniformity, preventing temperature drift.
2Measurement precision
If temperature probes are introduced near the wafer for accurate temperature sensing, then temperature measurement accuracy improves, but parasitic RF fields are created that distort the uniform environment
Solution Approach 1:
The patent uses an intermediary approach by positioning temperature sensors in locations that allow indirect measurement of wafer temperature. Sensors are placed near the wafer but not in direct contact with it, and near the electrostatic chuck to infer wafer temperature through thermal conduction relationships. This intermediary measurement method avoids creating parasitic RF fields while maintaining sufficient measurement accuracy for process control.
3Productivity
If RF power is increased to achieve higher etch rates, then productivity improves, but temperature non-uniformities increase that distort etch rate distribution
Solution Approach 1:
The patent implements preliminary thermal management by pre-cooling the electrostatic chuck and wafer before RF processing begins. The refrigeration system is activated in advance to establish the desired temperature profile, and temperature sensors continuously monitor conditions to make preliminary adjustments. This preliminary action ensures that when high RF power is applied for high-rate etching, the thermal foundation is already established to maintain etch rate uniformity.
Solution Approach 2:
The patent dynamically adjusts operational parameters including refrigerant flow rate, electrode bias power, and processing pressure to compensate for temperature non-uniformities that arise during high-power etching. By changing these parameters in response to real-time temperature measurements, the system maintains etch rate distribution uniformity even at high productivity levels.
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 significantly improves temperature uniformity across the wafer, maintaining etch rate uniformity even under high RF heat loads, enabling agile and accurate temperature control and preventing temperature drift, thus enhancing the reliability and efficiency of plasma processing.
Implementation Method 1
ensuring heat transfer through latent heat of vaporization for uniform temperature control
Implementation Method 2
using a thermally conductive gas for enhanced heat transfer
Data Source
AI summary
A plasma reactor having a reactor chamber and an electrostatic chuck with a surface for holding a workpiece inside the chamber includes a backside gas pressure source coupled to the electrostatic chuck for applying a thermally conductive gas under a selected pressure into a workpiece-surface interface formed whenever a workpiece is held on the surface and an evaporator inside the electrostatic chuck and a refrigeration loop having an expansion valve for controlling flow of coolant through the evaporator. The reactor further includes a temperature sensor in the electrostatic chuck and a memory storing a schedule of changes in RF power or wafer temperature. The reactor further includes a thermal model capable of simulating heat transfer between the evaporator and the surface based upon measurements from the temperature sensor, and a control processor coupled to the thermal model and to the memory and governing the backside gas pressure source in response to a prediction from the model of a change in the selected pressure that would compensate for the next scheduled change in RF power or implement the next scheduled change in wafer temperature.