Electrostatic Chuck Thermal Control for RF-Driven Wafer Temperature Drift
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Solution Overview
Problem
Conventional cooling systems for regulating the temperature of the wafer support pedestal or electrostatic chuck in capacitively coupled plasma reactors are inefficient, leading to temperature drift and non-uniformity across the wafer, which degrades etch rate uniformity and process control, especially under high RF heat loads.
Innovation Solution
A plasma reactor with an electrostatic chuck incorporating a backside gas pressure source, an evaporator, and a refrigeration loop with a thermal model and control processor to simulate heat transfer and adjust gas pressure for precise temperature control, using a two-phase cooling process to maintain uniform temperature through latent heat of vaporization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If conventional cooling systems with refrigeration cycles are used to regulate 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 refrigerant system where the refrigerant alternates between liquid and vapor phases within the electrostatic chuck to absorb and transport heat. The phase transition enables efficient heat removal from the wafer backside, maintaining uniform temperature distribution across the wafer surface even under high RF power conditions by distributing cooling capacity throughout the chuck structure.
Solution Approach 2:
The refrigeration system is nested within the electrostatic chuck structure itself, with refrigerant channels and phase change materials integrated into the chuck body. This nested configuration allows direct thermal coupling between the cooling system and the wafer support surface, eliminating temperature gradients that would otherwise develop in conventional separate cooling systems.
2Productivity
If RF source power is increased to maintain etch rate, then productivity is improved, but wafer temperature drift increases
Solution Approach 1:
The system incorporates temperature sensors that continuously monitor wafer temperature and feed this information back to the control system. Based on the temperature feedback, the system dynamically adjusts refrigerant flow rates and compressor operation to maintain optimal cooling capacity, enabling stable temperature control even as RF power levels fluctuate to maintain etch rate productivity.
3Speed
If rapid temperature changes are implemented for process control, then process agility is improved, but temperature uniformity degrades due to heat propagation delays
Solution Approach 1:
The electrostatic chuck is divided into multiple thermal zones with independent refrigerant channels, allowing selective and rapid temperature adjustment in different regions. This segmentation enables fast thermal response for process control while maintaining overall temperature uniformity by independently managing heat removal in each zone according to local requirements.
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 solution achieves rapid and uniform temperature control across the wafer, maintaining etch rate uniformity even under high RF heat loads, enhancing process stability and efficiency by eliminating temperature drift and non-uniformities.
Implementation Method 1
using a two-phase cooling process to maintain uniform temperature through latent heat of vaporization
Implementation Method 2
two-phase cooling process
Implementation Method 3
a backside gas pressure source coupled to the electrostatic chuck for applying a thermally conductive gas
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.