Cryogenic Anisotropic Silicon Etch Method
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Solution Overview
Problem
Conventional methods for anisotropic etching of silicon wafers face issues such as grooved trench walls, polymer deposition difficulties, inefficient etching rates, temperature sensitivity, and significant underetching, which affect the quality and reproducibility of microcomponents.
Innovation Solution
A novel plasma etching method involving alternated steps of fluorinated gas injection and passivation with silicon tetrafluoride and oxygen at cryogenic temperatures, with specific gas flow ratios and temperatures between −40° C. and −120° C., to achieve a balanced etch rate and reduced temperature sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If the alternated method is used for anisotropic etching, then the etching can be performed at ambient temperature, but the trench walls become grooved and polymer deposits form on the reactor walls
Solution Approach 1:
The patent changes the temperature parameter from ambient to cryogenic (−40° C. to −120° C.) to fundamentally alter the etching chemistry. This temperature change prevents polymer formation on trench walls while maintaining anisotropic etching capability, directly resolving the contradiction between temperature and trench wall quality
Solution Approach 2:
The patent utilizes phase transition of the passivation layer material. The passivation layer is deposited at cryogenic temperatures and then evaporates when the wafer is warmed to ambient temperature, providing a self-cleaning effect that eliminates polymer deposits and improves trench wall quality
2Productivity
If the alternated method is used for anisotropic etching, then etching can be performed, but the etch rate is substantially twice as slow as the cryogenic method
Solution Approach 1:
The patent optimizes the duty cycle parameters of the alternated etching process at cryogenic temperatures. By adjusting the ratio and duration of etching versus passivation steps, the patent achieves both high etch rates (comparable to continuous cryogenic etching) and smooth trench walls, resolving the productivity-precision contradiction
3Productivity
If the cryogenic method is used for anisotropic etching, then continuous etching is achieved, but the method is very sensitive to temperature variations
Solution Approach 1:
The patent employs periodic alternation between etching and passivation steps instead of continuous etching. This periodic action provides inherent process control and compensation for temperature variations, improving reliability while maintaining productivity. The cyclic nature allows for self-correction of temperature drift effects
Solution Approach 2:
The passivation step acts as a feedback mechanism that compensates for temperature variations. If temperature increases cause excessive etching, the subsequent passivation step deposits additional protective layer to compensate, thereby stabilizing the final trench dimensions and reducing temperature sensitivity
4Productivity
If the alternated method is used for anisotropic etching, then etching can be performed, but polymer deposits on reactor walls require frequent cleaning
Solution Approach 1:
The patent changes the temperature parameter to cryogenic conditions, which fundamentally alters the deposition chemistry. This prevents polymer formation on reactor walls entirely, eliminating the need for frequent cleaning while maintaining efficient etching speeds
Solution Approach 2:
The patent uses a temporary passivation layer that is intentionally designed to be short-lived. This layer protects trench walls during etching but is easily removed by warming the wafer to ambient temperature, creating a self-eliminating protective mechanism that leaves no persistent deposits requiring cleaning
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
The method achieves an intermediary etch rate with grooved trench structures similar to the alternated method, minimal underetching, and reduced temperature sensitivity, with a self-evaporating passivation layer, improving the efficiency and reliability of silicon wafer etching.
Implementation Method 1
a first recess 3 is etched by plasma in the trench. For this etching, a fluorinated compound, for example, sulfur hexafluoride, SF6, is injected into the plasma reactor
Implementation Method 2
the fluorinated plasma first etches by ion bombarding the polymer layer at the bottom of recess 3
Implementation Method 3
a gas such that the resulting active components in the reactor are likely to form a polymer 4 on the silicon walls of recess 3 is introduced into the plasma reactor
Implementation Method 4
This polymer forming gas is for example trifluoromethane, CHF3 or C4F8. The formed polymer is a film of a material that can be assimilated to Teflon (CF2)n
Implementation Method 5
The wafer is placed in a plasma reactor on a susceptor cooled down to a very low temperature, for example, around −100° C.
Implementation Method 6
deposited material 9 becomes gaseous when the wafer is brought back from the deposition temperature (approximately −100° C.) to the ambient temperature and eliminates by itself
Data Source
AI summary
A method of anisotropic plasma etching of a silicon wafer, maintained at a temperature from −40° C. to −120° C., comprising alternated and repeated steps of:etching with injection of a fluorinated gas, into the plasma reactor, andpassivation with injection of silicon tetrafluoride, SiF4, and of oxygen into the plasma reactor, the flow rate of the gases in the plasma reactor being on the order of from 10% to 25% of the gas flow rate during the etch step.