CMP Process for Thin SiC Wafer Stress Release

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Solution Overview

Problem

The grinding process for thinning Silicon Carbide wafers causes deep sub-surface damage, cracks, and compressive stress, leading to wafer warpage and fragility, which existing stress relief methods like thermal and laser annealing are inadequate in addressing, especially for wafers with reduced thickness below 180µm.

Innovation Solution

A Chemical Mechanical Polishing (CMP) process with specific parameters such as controlled polishing pad and wafer rotation speeds, pressure, polishing time, and the use of an alumina-based slurry with a pH of 2-3, applied to the back side of the wafer to remove defects and cracks, maintaining a temperature below 50°C.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Manufacturing precision

If thermal annealing is used to recover grinding damages, then surface recovery is attempted, but thermal constraints limit temperatures below 1000°C which is insufficient for complete recovery

Engineering Contradiction:

Improvesurface recovery qualityVSAvoidthermal budget limitation

Core Design Contradiction:

Manufacturing precisionVSTemperature

Solution Approach 1:

The patent replaces the thermal annealing process with a chemical-mechanical polishing (CMP) process. Instead of using heat to recover surface damages, the invention uses a slurry containing specific chemicals (HF, HNO3, CH3COOH) combined with mechanical polishing action to remove damaged layers and recover the wafer surface, thereby avoiding thermal budget constraints entirely.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

2Stability of the object's composition

If laser annealing is used for local surface heating, then crystal rearrangement occurs, but thermal gradient creates new defects and cracks

Engineering Contradiction:

Improvecrystal structure recoveryVSAvoidthermal gradient induced defects

Core Design Contradiction:

Stability of the object's compositionVSObject-generated harmful factors

Solution Approach 1:

The patent substitutes laser annealing with chemical-mechanical polishing. The CMP process uses chemical dissolution from the slurry and mechanical removal to achieve crystal structure recovery without introducing thermal gradients, thereby eliminating the harmful effect of thermally induced defects and cracks.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Manufacturing precision

If grinding process is used to thin the wafer, then optimal device performance is achieved, but deep sub-surface damage and wafer warpage occur

Engineering Contradiction:

Improvedevice performance optimizationVSAvoidsub-surface damage and warpage

Core Design Contradiction:

Manufacturing precisionVSObject-affected harmful factors

Solution Approach 1:

The patent applies chemical-mechanical polishing as a preliminary recovery step immediately after grinding. The CMP process removes the damaged sub-surface layer created by grinding, recovers the wafer flatness, and eliminates warpage before subsequent processing steps, thereby preserving device performance while eliminating harmful effects.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent converts the damaged surface layer created by grinding into a removable sacrificial layer. The CMP process selectively removes this damaged layer using chemical dissolution and mechanical polishing, transforming the harmful sub-surface damage into a controlled material removal process that restores wafer quality.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

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

Effectively relieves stress and recovers the wafer surface to its original state, improving mechanical and electrical quality by reducing warpage and surface roughness, enhancing device yield and surface precision.

Implementation Method 1

a slurry, in particular an alumina-based slurry, with a pH of 2-3

Methodology Applied

Scientific EffectChemical etching: Oxidation

Implementation Method 2

Chemical Mechanical Polishing (CMP) process

Methodology Applied

Scientific EffectMechanical abrasion: Abrasion

Implementation Method 3

maintaining a temperature below 50°C

Methodology Applied

Scientific EffectThermal control: Cooling

Data Source

PatentEP4174915A1CMP process applied to a thin sic wafer for stress release and damage recovery

Publication Date: 2023.05.03 STMICROELECTRONICS SRL

Figure 1~2

AI summary

A Chemical Mechanical Polishing, CMP, process applied to a wafer (20) of Silicon Carbide having a thickness of, or lower than, 200µm, comprising the steps of: arranging the wafer (20) on a supporting head (14) of a CMP processing apparatus (10), the wafer (20) having a front side (20a) and a back side (20b) opposite to one another, the front side (20a) housing at least one electronic component and being coupled to the supporting head (14); deliver a polishing slurry on the wafer (20), wherein the polishing slurry has a pH in the range 2-3; pressing the back side (20b) of the wafer (20) against a polishing pad (16) of the CMP apparatus (10) exerting, by the supporting head (14), a pressure on the polishing pad (16) in the range 5-20 kPa; setting a rotation of the polishing pad (16) in the range 30-180 rpm, and setting a rotation of polishing head (14) in the range 30-180 rpm; setting and maintaining a CMP process temperature equal to, or below, 50°C.