Monolithic CMOS Pixel Detector Covalent Bonding

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Solution Overview

Problem

Current digital imaging devices for electromagnetic radiation detection, such as X-ray imaging, face limitations in conversion efficiency and spatial resolution due to indirect conversion processes and incompatibility of single crystal absorbers with CMOS readout electronics, which restricts pixel size and detector performance.

Innovation Solution

A monolithic CMOS integrated pixel detector is developed, featuring a CMOS processed silicon readout wafer covalently bonded to a single crystal absorber wafer, allowing for direct conversion of electromagnetic radiation into electrical signals with high spectral and spatial resolution, and enabling operation in both detection and display modes by adjusting doping levels and biasing.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If single crystal absorbers are used for direct conversion, then conversion efficiency and spatial resolution are improved, but compatibility with CMOS readout electronics deteriorates due to high thermal budget requirements

Engineering Contradiction:

Improvespatial resolutionVSAvoidcompatibility with CMOS readout electronics

Core Design Contradiction:

Measurement precisionVSEase of manufacture

Solution Approach 1:

The detector is divided into two separate wafers: a single crystal absorber wafer and a CMOS readout wafer. These wafers are bonded together through a low-temperature bonding interface, allowing each component to be optimized independently - the absorber for high resolution and the readout for electronic compatibility

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

A bonding interface is introduced as an intermediary between the single crystal absorber and CMOS readout electronics. This interface allows thermal isolation during fabrication, enabling the absorber to be processed at high temperatures for crystal quality while the CMOS electronics are processed at lower temperatures for compatibility

Inventive Principle:

Principle #24Intermediary (Mediator)

2Ease of manufacture

If indirect conversion with scintillation layer is used, then compatibility with readout electronics is improved, but conversion efficiency and spatial resolution deteriorate due to losses and scattering

Engineering Contradiction:

Improvecompatibility with readout electronicsVSAvoidconversion efficiency

Core Design Contradiction:

Ease of manufactureVSMeasurement precision

Solution Approach 1:

The scintillation layer, which causes energy loss and scattering, is completely removed from the detection path. Instead, the patent uses direct conversion in a single crystal absorber that converts X-rays directly to electron-hole pairs without an intermediate optical conversion step, eliminating the associated losses

Inventive Principle:

Principle #2Taking out (Extraction)

3Temperature

If bump bonding is used to bond absorber and readout wafers, then low temperature processing is achieved, but pixel size cannot be reduced below about 50 μm, limiting spatial resolution

Engineering Contradiction:

Improvebonding temperatureVSAvoidpixel size

Core Design Contradiction:

TemperatureVSMeasurement precision

Solution Approach 1:

The patent employs fusion bonding techniques that create a monolithic-appearing bond between wafers, copying the advantages of direct bonding while avoiding the geometric constraints of bump bonding. This allows for much smaller pixel dimensions while maintaining structural integrity

Inventive Principle:

Principle #26Copying

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 solution provides high spectral and spatial resolution, enabling efficient detection and imaging of electromagnetic radiation with improved pixel size and detector performance, suitable for various medical and non-medical applications, while maintaining compatibility with CMOS readout electronics.

Implementation Method 1

semiconductor absorbers permit the direct conversion of X-rays into electron-hole pairs which can then be measured as an electrical signal by readout electronics

Methodology Applied

Scientific EffectPhotoelectric effect: Photoelectric Effect

Implementation Method 2

One of them is, for example, known from the vertical integration of integrated circuits, so-called 3D-IC technology. Here, bump bonding is replaced by fusion bonding, comprising oxide-to-oxide fusion bonding along with metal-to-metal bonding of metallic pads surrounded by oxide.

Methodology Applied

Scientific EffectOxide-to-oxide fusion bonding:

Data Source

PatentEP3507835B1Electromagnetic radiation detector comprising charge transport across a bonded interface

Publication Date: 2021.03.31 G RAY SWITZERLAND SA

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AI summary

Monolithic CMOS integrated pixel detector (10, 20, 30, 260, 470, 570), and systems and methods are provided for the detection and imaging of electromagnetic radiation with high spectral and spatial resolution. Such detectors comprise a Si wafer with a CMOS processed readout bonded to an absorber wafer in an electrically conducting covalent wafer bond. The pixel detectors, systems and methods are used in various medical and non-medical types of applications.