Pirani Sensor for Wafer-Level Package Leak Detection
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Solution Overview
Problem
Current methods for detecting leaks in hermetically sealed wafer-level package (WLP) chips are ineffective, particularly in monitoring pressure changes and detecting small leak rates, as existing techniques such as radioactive gas soaking and humidity capacitive sensors are either non-invasive or impractical for integrated circuit applications.
Innovation Solution
A Pirani-type sensor assembly is integrated into the WLP chip, featuring a low thermal conductance membrane with a resistive element that heats and detects temperature changes due to pressure variations within the sealed cavity, allowing for the detection of leaks as small as 1×10−10 atm cc/sec by maintaining a constant current or power through the element.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If radioactive gas soaking technique is used to detect leaks, then leak detection capability is provided, but the technique cannot detect very small leak rates and cannot monitor pressure changes in-situ
Solution Approach 1:
The patent replaces the mechanical/chemical radioactive gas soaking method with a thermal-based Pirani sensor system. The sensor uses a heated element whose temperature or power consumption changes in response to pressure changes inside the sealed cavity, enabling continuous in-situ monitoring and detection of very small leak rates that were undetectable by previous methods.
Solution Approach 2:
The invention monitors pressure changes by measuring temperature or power consumption parameters of a heated element. As pressure inside the cavity changes due to leaks, the thermal dissipation from the heated element changes, causing measurable variations in temperature or required power to maintain constant temperature, thereby enabling precise leak detection.
2Measurement precision
If humidity capacitive sensors are employed to detect leaks, then leak detection is enabled, but active circuits are required and humidity must be correlated with pressure changes
Solution Approach 1:
The patent extracts the leak detection function from complex active circuitry and humidity sensing mechanisms, implementing it instead through a simple passive thermal sensor. The Pirani sensor requires no active circuits, power management, or humidity-pressure correlation algorithms, significantly reducing device complexity while maintaining leak detection capability.
Solution Approach 2:
The invention uses a simple resistive heating element that can be fabricated using standard MMIC processes. This passive element replaces expensive, complex active humidity sensors and their associated circuitry, providing a cost-effective and simpler solution for leak detection.
3Measurement precision
If titanium oxide TiO sensor is used to detect ambient pressure, then pressure detection is provided, but the resistance responds very slowly to pressure changes at room temperature
Solution Approach 1:
The patent changes the operating parameter of the sensor by heating the sensing element to elevated temperatures. This thermal activation dramatically increases the response speed of the sensor to pressure changes, overcoming the slow response characteristic of room-temperature titanium oxide sensors while maintaining pressure detection capability.
Solution Approach 2:
The invention utilizes thermal energy to transition the sensor material into a high-response state. By maintaining the sensing element at elevated temperature through continuous or periodic heating, the material exhibits rapid response characteristics to pressure changes, effectively using thermal phase or state changes to improve performance.
4Measurement precision
If known micro-machined Pirani sensors are integrated, then pressure monitoring is enabled, but the materials and fabrication processes are not compatible with MMIC fabrication processes
Solution Approach 1:
The patent achieves homogeneity by using the same material system and fabrication processes for both the MMIC circuit and the pressure sensor. Both are fabricated using standard semiconductor processing techniques on the same wafer, ensuring material compatibility and simplifying manufacturing integration.
Solution Approach 2:
The invention merges the pressure sensing function with the MMIC fabrication process itself. The sensor structure is formed as part of the MMIC manufacturing sequence, combining what were previously separate devices into a single integrated structure that can be produced using existing MMIC fabrication capabilities.
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 sensor assembly effectively monitors the integrity of the seal and pressure within the cavity, enabling the detection of very small leaks and ensuring the hermeticity of the WLP chip, compatible with MMIC fabrication processes and suitable for in-situ pressure monitoring.
Implementation Method 1
A Pirani-type sensor assembly is integrated into the WLP chip, featuring a low thermal conductance membrane with a resistive element that heats and detects temperature changes
Implementation Method 2
detecting temperature or heating power changes of the layer as a result of an increase in pressure in the cavity
Data Source
AI summary
A sensor assembly for determining whether a hermetically sealed cavity between opposing substrate wafers in a wafer level packaged (WLP) chip is leaking. The sensor assembly includes a thermal insulating layer provided within the cavity, and a heater and temperature sensor deposited on the insulation layer. The thermal insulating layer is made of a suitable dielectric that is compatible with WLP and MMIC fabrication processes and can be, for example, benzocyclobutene (BCB) or polyimide. The sensor is responsive to a current that heats the thermal insulation layer so that heat dissipated by the thermal insulation layer is drawn away by gas between the layer and the substrate that determines the temperature of the sensor, which is detected.