Back-Side Metallization for Semiconductor Heat Sinks
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Solution Overview
Problem
Poor heat conduction between semiconductor substrates and heat sinks due to susceptibility of soldered connections to cavity formation, which can lead to functional failures in high-frequency power components.
Innovation Solution
A method involving a division of the back-side metallization into a first Au layer, an intermediate barrier layer composed of materials like Ti, W, or Mo, and a thinner second Au layer to prevent Sn diffusion and enhance heat conduction, with the intermediate layer reducing diffusion of contaminants and promoting oxidation to prevent solder wetting in pass-through holes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a thick Au layer is used for back-side metallization to ensure good electrical conductivity, then electrical conduction is improved, but Sn diffusion from solder into the Au layer occurs, forming cavities and impairing heat transfer
Solution Approach 1:
A diffusion barrier layer composed of Ti, W, or Mo is introduced between the Au layer and the solder to prevent Sn diffusion. This intermediary layer blocks the harmful interaction between Sn and Au while maintaining good electrical and thermal conductivity through the Au layer.
Solution Approach 2:
The back-side metallization is structured as a composite system with multiple layers: a thick Au layer for conductivity, an intermediate diffusion barrier layer to prevent Sn diffusion, and a thin final Au layer for soldering. This composite structure combines the benefits of each material while mitigating their individual drawbacks.
2Manufacturing precision
If soldering parameters are tightly controlled to prevent cavity formation, then connection quality is improved, but manufacturing complexity and process difficulty increase
Solution Approach 1:
The diffusion barrier layer is deposited in advance before soldering to preemptively prevent Sn diffusion. This preliminary protective measure eliminates the need for tight control of soldering parameters to prevent cavity formation, simplifying the manufacturing process.
Solution Approach 2:
The problematic interaction between Sn and Au is extracted and isolated by removing the direct contact interface. The diffusion barrier layer separates the solder from the thick Au layer, eliminating the source of cavity formation and reducing process complexity.
3Ease of manufacture
If the final Au layer is made thicker to improve solderability, then ease of soldering is improved, but Sn diffusion into the Au layer increases, forming more cavities
Solution Approach 1:
The diffusion barrier layer serves as an intermediary that allows a thicker final Au layer to be used for improved solderability without increasing Sn diffusion. The barrier layer blocks Sn penetration while the thicker Au layer provides better solder wetting and connection.
4Device complexity
If a single-layer Au metallization is used to simplify the structure, then device complexity is reduced, but heat conduction is impaired due to cavity formation
Solution Approach 1:
The metallization structure is designed as a composite with specific layer thicknesses and materials optimized for both simplicity and performance. The diffusion barrier layer is thin enough not to significantly impede heat conduction while effectively blocking Sn diffusion, and the thick Au layer provides excellent thermal and electrical conductivity.
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 approach results in low-interference, good heat conduction from substrate to heat sink with easier control over soldering process parameters, reducing defects and mechanical stresses, and allowing greater variation in process conditions.
Implementation Method 1
an intermediate layer that lies between these, which layer consists of a material different from Au and Sn, and forms a diffusion barrier for Sn into Au
Implementation Method 2
the solder pre-form melts, and gold from the second Au layer dissolves in the solder
Implementation Method 3
a eutectic AuSn4 alloy, particularly in the form of a thin pre-form film, is used as the solder
Implementation Method 4
the waste heat is conducted away through the substrate, by way of its back side, to a heat sink
Implementation Method 5
an adhesion-imparting layer, which can contain Ge, Ti, W, Pd, or Cr, for example, is usually applied for a firm connection of the Au layer with the surface of the semiconductor substrate
Data Source
AI summary
The invention relates to an electronic component having a circuit integrated on a semiconductor substrate, and a heat-conducting connection of the substrate by soldering using a carrier serving as a heat sink, wherein the invention proposes depositing a first, thicker Au layer (23) in the conventional back-side metallization of the substrate, thereafter a barrier coating (24), and, as the last layer, a thinner, second Au layer (25), wherein the material of the barrier coating is selected such that the barrier coating prevents the penetration by means of a diffusion barrier of Sn or AuSn from a liquid Au—Sn phase in the region of the second Au layer into the first Au layer (23) during the soldering process. The layer sequence of the back-side metallization is also deposited in the pass-through openings of the substrate, wherein the surface of the second Au layer comprises a reduced coatablity for the solder material due to the material diffused out of the barrier coating.