Copper-to-Copper Bonding With Grain Growth at Low Annealing Temperature
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Solution Overview
Problem
Current copper-to-copper bonding technologies face challenges in achieving high bond formation and bonding strength, particularly in miniaturized applications, with issues related to nonuniformities, electrical shorts, and temperature sensitivity, as well as the need for reduced annealing temperatures and simplified process steps.
Innovation Solution
A method involving electrochemical copper deposition with nanocrystalline grain sizes, followed by an annealing step at temperatures equal to or less than 200°C, which allows for grain growth across the bonding interface, eliminating distinct interfaces and enhancing bonding strength and electromigration performance without additional surface modification or CMP steps.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional solder technologies are used to bond copper bumps and pillars, then bonding strength is achieved, but the solder material can be squeezed out during bonding and create electrical shorts in fine pitch applications
Solution Approach 1:
The invention extracts and removes the solder material from the bonding process, transitioning from solder-based bonding to direct copper-to-copper bonding. This eliminates the harmful effect of solder being squeezed out and creating electrical shorts, while maintaining bonding reliability through direct metal-to-metal contact and diffusion bonding mechanisms.
Solution Approach 2:
The invention changes the bonding parameters by applying specific contact pressures (1-1000 bar) and temperatures (150-400°C) to enable direct copper bonding without solder. These parameter changes allow copper atoms to diffuse across the interface and form strong bonds, replacing the need for solder material while preventing electrical shorts.
2Ease of manufacture
If copper pillars are produced by electrolytic copper deposition, then copper interconnects are formed, but relatively large nonuniformities in coplanarity and total thickness variation occur
Solution Approach 1:
The invention applies preliminary chemical-mechanical polishing (CMP) to the copper deposits before bonding to correct nonuniformities in coplanarity and thickness. This preliminary action removes surface irregularities and creates uniform bonding surfaces, enabling precise alignment and contact during the bonding process while maintaining the ease of electrolytic copper deposition.
3Strength
If high annealing temperatures are applied during copper-to-copper bonding, then bond formation and bonding strength are improved, but temperature-sensitive devices are damaged
Solution Approach 1:
The invention changes the temperature parameter by implementing bonding at reduced temperatures (150-400°C, preferably 150-300°C) compared to conventional high-temperature processes. This parameter change enables sufficient copper diffusion and bond formation while preventing damage to temperature-sensitive devices, achieving both bonding strength and device safety.
Solution Approach 2:
The invention substitutes thermal energy with mechanical pressure (1-1000 bar) as the primary driving force for bonding. By applying high contact pressure, copper atoms are forced into close contact and diffuse across the interface at lower temperatures, replacing the need for high thermal energy and protecting temperature-sensitive devices from thermal damage.
4Reliability
If additional surface modification steps are applied to improve bonding, then bond formation is enhanced, but the number of process steps and complexity increase
Solution Approach 1:
The invention merges multiple process steps into fewer operations. Chemical-mechanical polishing (CMP) serves dual purposes: it planarizes the copper surface for uniform bonding contact and simultaneously activates the surface for bonding. This merging eliminates the need for separate surface activation steps, reducing process complexity while enhancing bond formation.
Solution Approach 2:
The invention makes the CMP process multi-functional by using it for both surface planarization and surface activation. The same CMP step that removes nonuniformities also creates a fresh, reactive copper surface that is highly suitable for bonding, eliminating the need for additional dedicated activation steps and reducing overall process complexity.
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 improved bond formation, increased bonding strength, and reduced electromigration issues, while maintaining low annealing temperatures and simplifying the process, making it suitable for temperature-sensitive devices and fine-pitch applications.
Implementation Method 1
converting the first deposit and the second deposit of the connected deposit into a connected and converted deposit, wherein the first deposit and the second deposit are formed by an electrochemical copper deposition step and having copper grains with a grain size which is smaller than a grain size after the converting in step d)... wherein the connected and converted deposit is having grains with a grain size which is larger than the grain size before the converting in step d)... wherein the step d) is conducted by applying an annealing step having an annealing temperature equal to or less than 200° C.
Implementation Method 2
The copper-to-copper bonding bases on surface diffusion of copper atoms which diffuse across the interface to form permanent bonds.
Implementation Method 3
providing a first substrate comprising a first pure copper deposit having a bonding surface... providing a second substrate comprising a second pure copper deposit having a bonding surface... wherein the first deposit and the second deposit are formed by an electrochemical copper deposition step
Data Source
AI summary
The invention relates to method for copper-to-copper direct bonding comprising the steps:a) providing a first substrate comprising a first pure copper deposit having a bonding surface;b) providing a second substrate comprising a second pure copper deposit having a bonding surface;c) connecting the bonding surface of the first deposit with the bonding surface of the second deposit and obtaining a connected deposit; andd) converting the first deposit and the second deposit of the connected deposit into a connected and converted deposit,wherein the first deposit and the second deposit are formed by an electrochemical copper deposition step and having copper grains with a grain size which is smaller than a grain size after the converting in step d),wherein the connected and converted deposit is having grains with a grain size which is larger than the grain size of the first deposit and the second deposit before the converting in step d); and to an assembly and a device produced by the method. (FIG. 1)