Operating instructions

Bond Module
18 Copyright © 2013 EVG
2.6 Transient Liquid Phase (TLP) Wafer Bonding
For some applications the process temperatures must be lower than the
bonding temperatures of the most usual eutectic alloys (300°C - 400°C).
In such situations an alternative process can be used, which results in
an inter-metallic compound bonding layer. In literature this process is
known under different names among which can be mentioned “diffusion
soldering” or Transient Liquid Phase (TLP) bonding.
This bonding process is an advanced type of solder bond that can form
high-quality hermetic seals at lower temperatures than other bonding
technologies. This technique uses one thin layer of metal (typically 1-
10µm thick) which during a thermal process diffuses into its bonding
partner forming an inter-metallic compound layer with re-melting
temperature higher than the bonding temperature (table 2). The process
flow and recommended thermal profile are same as for eutectic wafer
bonding (fig. 6).
Component 1
(thick)
Component 2
(thickness)
Bonding
temperature
Diffusion time
Remelting
temperature
Cu
Sn (1µm)
Sn (5µm)
280°C
300°C
4 min.
20 min.
>415°C
>676°C
Au
In (2µm)
In (5µm)
In (2µm)
260°C
200°C
160°C - 240°C
15 min.
30 sec.
10 min.
>278°C
>495°C
>495°C
Ag
Sn (2µm)
Sn (5µm)
250°C - 350°C
250°C
10 min.
60 min.
>600°C
>600°C
Table 2 - Examples of metals which can be used for Transient Liquid Phase (TLP)
* Table adapted from [G. Humpston, and D. Jacobson, in Principles of
Soldering, ASM International 2004, p. 231]. Times correspond to full
diffusion of the specified thickness of Component 2.
Same as eutectic wafer bonding, diffusion soldering bonding is
attractive for MEMS vacuum packaging as the process is completed at
low temperatures (150°C - 300°C) and can withstand much higher
temperatures after bonding (see Table 2), bonding layers are made out
of metals (low permeability), and they can planarize over surface
defects or particles resulting from prior processes.
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