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Sputtering is commonly used because the adhesion of deposited metals is excellent. The basic bondable metallization scheme for thin-film substrates contains TiW as the adhesion layer and Au as the conductor layer (TiW/Au). When resistors are required Tantalum Nitride is added (TaN/TiW/Au). TaN is available from 10 to 200 Ohms/square sheet resistivities. Multi-Ohms per square films are also available on single circuit designs.
Solderable metallization shemes are also available by adding Ni and/or Cu to these films (TiW/Au/Cu/Ni/Au). Bondable Au is typically >120µ" and solderable Au is typically <40µ" to prevent Au embrittlement. Palladium can also be added as a solderable film when using high-temperature eutectics, such as SAC305. Many different combinations are available for your design. Please contact ATP Sales for more information.
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| TiW/Au |
Yes |
Not Recommended |
No |
Yes |
Run Out issue |
| TaN/TiW/Au |
Yes |
Not Recommended |
No |
Yes |
Run Out issue |
| TiW/Pd/Au |
Yes |
No |
Yes |
Yes |
Yes |
| TaN/TiW/Pd/Au |
Yes |
No |
Yes |
Yes |
Yes |
| TiW/Ni/Au |
Yes |
Yes |
No |
Yes |
Yes |
| TaN/TiW/Ni/Au |
Yes |
Yes |
No |
Yes2 |
Yes |
| TiW/Au/Cu/Ni/Au |
Yes |
Yes |
No |
Yes |
Yes |
| TaN/TiW/Au/Cu/Ni/Au |
Yes |
Yes |
No |
Yes |
Yes |
| TiW/Au/Ni/Au |
Yes |
Yes |
No |
Yes |
Yes |
1 For die or wire attachment only.
2 Pb/Sn solderable and wire bondable on the same circuit can be achieved with special select processes.
Note: Dicing through metals will typically cause burrs/tails and on polished materials could affect adhesion. Another solution ATP recommends to prevent these type of issues is by putting a pullback typical 0.001" (0.0254mm) in the design of the circuit. Contact ATP Sales for more information. |
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Metallizations Offered
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Other metallizations and thicknesses are available. Please contact ATP Sales for information.
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SEM Profile
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Metal Stack
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Layers
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| Conductor and resistor applications that require traditional processing |
Typical Applications |
| Cost effective standard assembly practices; can integrate TaN into the film; bondable |
Advantages |
| Not Pb/Sn or SAC305 compatible |
Disadvantages |
| Epoxies; Au/Ge eutectic; Au/Si eutectic; Au/Sn eutectic |
Allowble Die Attach Method |
| -100 ±50ppm/°C |
Typical TCR |
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Sputtered 10–200 Ohms/sq TaN if resistors are required
Sputtered TiW: 300–800Å (0.03–0.08µm)
Sputtered Au: 20–200µ", typical = 120µ" (0.5–5µm, typical = 3µm)
Plated Au: 20–500µ", typical = 120µ" (0.5–12.7µm, typical = 3µm)
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Recommended Front Side Metal |
| Same as front side without the TaN layer |
Backside Metal |
| Conductor - resistor applications that allow bonding and soldering |
Typical Applications |
| Best for Au/Si assemblies and limited eutectic leaching; can integrate TaN into the film |
Advantages |
| Best |
Au/Si Solderability |
| Good |
Pb/Sn Solderability |
| Epoxies; Au/Si eutectic; Au/Sn eutectic; Au/Ge eutectic; Pb/Sn |
Allowble Die Attach Method |
| -100 ±50ppm/°C |
Typical TCR |
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Sputtered 10–200 Ohms/sq TaN if resistors are required
Sputtered TiW: 300–800Å (0.03–0.08µm)
Sputtered Pd: 1000–1500Å (0.10–0.15µm)
Sputtered Au: 20–200µ", typical = 120µ" (0.5–5µm, typical = 3µm)
Plated Au: 20–500µ", typical = 120µ" (0.5–12.7µm, typical = 3µm)
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Recommended Front Side Metal |
| Same as front side without the TaN layer |
Backside Metal |
| Conductor applications that require Pb/Sn soldering |
Typical Applications |
| Solderable for Pb/Sn assemblies; can be bondable as long as TaN is not present |
Advantages |
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Wire bonding problems may be experienced due to Ni-Au diffusion when devices processed > 300°C;
TaN is not recommended due to processing Ni-Au diffusion > 300°C
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Disadvantages |
| Best for Pb/Sn assemblies |
Pb/Sn Solderability |
| Epoxies; Au/Si eutectic; Au/Sn eutectic; Au/Ge eutectic; Pb/Sn |
Allowble Die Attach Method |
| -100 ±50ppm/°C |
Typical TCR |
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TaN is not recommended due to passivation above > 300°C, Ni diffusion
Sputtered TiW: 300–800Å (0.03–0.08µm)
Sputtered Ni: 1500–2000Å (0.15–0.2µm)
Sputtered Au: 20–40µ" (0.5–1.0µm) solderable (use thin gold; to prevent Au embrittlement, Au must be thin)
Plated Au: 80–500µ" (2.0–12.7µm) bondable (use thick gold)
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Recommended Front Side Metal |
| Same as front side without the TaN layer |
Backside Metal |
| Conductor-resistor applications with high conductivity film that requires Pb/Sn soldering |
Typical Applications |
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High conductivity film; can integrate TaN into the film;
Solderable Gold/Bondable Gold can be achieved on the same ciruit
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Advantages |
| Best |
Pb/Sn Solderability |
| Epoxies; Au/Si eutectic; Au/Sn eutectic; Au/Ge eutectic; Pb/Sn |
Allowble Die Attach Method |
| -100 ±50ppm/°C |
Typical TCR |
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Sputtered 10–200 Ohms/sq TaN if resistors are required
Sputtered TiW: 300–800Å (0.03–0.08µm)
Sputtered Au: 5µ" (0.127µm)
Electroplated Cu: 20–1000µ" (0.5–25.4µm)
Electroplated Ni: 20–500µ" (0.5–12.7µm)
Electroplated Au: 20–40µ" (0.5–1.0µm) solderable (use thin gold; to prevent Au embrittlement, Au must be thin)
Electroplated Au: 80–500µ" (2.0–12.7µm) bondable (use thick gold)
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Front Side Metal |
| Same as front side without the TaN layer |
Backside Metal |
| Conductor-resistor applications that require Pb/Sn soldering |
Typical Applications |
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Best for Pb/Sn assemblies; can integrate TaN into the film;
Solderable Gold/Bondable Gold can be achieved on the same ciruit
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Advantages |
| Best |
Pb/Sn Solderability |
| Epoxies; Au/Si eutectic; Au/Sn eutectic; Au/Ge eutectic; Pb/Sn |
Allowble Die Attach Method |
| -100 ±50ppm/°C |
Typical TCR |
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Sputtered 10–200 Ohms/sq TaN if resistors are required
Sputtered TiW: 400–800 Ansgtroms
Sputtered Au: 20–40µ" (0.5–1.0µm)
Electroplated Ni: 20–500µ" (0.5–12.7µm)
Electroplated Au: 20–40µ" (0.5–1.0µm) solderable (use thin gold; to prevent Au embrittlement, Au must be thin)
Electroplated Au: 80–500µ" (2.0–12.7µm) bondable (use thick gold)
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Front Side Metal |
| Same as front side without the TaN layer |
Backside Metal |
* Selective Ni/Au solder pads can be manufactured while leaving low loss TiW/Au in critical RF paths.
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Barrier Metal for Gold Tin Eutectic |
The 80:20 Gold Tin (AuSn) eutectic is a favored material for attaching laser diodes to Aluminum Nitride (AlN) standoffs. This eutectic melts at the relatively low temperature of 278°C. It has a high thermal conductivity compared with practical alternatives and is not prone to fatigue during thermal cycling.
An important metric for AuSn die attachment is the time the AuSn remains molten. The melt time is affected by several factors, one of the most important being the choice of barrier metal separating the molten AuSn from underlying Gold or Copper. The careful study described in [1] shows clearly that Nickel is superior to Platinum in every respect. The following Table summarizes key attributes for the two metals.
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Note that Nickel has lower electrical resistivity and higher thermal conductivity than Platinum. The combination of high bulk cost for Platinum combined with its relatively difficult processing greatly increases its out-the-door cost when compared with Nickel.
However, the reactivity of the two metals with Tin is by far the most important factor involving their use as a barrier metal for AuSn eutectic. The Platinum/Tin system is quite reactive. Platinum removes significant amounts of Tin from the molten AuSn, thus reducing the melt time. In contrast, the Nickel/Tin system has a low reactivity, thus optimizing the melt time.
Summary: Nickel as a barrier metal for AuSn die attachment of laser diode chips to AlN standoffs has every advantage over Platinum, both in terms of cost and on technical grounds. Of particular interest is the AuSn melt time with a Nickel barrier.
[1] Lee, C.H., et al, “Study of Ni as a barrier metal in AuSn soldering application for laser chip/submount assembly”, J. Appl. Phys. 72 (8), 16 October 1992 |
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