Plasma Cleaning in Device Packaging
Plasma cleaning is widely used in microelectronics fabrication and material research due to its versatility for various materials and sample feature size. The common plasma cleaning process is to use O2 plasma, Ar plasma or H2 plasma. However, the conventional plasma cleaning methods are facing critical challenges in device packaging. This page describes the details of process challenges and Samco’s process solutions on plasma cleaning.
Plasma Cleaning Process Challenges in Device Packaging
The key materials in device packaging are metals such as copper (Cu), silver (Ag) and gold (Au). Traditionally, gold was a mainstream material of microelectronics electrodes. It is because gold is highly stable against chemicals and environment and exhibits excellent conductivity. One drawback of gold electrodes is its material cost. As an alternative of gold electrodes, new metal electrodes have been developed over years. Currently silver and copper attract more attention from back-end community because material cost reduction is highly required to make more profits at device fabs. In fact, silver is relatively cheaper and also copper is extremely cheaper compared to gold. However, there are several challenges to use silver and copper in back-end process in production line. For example, the performance of these materials are generally inferior to gold in terms of corrosion and migration. Furthermore, copper wire-bonding needs to be operated in inert gases such as argon (Ar) and nitrogen (N2) or reduced gases such as hydrogen (H2) in order to prevent surface oxidation.
Plasma cleaning processes are required in silver and copper wire-bonding to remove contamination from the surfaces. There are several options as a plasma cleaning method.
O2 plasma generates atomic oxygen and oxygen radicals. Even though organic contamination is removed by the oxygen-containing reactive species, silver and copper surface is oxidized. The oxidized lead-frame deteriorates intrinsic brightness and reflectivity of the metals. In the case of silver, the color change of siliver leadframe by O2 plasma weakens light intensity of high-brightness LEDs.
Argon is noble gas and does not cause chemical change on the metal surfaces. However, argon ion bombardment physically attacks the metal surfaces, and redeposition of sputtered metals occur in the vicinity of metal components. As a result, the redeposition causes lower device performance of electrical and optical properties.
H2 is one of the reduced gases. H2 plasma is used for back-end process for surface cleaning. Installation of H2 plasma process in production line is accompanied by potential safety issue. Fab managers need to take care of H2 gas leak which causes explosion.
Using H2O for Plasma Treatment
Aqua Plasma is plasma treatment process which uses H2O (water vapor) as process gas.
Water vapor generates reactive oxygen species and hydroxyl radicals.
There are several process features of Aqua Plasma.
1. Surface cleaning + reduction
Contamination removal and reduction (oxidized layer removal) simultaneously occur on metal surfaces.
2. Inexpensive and safe
Water as process gas source is inexpensive and safe. Easy to install compared to H2 which is used for gentle plasma cleaning.
3. Exclusive for Samco Plasma Cleaner AQ-2000
Aqua Plasma process is specially developed by Samco to provide plasma cleaning solutions to back-end customers at device fabs.
Copper Plasma Cleaning + Surface Reduction
The picture on the right shows a copper-made coin before and after Aqua Plasma cleaning. Oxidized layer was removed and the surface showed intrinsic color of copper. As seen in the picture, copper leadframe can be cleaned in Aqua Plasma without surface oxidation and redeposition of copper particles.
Copper Coin before/after Aqua Plasma
The figures on the right show ESCA analysis data of O2 plasma-treated copper surfaces before/after Aqua Plasma. The sample before Aqua Plasma treatment showed thick oxidized layer of 79.2 nm in Ar beam sputtering. The oxidized layer was formed in O2 plasma treatment. It took 7 min to remove the layer completely (no O1s signal). On the other hand, the sample processed by Aqua Plasma showed only 1.6 nm thick layer, and it took just 30 sec to remove the layer.
This data indicate that the copper surface was reduced by Aqua Plasma.
Copper Surface after O2 Plasma
Copper Surface after Aqua Plasma
Silver Plasma Cleaning + Surface Reduction
The picture on the right shows silver LED package before and after Aqua Plasma cleaning. Intrinsic silver color and brightness was recovered after Aqua Plasma cleaning. This plasma cleaning process of silver LED package is beneficial to maintain light intensity of LED devices.
Silver LED Package before/after Aqua Plasma
More yields with reliable cleaning
Enhances die yields without deterioration of device performance
Low-cost & safe
Water vapor as plasma gas source is low cost and safe.
Expandable to production
Batch substrate processing on multiple shelves allows high throughput.