A plasma process that lowers the risk of overtreatment or heat related issues.
By Daniel Chir and Johnson Toh
Lead frame surface oxidation can lead to surface delamination after molding or wire bonding issues. The application of plasma treatment has been proven to be safe and effective solution to address these issues. However, the effectiveness of plasma treatment for removing oxide is dependent on the correct use of recipe parameters, gas chemistry and electrode configuration.
In this paper, analytical techniques such as contact angle measurement, high magnification optical inspection and SEM-EDX are carried out on copper lead frames to evaluate the impact of using different plasma gas chemistries and electrode configurations. It is concluded that the use of Ar/H2 is better than Ar gas chemistry in removing oxide from copper lead frames. Another conclusion is the placement of copper lead frames on ground electrode is showing higher oxide removal rate than the placement on powered electrode.
Copper alloy is a major substrate material used in IC packaging due to its good thermal and electrical performance, good manufacturability, and low cost. However, it has very high affinity with oxygen in the atmosphere, which can cause oxidation during the assembly process. The risk of oxidation on copper surface increases during manufacturing process that require heating, especially during the wire bonding step [1]. The oxidation can lead to higher probability of delamination or bonding issues between the mold and metal surface during post mold curing [2] [3]. The optimum plasma configuration is known to be used for removing oxide from the copper surface and can increase the surface energy by up to 84% of its original value [4]. Placement of a product to be treated on a ground electrode subjects it to a more isotropic treatment than when it is placed on a powered electrode, which gives a more anisotropic treatment. Using the powered electrode will produce higher temperature surface and give lower treatment uniformity compared to treatment on the ground electrode [5].
Active plasma species which include ions, electrons, radicals and photons are formed during ionization of the process gases by RF energy. Ions will physically collide with the target surface, breaking bonds and releasing the surface material in the process known as sputtering. The H2 radical formed in plasma is a good reducing agent that reacts with oxygen atoms on the oxidized surface to form water vapor. It is a useful process gas for removing oxides from the metallic surfaces.
Material used: Two types of commercial copper alloy lead frames (bare copper and rough copper) were tested. The lead frames were exposed to temperatures that simulate the die attach, glue curing and wire bonding steps before they were sent for plasma treatment. This was done to simulate the oxidation that the lead frame will encounter during a typical assembly process.
Plasma equipment: The Nordson MARCH plasma system uses 13.56 MHz RF to generate plasma. The equipment had a H2 safety kit that allows the use of up to 100% H2 gas flow, but for this evaluation premixed gas of 95% Ar and 5% H2 (Ar/H2) was used. The strip is placed directly on the electrode in a configuration that is known as OTE (On-The-Electrode). This configuration is proven to give better uniformity and focuses the plasma energy on the top surface of the lead frame since the back of the lead frame sits flat on the electrode base and is not exposed to plasma. The process parameters of 600W RF power, 180s plasma time were used for this evaluation. The variables were the different electrode configurations and the gas chemistry.
Analysis method: The analysis of the copper lead frame surface was performed using two types of equipment: SEM-EDX and contact angle goniometer. The SEM-EDX was utilized in this study to analyze the oxide concentration on the sample surface The goniometer was used to analyze the wettability and surface energy.
Contact Angle result of different configuration and material.
SEM-EDX result on the copper lead frame.
Appearance of bare copper lead frame before and after plasma treatment.
High magnification scope for bare copper lead frame after plasma treatment.
Appearance of rough copper lead frame after plasma treatment.
High magnification scope for rough copper lead frame after plasma treatment.
The pre-plasma treated lead frame was observed to be discolored after the heat exposure in the different process steps. Discoloration was still found on samples treated with Ar on powered electrode and with Ar/H2 on a powered electrode setup. However, there was no discoloration observed on the lead frame after treatment with Ar/H2 on the ground electrode setup.
Appearance of the rough copper lead frame after plasma treatment shows different discoloration from the treated bare copper lead frame. This could be the result of the thicker oxide layer on the rough copper compared to the bare copper lead frame. On both types of lead frames there was a trend of reduction in discoloration intensity with reducing oxide thickness. There is more discoloration remaining on the rough copper compared to the bare copper lead frames that were processed with Ar/H2 on ground electrode. Further recipe optimization is required to remove the remaining oxide and attain lower discoloration on the surface.
In summary, the use of H2 and the placement on the ground electrode can increase the rate of oxide removal from the copper surface. It has produced the lowest contact angle, the most effective removal of the discoloration and the lowest oxide concentration on the surface based on the result obtained in this study. This is explained by the lower activation energy required to reduce copper oxide to copper when H2 radicals are formed in the plasma. In contrast, with a physical process on the powered electrode where ion sputtering is dominant, the removal of oxide is less effective.
The key takeaway from this report is that a correct gas chemistry and a suitable electrode configuration are critical to obtain an optimum plasma process that requires a shorter process time and lowers the risk of overtreatment or heat related issues. From a manufacturing perspective, this results in higher production throughputs and better yields.
Johnson Toh is a senior application supervisor in RF Plasma at Nordson MARCH.
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