Sep 03 2013

Plasma Surface Treatment for Adhesion in Semiconductor Packaging

In this blog, I will address the application of atmospheric pressure plasma’s to semiconductor packaging applications.  If after reading this note you have questions, please contact me through the Surfx website.

The Semiconductor Package

As you may already know, the package is the physical structure that integrated circuits (ICs) are mounted on so that they may be attached to the printed circuit board (PCB).  The package has several functions: it provides the electrical input and output connections between the microelectronic chip and the PCB; it provides environmental isolation and protection to the IC; and it efficiently conducts heat away from the device.  The chip may be mounted onto a lead frame that has bond pads around the perimeter of the device.  In this case, the electrical connections are made by attaching wire bonds between the metal pads on the chip and those on the lead frame.  Alternatively, the chip may be mounted onto a ball-grid array, where solder bumps are directly attached to a two-dimensional array of pads on one side of the chip.  This latter case is the “flip chip” design, and provides a higher density of I/O connections than traditional wire bonding.

After the I/O connections are made by wire bonding, or by solder reflow of the ball-grid array, then the chip must be covered by epoxy to isolate the device from the atmosphere.  This means coating the device and the electrical connections with an epoxy overmold.  For flip-chip, one must also fill in all the space around the ball-grid array with an epoxy underfill.  The epoxy overmold and underfill are cured in an oven.  The last step is to punch or cut out each die, yielding the final package for mounting onto the printed circuit board.

Adhesion Problems Encountered in Packaging

            Loss of adhesion can occur at multiple steps in the semiconductor packaging operation.  The most common are:

  • Attachment of the die to the frame.
  • Dispense of the overmold to the chip, the metal frame, or the polymer substrate.
  • Dispense of the underfill around the ball-grid array or along the edge of the chip.
  • Attachment of the wire bond to the bond pad.

Adhesion problems occur with epoxy adhesives when an insufficient number of strong chemical bonds are made from the glue to the substrate surface.  This can result from contamination by flux residue or other organic or inorganic matter.  On clean surfaces, the adhesion may not be sufficient because the substrate does not contain enough active sites for bonding to the glue.  This is a common problem with polymers, such as polyimide and flame retardant 4 (FR-4), used in microelectronics packaging.  These materials do not have functional groups on their surface that will react with the epoxy when it cures.

Metal-metal adhesion is a problem when there are contaminants on the metal surface where a solder bond is made.  The contaminant can be a flux residue or an organic material deposited in one of the previous processing steps.  In addition, the contaminant can be the native metal oxide formed on the metal surface when the package is heated in an oxidizing environment.  Copper oxide on copper bond pads is a widespread problem in the packaging industry.

Plasma Cleaning and Surface Activation

Adhesion problems can be solved by cleaning away the contaminant and activating the surface to form strong chemical bonds to the epoxy, metal wire, or solder.  Plasma processing is a well-accepted method of doing this.  Organic contamination can be removed in a matter of seconds using oxygen plasmas.  The oxygen plasma will also activate polyimide and FR-4 for adhesion to the epoxy.  Reactive species in the plasma react with the polymer and add hydroxyl, carbonyl and carboxylic acid groups to the surface.  The epoxy rings in the adhesive molecules make strong bonds to these functional groups, particularly to the carboxylic acids.

For metal-metal bonding and soldering, the plasma must remove the contaminants as well as the native metal oxide.  In this case, hydrogen plasmas, or other reducing-gas plasmas, must be used.  Here, the surface reaction is:  MOxs + 2xH = Ms + xH2O; where MOxs is the native metal oxide, H is a hydrogen atom, and Ms is the clean metal surface.  The chemistry of the plasma must be tailored to provide a fast rate of removal of the contaminants, plus leave the surface is a state that is active for bonding.

The advantages of plasma cleaning and activation are as follows:

  • Speed – hundreds of lead frames can be cleaned in one hour.
  • Low-cost – less than one cent per frame.
  • Versatility – process chemistry may be tailored to address different cleaning jobs.
  • Minimal consumables – only process gas and electricity.
  • Fully automated – manual operations are avoided, which are inherently low yield.
  • Green – no hazardous waste to dispose of, and no environmental, health and safety (EH&S) concerns.

Atmospheric Pressure Plasmas versus Vacuum Plasmas

Vacuum plasmas have been used for over 20 years in microelectronics packaging.  The semiconductor industry is very comfortable using vacuum process, and acceptance is high.  Vacuum plasmas come in two configurations: oven systems, where you “rack and stack” the substrates inside a big chamber; and track systems, where you feed the substrates on a conveyor, and one part is treated at a time inside a small clamshell container.  Extensive automation is provided for cassette-to-cassette processing of lead frames.

When semiconductor packages are processed in vacuum plasmas, the entire substrate is immersed in the ionized gas.  This means that everything, the chip, the frame, the substrate, the tray, etc., are exposed to the plasma.  The plasma contains energetic electrons, energetic ions, reactive species and ultraviolet light.  Positively charge ions, such as Ar+, are accelerated towards the substrate, smash into it at high velocities, and non-selectively sputter away material.  This means that sputtering can occur on the chip, the frame, the substrate, the tray, etc.

The plasma is part of a complicated electrical circuit that includes the radio-frequency power supply, the impedance matching network, and the cabling.  When the substrate is immersed in the plasma, it becomes part of the electrical circuit.  If the substrate contains, metallic, semiconducting, and insulating components, then each of these will interact with the ionized gas in a different way.  The metal for example, will focus a lot of the ion energy, whereas the insulators will not.  This can mean different rates of cleaning (or sputtering) of these materials.  If the substrate contains magnets, then this will interact with the plasma in highly unpredictable ways.

Atmospheric pressure plasmas are a relatively young technology, and have not been used extensively in semiconductor packaging.  Unlike vacuum plasmas, the substrate to be treated is not immersed inside the ionized gas.  Instead, the atmospheric plasma applicator confines the ionized gas to its housing.  Neutral reactive species, stripped of the energetic electrons and ions, flow out of the applicator, and contact the substrate placed a short distance downstream.  Since only neutral species contact the semiconductor package, no electrical circuit is made with it.  Metallic, semiconducting, and insulating components interact with the neutral reactive gas in the same way.  Surface reaction chemistry governs the speed of cleaning and activation.

The reactive species produced in atmospheric pressure plasmas are many thousands of times higher than in vacuum plasmas.  This can be understood if we look at the rate of generation of ground-state oxygen atoms in oxygen plasmas.  The O atoms are produced by electron impact ionization:  O2 + e = O + O + e.  The rate of this reaction is proportional to the concentrations of oxygen molecules and free electrons in the plasma.  In a vacuum plasma, the oxygen pressure is about 0.05 Torr and the electron concentration is about 1011 cm-3.  In a Surfx atmospheric pressure plasma, the oxygen pressure is about 30 Torr and the electron concentration is approximately 1012 cm-3.  This means that the rate of oxygen atom generation in the Surfx atmospheric plasma relative to the vacuum plasma is (30∙1012)/(0.05∙1011) = 6,000!

Recent studies at the University have revealed that the rate of cleaning and activation of polyimide and FR-4 with the Surfx atmospheric pressure plasma is extremely fast.  For example, the water contact angle of polyimide drops from 75° to 20° after only two tenths of a second exposure to the atmospheric pressure plasma.  Similar speeds of activation are seen with FR-4.  After about 1 second of exposure to the AP plasma, the oxygen concentration on the surface, as measure by X-ray photoemission spectroscopy, has risen to 25%.  In addition, the lap-shear strength of the FR-4 substrate to epoxy underfill adhesive is measure at 11.0 MPa.  Analysis of the fracture surface after the lap-shear tests, reveals that failure is 100% cohesive, meaning that shear occurred entirely within the glue, not at the glue-polymer interface.  This is basically the best adhesion you can get.

The results presented above indicate that atmospheric pressure plasmas are extremely effective for cleaning and activating surfaces for adhesion.  Knowing that, are there compelling reasons for switching from vacuum plasmas to atmospheric pressure plasmas in semiconductor packaging?  I think there are, and these are my reasons:

  1. Speed – a lead frame is processed underneath the plasma beam at 1 cm/s; whole frames in less than 10 secs.
  2. Selective area processing – only the parts of the substrate you want processed are processed with the plasma beam.  For example, no erosion of substrate trays.
  3. No damage – there is no ion bombardment of the substrate, so you cannot remove layers of material non-selectively.  You cannot sputter away your thin gold layer on copper.
  4. Simplified automation – you don’t have to deal with pump cycles, seals, and all that maintenance required of a vacuum chamber.
  5. No cross-contamination – there is no chamber to gunk up with etch residue.  You can switch from one job to the next, and not have to take apart and clean the tool.
  6. No electrical interactions – the plasma and substrate are separate and do not interact with each other.  Put anything you want under the plasma beam.
  7. Substrate size does not matter – since the substrate does not go into a chamber, it does not have to be a specific size.  Surfx’s plasma machines process parts ranging from 1×1 mm2 to 400×400 mm2 in size.  Automation is straightforward for cassettes, trays, etc.
  8. Single part traceability – the entire process, including gas flow rates, power, and plasma emission intensity are logged in real time as individual parts are scanned with the beam.

There you have it; you have eight good reasons to try out Surfx’s atmospheric pressure plasmas for improving adhesion in semiconductor packaging.  These reasons are backed by sound science, so I am certain you will be pleasantly surprised with the results.

2 Responses to Plasma Surface Treatment for Adhesion in Semiconductor Packaging

  1. Caseynpcgdkw Wednesday February 15th, 2017 at 05:22 AM

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  2. Anonymous Thursday November 30th, 2017 at 03:06 AM

    Can the plasma cleaning also remove the flux residue after clip bonding ?

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