March 1998
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Polymers for encapsulation: Materials Processes and Reliability
Today'S Complex Circuits Require Encapsulants That Do More Than Simply Protect The Device Inside The Package.By Prof. C.P. Wong, Georgia Institute of Technology, Atlanta, Georgia
The advances in very-large-scale integrated circuit technology can be largely attributed to the many improvements in polymeric materials. This article describes and discusses today's leading encapsulant materials, their processes and their reliability, as these issues relate to future generations of electronic packaging.
An ultra-large-scale integrated circuit (USLIC) is a very complex and delicate three-dimensional structure. It consists of millions of components on a single IC. These components are densely packaged in a multilayer structure with metallized (Al or Cu) conductor lines separated by dielectric (organic and inorganic) insulating layers.
The rapid growth in the number of components per chip, the decrease of device dimensions coupled with the steady increase in IC chip size, and the growing number of I/Os have imposed stringent requirements on the chip's physical design and fabrication which are reflected by the properties of encapsulants employed in assembly.
Why Encapsulate?
The obvious purpose of encapsulation is to protect electronic devices from an adverse environment and to increase their long-term reliability. The ultimate goal of encapsulation, however, is to ensure device reliability and to increase production yield at the lowest cost.Moisture, contaminants, mobile ions, radiation (ultraviolet, visible and
-particle), and hostile environments, such as low and high temperature cycling (temperature ranges -65 °C to 175 °C), are some of the factors which can negatively affect device performance or lifetime. Furthermore, passivation layers on ICs are not 100% crack-free and pinhole-free; as such, they need additional encapsulant to enhance their reliability. The chip bond-pad areas are normally
etched out for interconnect and need protection, as well, which is why passivated ICs need additional coating materials to enhance reliability. The following adverse elements and conditions can negatively affect the chip's performance
Diffused Moisture
Moisture is one of the major sources of corrosion in ICs (Figure 1). Electro-oxidation and metal migration are associated with the presence of moisture However, the diffusion rate of moisture depends on the encapsulant material and is a function of the encapsulant thickness and the exposure time to various materials.Pure crystals and metals are the tees materials to employ as moisture barriers. Glass (silicon dioxide) is an excellent moisture barrier, but it is slightly inferior to pure crystals and metals Compared to glass, organic polymers such as fluorocarbons, epoxies and sill
cones are a few orders of magnitude more permeable to moisture. Silicone materials have the highest moisture permeability of most polymers, yet they are of one of the best device encapsulants.
In general, for each particular material, the moisture diffusion rate is proportional to the water vapor partial pressure and inversely proportional to the material thickness. This is accurate when moisture diffusion rates are in steady-state permeation. However, moisture transient penetration rates are inversely proportional to the square of material thickness.
Mobile Ion Contaminants
Mobile ions, such as sodium or potassium, tend to migrate to the p-n junction of the device, where they acquire an electron and deposit as the corresponding metal on the pen junction, destroying the device. Furthermore, mobile ions will also support leakage currents between biased device features which degrade device performance and ultimately destroy the device by electrochemical processes, such as metal conductor dissolution.The protection of ICs from the effects of mobile ions is an absolute requirement. The use of an ultra-high-purity material to encapsulate the passivated IC is the answer to some mobile ion contamination problems.
Light and
- Particle RadiationUV-VIS light radiation can damage opto-electronic devices. However, UV-VIS protection can be achieved by choosing an opaque encapsulant. Furthermore, impurities, such as low levels of uranium in the encapsulant or in the ceramic or plastic package, can cause appreciable of-particle radiation. Cosmic radiation in the atmosphere may also damage IC electronics.
The of-radiation can generate a temporary "soft error" in operating dynamic random access memory (DRAM) devices.
-particle radiation has become a major concern, especially in high density memory devices. Good encapsulants must have
- radiation levels below 0.001 -particles/cm2/hour, and must be opaque to protect ICs from UV-VIS radiation. Since the particle is weak radiation, a high purity encapsulant which is a few micrometers thick will usually prevent radiation damage to DRAM devices. Hostile Environments
Hostile device operating environments, such as extreme cycling temperatures (values from -65 °C to +150 °C in Military Standard 883-C), high relative humidity (85% to 100%), shock, vibration and high-temperature operating bias are part of real life operating conditions.It is critical for the device to survive these operation-life cycles. In addition, encapsulants must also have suitable mechanical, electrical and physical properties, such as minimal stress and matching thermal expansion coefficient, etc., which are compatible with the devices. Furthermore, the encapsulant must also be ultrapure, with extremely low ionic contaminants (< a few ppm). Since encapsulation is the final process step, the materials must be easy to apply and repair in production and service, particularly for expensive devices.
Cleaning Prior to Encapsulation
Prior to IC encapsulation, cleaning is the most critical step to ensure long-term device reliability, since encapsulating a dirty device is a guarantee of circuit failure. It is imperative that even trace amounts of contamination from the device surface be removed prior to the encapsulation process.The three main cleaning processes are conventional cleaning, reactive oxygen cleaning and hydrogen cleaning, described as follows.
Conventional Cleaning - This process includes the use of organics, such as detergents and solvents (i.e. chlorofluorobydrocarbons (CFCs), freons and chlorohydrocarbons (trichloroethane, methylene chloride, etc.), to remove organic contaminants. Reactive Oxygen Cleaning-In addition to conventional cleaning, reactive oxygen cleaning is very effective in removing low-level organic contaminants. There are three types of reactive oxygen gas processes used in cleaning: UV-Ozone, Plasma Oxygen and Microwave Discharge. UV-ozone is very effective in removing a few monolayers of organics from the substrate surface; however, the device being cleaned must be placed directly under the UV source.
Plasma oxygen operates at 13.6 MHz radio frequency (rf), is fast and effective in cleaning MOS devices and preserves the aluminum metallization. However, the thermal stress associated with the plasma process may damage some device structures.
Microwave discharge cleaning with oxygen at 2.5 GHz rf is also a powerful device cleaning technique. This process is similar to the oxygen plasma process, except that the microwave frequency is used.
Reactive dc (Hydrogen Plasma Cleaning)-Recently, dc-hydrogen plasma cleaning has been reported as an alternative cleaning process for high performance IC packages. The plasma process is based on an argon-hydrogen discharge generated between the heated filament (cathode) and the reactor wall (anode). The discharge is based on a current density from 10-100 amps and a low voltage of 20-30 volts. As such, it will only mildly clean the interfacial contaminant, eliminating the sputtering that damages the IC. The process is simple and environmentally friendly as it reduces organic and inorganic contaminants. Furthermore, the hydrogen cleaning process eliminates oxide formations, such as those generated by UV ozone.
Figure 1 - Moisture Permeabiiity of Various Materials.
Polymeric Materials for Microelectronics
There are numerous organic polymeric materials that are used as encapsulants. These materials are typically used for on-chip as well as off-chip encapsulation and packaging.Passivating materials are deposited on devices while they are still in wafer form. This is usually done at the completion of the fabrication process. These materials are mainly used for the mechanical protection of the IC during wafer dicing. Furthermore, the passivation layer also serves as corrosion protection. Inorganic polymers, such as silicon dioxide, silicon nitride and silicon-oxy-nitride, are usually employed by the semiconductor industry.
Although silicon dioxide and silicon nitride are both excellent moisture barriers, silicon dioxide is still permeable to mobile ions such as sodium, particularly when it is under bias conditions. Recently, organic polymers (polyimides, benzocyclobutenes and silicone-polyimides), particularly, the photo-definable derivatives of these, have increasingly been used as passivating materials.
Because the passivation layers are never 100% defect-free, corrosion of devices still occurs. For protection, a second layer of high performance organic encapsulant is needed. This layer will also act as a buffer coating for stress relief on large ICs.
There are numerous organic polymers that are used as electronic encapsulants. These materials are divided into (1) thermosettingpolymers, (2) thermoplastics, and (3) elastomers.
Thermoplastic polymers are materials which, when subjected to heat, will flow and solidify upon cooling without crosslinking. These thermoplastic processes are reversible and the polymers become suitable plastic engineering materials.
Polyvinyl chloride, polystyrene, polyethylene, fluorocarbon polymers, asphalt, acrylics, tars, Parylene (Union Carbide's poly-para-xylylene) and preimidized silicone-modified polyimides (originally developed by General Electric, and subsequently developed by Hitachi, M&T Chemicals, National Starch and Chemical, Occidental Chemical, Sumitomo, etc.) are examples of high performance thermoplastic polymers.
Thermosetting materials are cross- E linking polymers which cannot be | reversed to the original polymer after curing. Silicones, polyimides, epoxies, silicone-modified polyimides, silicone-epoxies, polyesters, butadiene-styrenes, alkyd resin, allyl esters, silicon-carbons (Sycars) by Hercules and polycyclicolefins by B.F. Goodrich are examples of electronic thermosetting encapsulants.
Elastomers are thermosetting materials that have high elongation or elasticity. These materials consist of a long, linear, flexible molecular chain which is joined by internal covalent chemical crosslinking. Silicone rubbers, silicone gels, natural rubbers and polyurethanes are examples. However, for IC technology applications, only a few of the materials in the above three groups can be made ultrapure in order to serve as acceptable encapsulation. Candidate materials include epoxies, silicones, polyurethanes, polyimides, silicone-polyimides, Parylenes,; polycyclicolefins, silicon-carbons and benzocyclobutenes, as well as recently-developed high performance liquid crystal materials (high performance engineering plastic materials)
Silicones (Polyorganosiloxanes)
Silicone, with a repeating unit of alternating silicon-oxygen (Si-O) siloxane backbone, has some unique chemistry. The major types are discussed as follows.Room Temperature Vulcanized (RTV) Silicones-RTV silicone is typical condensation-cure system material. The moisture-initiated catalyst such as the organotitanate, tin dibutyldilaurate-assisted process generates water or alcohol by-products which can cause outgassing and voids However, by carefully controlling the curing process, one can achieve a very reliable encapsulation. Since this silicone has a low surface tension, it tends to creep and run over the encapsulated IC circuits. To control the rheological properties of the material better, a thixotropic agent (such as fumed silica) is usually added to the formulation. The thixotropic agent provides a yield stress, increases the suitable G'' (storage modulus), G"" (loss modulus) and
* (dynamic viscosity) of the encapsulant. The ability of the RTV silicone to form chemical bonds with the coated substrate is one of the key reasons the material achieves excellent electrical performance. The reactive alkoxy functional groups of the silicone react with the surface hydroxyl groups to form a stable inert silicon-oxygen-substrate bond.
Heat Curable Hydrosilation Silicones-Heat-curable hydrosilation silicone (either elastomer or gel) has become an attractive device encapsulant. Its curing time is much shorter than the RTV-type silicone. Heat curable silicones also tend to have slightly better stability at elevated temperatures than the conventional RTV silicone. With its jelly-like (very low modulus) intrinsic softness, silicone gel (Table 1) is a very attractive encapsulant in wirebonded large chip size IC devices. The two-part heat curable system which consists of the vinyl and hydride reactive functional groups, and the platinum catalyst hydrosilation addition cure system provides a fast cure system without any byproducts .
The low viscosity hydride resin usually blends in with the higher viscosity vinyl resin to achieve an easier mixing ratio of part A (only vinyl portion) and part B (hydride plus some vinyl portion for ease of mixing).
The key to formulating a low modulus silicone is the deliberate undercrosslinking of the silicone system. A few ppm of a platinum catalyst, such as chloroplatanic acid or organoplatinum, is used in this system. This catalyst is usually incorporated in the part A vinyl portion of the resin.
However, a highly deactivated platinum catalyst system (by premixing a chelating compound such as 2-methy-3-butyn-2-ol to coordinate the reactive platinum catalyst) is used to formulate a one-component system. This one-component silicone gel system provides less mixing and a problem-free production material. This solventless type of heat curable silicone gel will have increased use in electronic applications.
Silicones in Electronic Coatings- Since World War II, silicones have been used in a variety of applications where high thermal stability, hydrophobicity and low dielectric constant are necessary, e.g., as encapsulants or conformal coatings for integrated circuits.
Epoxies
Epoxies are one of the most frequently-used polymers in electronics. This class of materials was first prepared in early 1930.Their unique chemical and physical properties, such as excellent chemical and corrosion resistance, electrical and physical properties, excellent adhesion, thermal insulation, low shrinkage and reasonable material cost have made epoxy resins very attractive in electronic applications.
Figure 3 - Manufacturing process for Bis A-based epoxy resins.
The commercial preparation of epoxies is based on bisphenol A, which reacts with epichlorohydrin producing diglycidyl ethers (Figure 2).
In addition to the bisphenol A resins, the Novolac resins have gained increasing acceptance due to their multifunctional groups which lead to higher cross-linked density and better thermal and chemical resistance. Typical epoxy curing agents are amines, anhydrides, dicyanodiamides , melamine/formaldehydes, urea/formaldehydes, phenol/ formaldehydes and catalytic curing agents. Anhydrides and amines are two of the most frequently used curing agents. Novolac is a phenol-formaldehyde, acid-catalyzed epoxy polymer. The phenolic groups in the polymer are linked by a methylene bridge which provides highly cross-linked systems which are resistant to high temperatures and chemicals. Resole is a base-catalyzed phenol-formaldehyde epoxy polymer. In most phenolic resins, the phenolic group is converted into an ether to give improved base resistance.
Epoxy Curing Agents-Selecting the proper curing agents is dependent on application techniques, curing conditions, required pot life and the desired physical properties. Besides affecting viscosity and reactivities of the epoxy formulations, curing agents determine the degree of cross-linking and the formulation of chemical bonds in the cured epoxy system.
The reactivity of some anhydrides with epoxies is slow; therefore an accelerator, usually a tertiary amine, is used to assist the cure. Novolac and Resole are two commonly used phenolformaldehyde epoxies.
New Epoxies - Recently developed high purity epoxies have become very attractive encapsulants for electronics. These new resins contain greatly reduced amounts of chloride and other mobile ions, such as sodium and potassium, and are being widely used in device encapsulation and molding compounds.
The incorporation of small, well-controlled, spherical silica particles with narrow size distribution as fillers in the epoxy systems has drastically reduce the thermal coefficient of expansion of these materials and makes them more compatible with the IC die-attache l substrate materials. The incorporation of a small amount of an elastomer) l material (such as, silicone elastomer domain particles) to the rigid epoxy ha l drastically reduced the elastic modulus l lowered the thermal stress and increase l the toughness of the epoxy material.
This new type of low-stress epoxy l encapsulant has great potential for l molding large IC devices. In addition l the newly developed "glob-top" (a glob of polymeric resin that covers the entire l IC device, including the wire bonding l is becoming increasingly more accept l able as an encapsulant for higher reliability chip-on-board and flip-chip type electronic devices and systems.
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