Optimization of the cu wire bonding process for ic assembly. The structural reinforcement of intermetal dielectrics shown. As a result of increasing gold au price, the use of copper cu bonding wire has gained popularity for mass production. Selecting the wire the choice of wire diameter depends on the wire bond pitch, current carrying capacity, and cost.
The diffusion between the copper bond wire and aluminum bond pad results in an intermetallic phase between the two materials. Wire bonding with bare cu and pd coated cu pdcu wire have been adopted quickly as a mainstream packaging technology for high pin count and fine pitch devices. Wire type wire diameter emc surface finish 2 3 1 1 for au, 2 for cu 3 types bga qfn au control bare cu pdcoated cu 0. More than 90% of the chip interconnections produced annually. Comparative analyses between bare cu wire and palladium. Figure 8 wire bond to alsi cu metallization using the revised bonding schedule. Copper cu wire bonding on the preplated leadframes with nipdauag plating has been applied extensively in the semiconductor industry for the interconnection of integratedcircuit ic packaging due to the lower material cost of cu and its excellent electrical properties. Copper ball bonding, an evolving process technology. Wire bonding services gold wire bonding copper wire. In applications where resistance can improve performance, using copper cu wire can offer real technical benefits. Cu wire bonding, failure mechanisms, failure modes, defects, process.
Evaluations on a wire bonder equipped with a cu kit, using a 17. Oxidation and corrosion of auai and cuai in wire bonding. Among the technical challenges in copper wire bonding are excessive deformation of the bond pad aluminum al splash and damage to sensitive structures. Wire bonding is the process of forming electrical connection between the integrated circuit ic and its structural package.
Wire bonding on a bare copper pad has always been a challenge to semiconductor packaging. Bonding force for 1st bond is the same range, but approx. The dice are fabricated in qualified 5pae and csoi7rf cmos processes. Due to the expectation on the arrival of copper wafer, various studies on copper wire bonding, be it copper wire on aluminum bond pad or gold wire on copper bond pad have been explored by various researchers recently. It also has higher thermal and electrical conductivity. Small precision tools gold ball wire bonding process. Introduction in order to fully solve the problems of wire bonding to chips with copper bond pads, it is necessary to protect the top surface of the copper from oxidation, sulfiding, etc. Ics made of material with low dielectric constant lowk and ultra lowk are porous in nature, and are prone to fracture induced failure during packaging process. The bonder sticks the ball onto the part device or substrate where it solidifies, then the capillary is lifted, pulling out more. Physically robust interconnect design in cup bond pads. Comparative reliability studies and analysis of au, pdcoated. Bondpad cratering, cu ball bond interface corrosion, imd intermetal dielectric cracking, and uncontrolled postwirebond staging are the key technical barriers in cu wire development.
Cu wire bonding university of california, berkeley. The high hardness of cu wire increases the occurrence of chip damage during bonding 1,3,4. Se2618f is being used as a test vehicle for the qualification of cu wire bonding on ibm. Overview of wire bonding using copper wire or insulated wire. The information contained herein provides the reader as well as the industry salient points needed to make a decision when migrating from au wire to cu bonding wire, and selecting the wire type between the two technology options. With cu wire bonding technology, the production yield increased from 98. Copper cu is the most preferred alterna tive material for wire bonding because of its lower cost, higher mechanical strength, lower electrical resistance, slower intermetallic growth on alumi num al pads, and higher thermal conductivity compared with au. Wire bonding is the method of making interconnections between an integrated circuit ic or other semiconductor device and its packaging during semiconductor device fabrication. Fine pitch copper wire bonding process and materials study. Paling dielectric layer is used to reduce the stress of pad bonding and to rise up the bonding yield 810. Use of harsh wire bonding to evaluate various bond pad structures.
Converting high volume ic manufacturing to cu wire. The capillary design selection process for ultra fine pitch copper wire bonding application did not encounter much difficulty as it is also dictated by the following the bond pad pitch bpp, bond pad opening bpo, and wire diameter wd basically, the same. Copper wire bonding offers several mechanical and electrical advantages as well as cost saving compared to its gold wire predecessor. Technical barriers and development of cu wirebonding in. Despite these benefits, silicon cratering, which completes the fracture and removal of bond pad underlayers, has been a major hurdle to overcome in copper wire bonding. Stitch bonding strength of cu wire on auagpdni preplated cu. This paper presents an extensive evaluation of electroless nipd and nipdau bond pads that offer a much more robust alternative to the standard al pad.
Copper ball bonding bonding copper wire to aluminum pads was previously evaluated as a method for reducing costs1,2,3. With a history of building its own chip assembly equipment, ti was able to experiment with copper wire bonding independently to evaluate the results and benefits. In an integrated circuit structure, the improvement comprising a wire bonded cu pad with cuwire component, wherein the cupad cuwire component is. Figure 7 wire bond to alsi cu metallization using the old bonding schedule. Different wire bondbond pad metal systems such as aluminumaluminum alal, goldaluminum aual, and copperaluminum cual require different. Bedoped wire is stronger than cudoped wire by about 1020% under most conditions, thus advantageous for automated thermosonic bonding where highspeed capillary movements generate higher stresses than in slow or manual bonders. Cratering on thermosonic copper wire ball bonding springerlink. This chapter examines the pad materials and finishes for wire bonding. Ase has 6 years experience in cu wire bonding, total shipment will exceeded 7 billion units at end of 2011 reliability demonstrated to exceeded up to 6x standard jedec testing. The common pad finishes, including niau, nipdau, pdau, electroless nickel immersion gold, electroless nickelelectroless palladiumimmersion gold, and electroplated silver, are considered. Largecuwire wedge bonding on wafers with cu pad metallization. Copper wire is harder than au wire and oxidizes in air. Direct gold and copper wires bonding on copper sciencedirect. The molten wire forms a ball due to surface tension.
Wire bonding is a solid state welding process, where two metallic materials are in intimate contact, and the rate of metallic interdiffusion is a function of temperature, force, ultrasonic power, and time. The increasing need to reduce integrated circuit ic. The different properties in hardness, oxidation and electro chemical potential between au and cu have a significant impact on the wire bonding process and reliability. In a materials stack of a thin inorganic film over a metal pad, one should choose the film to have both a low ultimate strength. Challenges and developments of copper wire bonding technology. Process quality improvement in thermosonic wire bonding. In both the stressed and unstressed samples, aluminum layers were identified that exhibited signs of cracking and voiding. Effective bonding can be achieved by increasing the bond force and ultrasonic energy used. This paper discusses the uhast unbiased hast reliability performance of cu wire used in finepitch bga package.
Nov 07, 20 intermetallic compound imc thickness measurement has been carried out to estimate the coefficient of diffusion d o under various aging conditions of different bonding wires. This paper focuses on oxidation and the complications asso. It uses force, power, time, temperature, and ultrasonic energy sometimes referred to as bonding parameters to form both the ball and stitch bonds. Using cu wire has some distinct advantages as well as disadvantages see next slide cu wire bonding has been explored at the ic interface and the substrate interface using standard pwb.
Fine pitch probing and wire bonding and reliability of aluminum capped copper bond pads page 6 backend challenges in cu technology cu bond pad. Among the challenges to copper wire bonding is the damage to bond pads that had been engineered for wire bonding with the softer gold wire. Since copper wire is harder than gold wire, it is more difficult to bond. Figure 7 wire bond to alsicu metallization using the old bonding schedule.
In recent years, there is increasing interest in copper wire bond technology as an alternative to gold wire. Us6515373b2 cupadbondedcuwire with selfpassivating cu. Using cu wire for wire bonding can be a solution to the wire short problem caused by small wire sizes, besides other solutions such as using insulated wire zhong, 2008 and having varying loop heights zhong, 2007. Se2618f with cu wire at carsem package qualification plan.
Bondable with al thick wire and alsi thin wire bondable with au gold bonding wire solderable for leadfree and leaded solder materials applicable on all. Tis journey to highvolume copper wire bonding production. Effect of gas type and flow rate on cu free air ball. Wire bonding to advanced copper, lowk integrated circuits. However, there is a limit to which these parameters can be increased, since excessive force and power can damage the silicon substrate under the bond pad, a phenomenon known as cratering. In this study, copper wire bonding samples were aged at 205 c in air from 0 h to 2000 h. Ballbonders are the most prevalent in the industry. Bedoped wire is stronger than cu doped wire by about 1020% under most conditions, thus advantageous for automated thermosonic bonding where highspeed capillary movements generate higher stresses than in slow or manual bonders. Nickelpalladium bond pads for copper wire bonding atotech. The capillary design selection process for ultra fine pitch copper wirebonding application did not encounter much difficulty as it is also dictated by the following the bond pad pitch bpp, bond pad opening bpo, and wire diameter wd basically, the same. Figure 8 wire bond to alsicu metallization using the revised bonding schedule. A stage temperature of 1502008c is also needed for bonding copper wire chen et al. Wire bonding length defines the resistance, capacitance, and inductance of the wire.
Fully processcompatible layout design on bond pad to. Copper wire bonding book lays out the challenges involved in replacing gold with copper as a wire bond material, and includes the bonding process changes. Copper wire is harder than gold, and thus needs greater ultrasonic power and bond force to. A ball bonder feeds wire through a capillary, where the tip is heated enough to melt the wire. This qualification is for assembly with palladiumcoated cu wire. Bond pad surface quality for reliable wire bonding. Tis extensive relationships with at tool and materials. Research focusing on methods of softening the cu wire and its free air ball fab during bonding were reported in 57. Copper cu wire bonding technical benefits anysilicon. Wire pull and ball bond shear strengths have been analyzed and we found smaller variation in pddoped cu wire compared to au and pddoped cu wire. Tis journey to highvolume copper wire bonding production 3 october 2014 operations in chip assembly and test at. Bonding capillary is peco granular type and it has changed every time. Oct 31, 2012 resistance r copper cu wire vs gold au wire.
With cu wire, one must also be concerned with added variables such as the age of the copper wire, softness or purity of the wire, thickness of the al bond pad, mix and flow rate of forming gas, and the composition of the overmold compound. Experimental characterization of cu freeair ball and. Cu wire bond microstructure analysis and failure mechanism. When wire bonding on multilayer substrates, wirebond pads should be a minimum of 10 mm from the edge of adjacent conductors to allow for registration, printing, and wire bonding tolerances. Intermetallic compound imc thickness measurement has been carried out to estimate the coefficient of diffusion d o under various aging conditions of different bonding wires. It has higher mechanical strength for better wire bond looping performance which is the key for high io count devices. Although less common, wire bonding can be used to connect an ic to other electronics or to connect from one printed circuit board pcb to another. Long wire bonds can be critical to the overall package performance. Bondpad cratering, cu ball bond interface corrosion, imd intermetal dielectric cracking, and uncontrolled postwirebond staging are the key technical barriers. Ase has 6 years experience in cu wirebonding, total shipment will exceeded 7 billion units at end of 2011 reliability demonstrated to exceeded up to 6x standard jedec testing. Converting high volume ic manufacturing to cu wire packaging. It was found that the bonding of a cu wire and an al pad formed cu 9 al 4, cual, and cual 2 intermetallic compounds, and an initial crack was formed by the ultrasonic squeeze effect during thermosonic wire bonding. To obtain the best wire bonding quality, we employed taguchi methods in optimizing the cu wire bonding process.
Advantages of cu wire bonding the main advantage of cu wire bonding is the lower cost. Additionally, cu forms intermetallics with an al bond pad more slowly than. Copper wire bonds can be so strong that the wire itself does not break during wire pull testing, resulting instead in bond pad metal lifting. D material properties that effect wire bonding to copper through thin inorganic films. A second proposal is to use different device structures to improve the bond wire stiffness. Experimental data from bonding with 1mil au or cu wires reveals dramatic differences in pad robustness against cracking, depending upon the underlying metal. Wire bonding lee levine, process solutions consulting, inc. Fully processcompatible layout design on bond pad to improve.
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