Challenges and developments of copper wire bonding technology. 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. Fine pitch probing and wire bonding and reliability of aluminum capped copper bond pads page 6 backend challenges in cu technology cu bond pad. Tis extensive relationships with at tool and materials. Oct 31, 2012 resistance r copper cu wire vs gold au wire. Wire bonding is the process of forming electrical connection between the integrated circuit ic and its structural package. 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. 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. Copper wire bonding offers several mechanical and electrical advantages as well as cost saving compared to its gold wire predecessor. Bond pad surface quality for reliable wire bonding. A second proposal is to use different device structures to improve the bond wire stiffness. In this study, copper wire bonding samples were aged at 205 c in air from 0 h to 2000 h. 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.
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. Nickelpalladium bond pads for copper wire bonding atotech. Bondpad cratering, cu ball bond interface corrosion, imd intermetal dielectric cracking, and uncontrolled postwirebond staging are the key technical barriers in cu wire development. The structural reinforcement of intermetal dielectrics shown. Direct gold and copper wires bonding on copper sciencedirect. Se2618f with cu wire at carsem package qualification plan. In both the stressed and unstressed samples, aluminum layers were identified that exhibited signs of cracking and voiding. 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.
Different wire bondbond pad metal systems such as aluminumaluminum alal, goldaluminum aual, and copperaluminum cual require different. Fully processcompatible layout design on bond pad to improve. Oxidation and corrosion of auai and cuai in wire bonding. Paling dielectric layer is used to reduce the stress of pad bonding and to rise up the bonding yield 810. Process quality improvement in thermosonic wire bonding. To obtain the best wire bonding quality, we employed taguchi methods in optimizing the cu wire bonding process. Wire bonding is the method of making interconnections between an integrated circuit ic or other semiconductor device and its packaging during semiconductor device fabrication. 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. Copper ball bonding bonding copper wire to aluminum pads was previously evaluated as a method for reducing costs1,2,3. Figure 8 wire bond to alsi cu metallization using the revised bonding schedule. D material properties that effect wire bonding to copper through thin inorganic films.
Wire bonding to advanced copper, lowk integrated circuits. Wire bonding services gold wire bonding copper wire. The high hardness of cu wire increases the occurrence of chip damage during bonding 1,3,4. 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. With cu wire bonding technology, the production yield increased from 98. Imc between cu bond and al pad change in thickness. 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. 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.
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. The bonder sticks the ball onto the part device or substrate where it solidifies, then the capillary is lifted, pulling out more. 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. Effect of gas type and flow rate on cu free air ball. Experimental data from bonding with 1mil au or cu wires reveals dramatic differences in pad robustness against cracking, depending upon the underlying metal. It has higher mechanical strength for better wire bond looping performance which is the key for high io count devices.
As a result of increasing gold au price, the use of copper cu bonding wire has gained popularity for mass production. In recent years, there is increasing interest in copper wire bond technology as an alternative to gold wire. Wire bonding on a bare copper pad has always been a challenge to semiconductor packaging. Optimization of the cu wire bonding process for ic assembly. Tis journey to highvolume copper wire bonding production 3 october 2014 operations in chip assembly and test at. Stitch bonding strength of cu wire on auagpdni preplated cu. This paper discusses the uhast unbiased hast reliability performance of cu wire used in finepitch bga package. Cratering on thermosonic copper wire ball bonding springerlink. 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. 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.
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. 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. Among the technical challenges in copper wire bonding are excessive deformation of the bond pad aluminum al splash and damage to sensitive structures. Fine pitch probing and wire bonding and reliability of.
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. In an integrated circuit structure, the improvement comprising a wire bonded cu pad with cuwire component, wherein the cupad cuwire component is. Se2618f is being used as a test vehicle for the qualification of cu wire bonding on ibm. The diffusion between the copper bond wire and aluminum bond pad results in an intermetallic phase between the two materials. Indepth failure analysis has been carried out to identify the failure mechanism under various. Figure 7 wire bond to alsi cu metallization using the old bonding schedule. Bondpad cratering, cu ball bond interface corrosion, imd intermetal dielectric cracking, and uncontrolled postwirebond staging are the key technical barriers. The common pad finishes, including niau, nipdau, pdau, electroless nickel immersion gold, electroless nickelelectroless palladiumimmersion gold, and electroplated silver, are considered. Cu wire bonding, failure mechanisms, failure modes, defects, process.
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. Cu wire bonding university of california, berkeley. Selecting the wire the choice of wire diameter depends on the wire bond pitch, current carrying capacity, and cost. Figure 8 wire bond to alsicu metallization using the revised bonding schedule. Use of harsh wire bonding to evaluate various bond pad structures. Overview of wire bonding using copper wire or insulated wire. Small precision tools gold ball wire bonding process. This chapter examines the pad materials and finishes for wire bonding. Fully processcompatible layout design on bond pad to. 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. Copper cu wire bonding technical benefits anysilicon. The increasing need to reduce integrated circuit ic. This qualification is for assembly with palladiumcoated cu wire. 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.
Advantages of cu wire bonding the main advantage of cu wire bonding is the lower cost. Converting high volume ic manufacturing to cu wire. More than 90% of the chip interconnections produced annually. Converting high volume ic manufacturing to cu wire packaging. Comparative reliability studies and analysis of au, pdcoated. 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. Wire bonding length defines the resistance, capacitance, and inductance of the wire. Physically robust interconnect design in cup bond pads. Cu wire bond microstructure analysis and failure mechanism. Research focusing on methods of softening the cu wire and its free air ball fab during bonding were reported in 57. Comparative analyses between bare cu wire and palladium. This paper focuses on oxidation and the complications asso. 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. The dice are fabricated in qualified 5pae and csoi7rf cmos processes.
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. 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. Since copper wire is harder than gold wire, it is more difficult to bond. Largecuwire wedge bonding on wafers with cu pad metallization. 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. 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. 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. Effective bonding can be achieved by increasing the bond force and ultrasonic energy used. Bonding force for 1st bond is the same range, but approx. Long wire bonds can be critical to the overall package performance. Fine pitch copper wire bonding process and materials study. Tis journey to highvolume copper wire bonding production. 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.
Us6515373b2 cupadbondedcuwire with selfpassivating cu. 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 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. Additionally, cu forms intermetallics with an al bond pad more slowly than.
Copper wire is harder than au wire and oxidizes in air. Evaluations on a wire bonder equipped with a cu kit, using a 17. 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. Experimental characterization of cu freeair ball and. Wire bonding lee levine, process solutions consulting, inc. It also has higher thermal and electrical conductivity.
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. Copper ball bonding, an evolving process technology. Ballbonders are the most prevalent in the industry. The molten wire forms a ball due to surface tension. Bonding capillary is peco granular type and it has changed every time. A ball bonder feeds wire through a capillary, where the tip is heated enough to melt the wire. Figure 7 wire bond to alsicu metallization using the old bonding schedule. In applications where resistance can improve performance, using copper cu wire can offer real technical benefits. 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. Technical barriers and development of cu wirebonding in. 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.