Ta/Au ohmic contacts to n-type ZnO

Ta/Au ohmic contacts are fabricated on n-type ZnO (∼1 × 10¹⁷ cm⁻³) epilayers, which were grown on R-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). After growth and metallization, the samples are annealed at 300°C and 500°C for 30 sec in nitrogen ambient. The specific contact resistance is measured to be 3.2×10⁻⁴ Ωcm² for the as-deposited samples. It reduces to 5.4×10⁻⁶ Ωcm² after annealing at 300°C for 30 sec without significant surface morphology degradation. When the sample is annealed at 500°C for 30 sec, the specific contact resistance increases to 3.3 × 10⁻⁵ Ωcm². The layer structures no longer exist due to strong Au and Ta in-diffusion and O out-diffusion. The contact surface becomes rough and textured.     

Zn-Al-Mg-Ga alloys as Pb-free solder for die-attaching use

Zn-based alloys have been investigated to replace Pb-5%Sn solder for die-attaching use. We have found that a Zn-4%Al-3%Mg-3%Ga alloy has a 309°C solidus and a 347°C liquidus. A die-attaching test was done with preforms of this alloy, Ag-plated lead-frames, and Au-plated dummy dies. Good die-attaching with a small amount of voids can be achieved at 320°C or higher. In subsequent reliability tests, no failure was observed until 1000 cycles between −65°C and 150°C or until 1000 h at 85°C and 85% humidity. Although the poor workability and poor ability of stress relaxation at room temperature of this alloy may somewhat limit its application areas, this solder is the first Pb-free solder for die-attaching use to our knowledge.     

Creep behavior of the ternary 95.5Sn-3.9Ag-0.6Cu solder: Part II—Aged condition

Compression creep tests were performed on the 95.5Sn-3.9Ag-0.6Cu (wt.%) solder. The specimens were aged prior to testing at 125°C, 24 h or 150°C, 24 h. Applied stresses were 2–40 MPa. Test temperatures were −25°C to 160°C. The 125°C, 24-h aging treatment caused the formation of coarsened Ag₃Sn particle boundaries within the larger ternary-eutectic regions. The 150°C, 24-h aging treatment resulted in contiguous Ag₃Sn boundaries in the ternary-eutectic regions as well as a general coarsening of Ag₃Sn particles. The 125°C, 24-h aging treatment had only a small effect on the strain-time curves vis-à-vis the as-cast condition. Negative creep was observed at 75°C for time periods >10⁵ sec and stresses of 3–10 MPa. The creep kinetics exhibited a sinh term (stress) exponent, p, of 5.3±0.6 and an apparent-activation energy, ΔH, of 49±5 kJ/mol when data from all test temperatures were included. A good data correlation was observed over the [−25–125°C] temperature regime. Steady-state creep kinetics exhibited a greater variability in the [125–160°C] regime because of the simultaneous coarsening of Ag₃Sn particles. The aging treatment of 150°C for 24 h resulted in a more consistent stress dependence and better reproducibility of the creep curves. Negative creep was observed in samples aged at 150°C for 24 h when tested at −25°C, 25°C, and 75°C. The values of p and ΔH were 4.9±0.3 kJ/mol and 6±5 kJ/mol, respectively. Only a slight improvement in the data correlation was observed when the analysis examined separated [−25−75°C] and [75–166°C] temperature regimes. Creep testing did not cause observable deformation in any of the sample microstructures.     

The effect of intermetallic compound morphology on Cu diffusion in Sn-Ag and Sn-Pb solder bump on the Ni/Cu Under-bump metallization

The eutectic Sn-Ag solder alloy is one of the candidates for the Pb-free solder, and Sn-Pb solder alloys are still widely used in today’s electronic packages. In this tudy, the interfacial reaction in the eutectic Sn-Ag and Sn-Pb solder joints was investigated with an assembly of a solder/Ni/Cu/Ti/Si₃N₄/Si multilayer structures. In the Sn-3.5Ag solder joints reflowed at 260°C, only the (Ni_1−x,Cu_x)₃Sn₄ intermetallic compound (IMC) formed at the solder/Ni interface. For the Sn-37Pb solder reflowed at 225°C for one to ten cycles, only the (Ni_1−x,Cu_x)₃Sn₄ IMC formed between the solder and the Ni/Cu under-bump metallization (UBM). Nevertheless, the (Cu_1−y,Ni_y)₆Sn₅ IMC was observed in joints reflowed at 245°C after five cycles and at 265°C after three cycles. With the aid of microstructure evolution, quantitative analysis, and elemental distribution between the solder and Ni/Cu UBM, it was revealed that Cu content in the solder near the solder/IMC interface played an important role in the formation of the (Cu_1−y,Ni_y)₆Sn₅ IMC. In addition, the diffusion behavior of Cu in eutectic Sn-Ag and Sn-Pb solders with the Ni/Cu UBM were probed and discussed. The atomic flux of Cu diffused through Ni was evaluated by detailed quantitative analysis in an electron probe microanalyzer (EPMA). During reflow, the atomic flux of Cu was on the order of 10¹⁶−10¹⁷ atoms/cm²sec in both the eutectic Sn-Ag and Sn-Pb systems.     

Thermal activation of shallow boron-ion implants

Boron implanted into n-type Si at 10¹⁵ cm⁻² dose and energies from 500 eV to 1 keV was activated by annealing in nominally pure N₂ and in N₂ with small admixtures of O₂. Effective process times and temperatures were derived by thermal activation analysis of various heating cycles. The lowest thermal budgets used “spike anneals” with heating rates up to 150°C/sec, cooling rates up to 80°C/sec, and minimal dwell time at the maximum temperature. Dopant activation was determined by sheet electrical transport measurements. Surface oxidation was characterized by film thickness ellipsometry. P-n junction depths were inferred from analysis of sheet electrical transport measurements and secondary ion mass spectroscopy profiles. Boron activation increases with boron diffusion from the implanted region. Electrical activation has a thermal activation energy near 5 eV, while boron diffusion has an activation energy near 4 eV. Surface oxide can retard boron diffusion into the ambient for high-temperature anneals.     

Effect of bump characteristics and temperature variation on the online contact resistance of anisotropic conductive joints

Anisotropic conductive film (ACF) suffers a major drawback in regard to reliability even though it has merits, such as reduction in interconnection distance, high performance, and environmental friendliness. The factor of thermal warpage may lead to a highly unreliable electrical connection in the assembly. The work presented in this paper focuses on the online contact-resistance behavior of the ACF joint during thermal shock and compares the results of two different types of dies (Au/Ni bump and bumpless). For this work, we used a flip chip of 11 × 3 mm² in dimension. The flex substrate used was made of polyimide film with an Au/Ni/Cu electrode and daisy-chained circuit for a matching die-bump pattern. The ACF that was used is an epoxy resin in which nickel and gold-coated polymer balls are dispersed. Tests for three different thermal-cycling profiles (125°C to −55°C, 140°C to −40°C, and 150°C to −65°C) were carried out. The samples bonded at a temperature of 180°C, and a pressure of 80 N was used. The initial contact resistances of Au/Ni bump and bumpless samples were 0.25 ω and 0.4 ω respectively. A comparative study was carried out from the results obtained. The results showed that for the flip-chip-on-flex (FCOF) packages having an Au/Ni bump, the increase in online contact resistance is higher than that of the FCOF packages having bumpless chips. For example, in the thermal-cycling profile of 140°C to −40°C, the online contact resistance for the Au/Ni bump raised to 4.6 ω after 180 cycles, whereas it was only 1.3 ω for the bumpless sample. The bump height and bump materials were found to be the main factor for such variation. Results show that, above the glass-transition temperature (T_g), the ACF matrix becomes less viscous, which reduces its adhesive strength and lets the higher bump height of the chip result in a higher standoff of the package and thus sliding is easier to take place. The responses by the assemblies in hot and cold conditions are examined, and in-chamber behavior of the assembly is studied and explained.     

Defects in annealed 1.5 MeV boron implanted p-type silicon

Effects of temperature and dosage on the evolution of extended defects during annealing of MeV ion-implanted Czochralski (CZ) p-type (001) silicon have been studied using transmission electron microcopy. Excess interstitials generated in a 1 10¹⁵ cm⁻²/1.5 MeV B⁺ implanted Si have been found to transform into extended interstitial {311} defects upon rapid thermal annealing at 800°C for 15 sec. During prolonged furnace annealing at 960°C for 1 h, some of the {311} defects grow longer at the expense of the smaller ones, and the average width of the defects seems to decrease at the same time. Formation of stable dislocation loops appears to occur only above a certain threshold annealing temperature (∼1000°C). The leakage current in diodes fabricated on 1.5 MeV B⁺ implanted wafers was found to be higher for a dosage of 1 10¹⁴cm⁻² and less, as compared to those fabricated with a dosage of 5 10¹⁴ cm⁻² and more. The difference in the observed leakage current has been attributed to the presence of dislocations in the active device region of the wafers that were implanted with the lower dosage.     

Low-Temperature Epitaxy of Compressively Strained Silicon Directly on Silicon Substrates

We report epitaxial growth of compressively strained silicon directly on (100) silicon substrates by plasma-enhanced chemical vapor deposition. The silicon epitaxy was performed in a silane and hydrogen gas mixture at temperatures as low as 150°C. We investigate the effect of hydrogen dilution during the silicon epitaxy on the strain level by high-resolution x-ray diffraction. Additionally, triple-axis x-ray reciprocal-space mapping of the samples indicates that (i) the epitaxial layers are fully strained and (ii) the strain is graded. Secondary-ion mass spectrometry depth profiling reveals the correlation between the strain gradient and the hydrogen concentration profile within the epitaxial layers. Furthermore, heavily phosphorus-doped layers with an electrically active doping concentration of ~2 × 10²⁰ cm⁻³ were obtained at such low growth temperatures.     

Density measurement of Sn-40Pb,Sn-57Bi,and Sn-9Zn by indirect Archimedean method

By the indirect Archimedean method, the density and the density-temperature relationship of the Sn-40Pb eutectic alloy and two Pb-free solders, Sn-57Bi and Sn-9Zn eutectic alloys, were measured from room temperature to about 250°C. The results showed that the density-temperature dependence for each alloy in both solid and melting states can be fitted linearly as ρ_S(Sn-40Pb)=8.51−8.94×10⁻⁴(T−25°C), ρ_L(Sn-40Pb)=8.15−13.8×10⁻⁴(T−T_m); ρ_S(Sn-57Bi)=8.54−5.86 × 10⁻⁴(T−25°C), ρ_L(Sn-57Bi)=8.51−10.9×10⁻⁴(T−T_m); and ρ_s(Sn-9Zn)=7.22−7.78×10⁻⁴(T−25°C), ρ_L(Sn-9Zn)=6.89−5.88×10 ⁻⁴(T−T_m), where the density unit was g/cm³. At the melting point, density of the melt of these solders is 8.15 g/cm³, 8.51 g/cm³, and 6.89 g/cm³, respectively. The density decreased 2.6% for Sn-40Pb eutectic alloy during melting, and 2.7% for Sn-9Zn eutectic alloy, but increased 0.5% for Sn-57Bi eutectic alloy. The excess molar volume for these alloys after mixing at their melting point is 0.03 cm³/mol for Sn-40Pb, 0.09 cm³/mol for Sn-57Bi, and 0.21 cm³/mol for Sn-9Zn.     

Exfoliation and blistering of Cd<Subscript>0.96</Subscript>Zn<Subscript>0.04</Subscript>Te substrates by ion implantation

As part of a series of wafer bonding experiments, the exfoliation/blistering of ion-implanted Cd_0.96Zn_0.04Te substrates was investigated as a function of postimplantation annealing conditions. (211) Cd_0.96Zn_0.04Te samples were implanted either with hydrogen (5×10¹⁶ cm⁻²; 40–200 keV) or co-implanted with boron (1×10¹⁵ cm⁻²; 147 keV) and hydrogen (1–5×10¹⁶ cm⁻²; 40 keV) at intended implant temperatures of 253 K or 77 K. Silicon reference samples were simultaneously co-implanted. The change in the implant profile after annealing at low temperatures (<300°C) was monitored using high-resolution x-ray diffraction, atomic force microscopy (AFM), and optical microscopy. The samples implanted at the higher temperature did not show any evidence of blistering after annealing, although there was evidence of sample heating above 253 K during the implant. The samples implanted at 77 K blistered at temperatures ranging from 150°C to 300°C, depending on the hydrogen implant dose and the presence of the boron co-implant. The production of blisters under different implant and annealing conditions is consistent with nucleation of subsurface defects at lower temperature, followed by blistering/exfoliation at higher temperature. The surface roughness remained comparable to that of the as-implanted sample after the lower temperature anneal sequence, so this defect nucleation step is consistent with a wafer bond annealing step prior to exfoliation. Higher temperature anneals lead to exfoliation of all samples implanted at 77 K, although the blistering temperature (150–300°C) was a strong function of the implant conditions. The exfoliated layer thickness was 330 nm, in good agreement with the projected range. The “optimum” conditions based on our experimental data showed that implanting CdZnTe with H⁺ at 77 K and a dose of 5×10¹⁶/cm² is compatible with developing high interfacial energy at the bonded interface during a low-temperature (150°C) anneal followed by layer exfoliation at higher (300°C) temperature.     

High-quality crystalline layer transfer from a silicon-on-insulator substrate onto a sapphire substrate using wafer bonding

We demonstrate layer transfer of 150 nm of Si from a 200-mm, silicon-on-insulator (SOI) substrate onto a sapphire substrate using low-temperature wafer bonding (T=150°C). The crystalline quality and the thermal stability of the transferred Si layer were characterized by x-ray diffraction (XRD). A broadening of the (004) Si peak is observed only for anneal temperatures T_A≥800°C, indicating some degradation of the crystalline quality of the transferred Si film above these temperatures. The measured electron Hall mobility in the bonded Si layer is comparable to bulk silicon for T_A≤800°C, indicating excellent material quality.     

Rapid thermal annealing of dual Si and P implants in InP

In this study we evaluate the effects of dual implantation with different doses of Si and P on dopant activation efficiency and carrier mobility in InP:Fe. The implants were activated by a rapid thermal annealing step carried out in an optimized phosphoruscontaining ambient. For high dose implants (10¹⁴–10¹⁵ cm⁻²), which are typically employed for source/drain regions in FETs, dual implantation of equal doses of Si and P results in a higher sheet carrier concentration and lower sheet resistance. For 10¹⁴ cm⁻² Si implants at 150 keV, the optimal P co-implant dose is equal to the Si dose for most anneal temperatures. We obtain an activation efficiency of ∼70% for dual implanted samples annealed at 850° C for 10 sec. The high activation efficiencies and low sheet resistances obtained in this study emphasize the importance of stoichiometry control through the use of P co-implants and a phosphorus-containing ambient during the thermal processing of InP.     

Lateral schottky GaN rectifiers formed by Si<Superscript>+</Superscript> ion implantation

Type conversion of p-GaN by direct Si⁺ ion implantation and subsequent annealing was demonstrated by the fabrication of lateral Schottky diodes. The Si⁺ activation percentage was measured as a function of annealing time (30–300 sec) and temperature (1,000–1,200°C), reaching a maximum of ∼30% for 1,200°C, 2-min anneals. The resulting n-type carrier concentration was 1.1×10¹⁸ cm⁻³ for a moderate Si⁺ ion dose of ∼2×10¹⁴ cm⁻². The lateral Schottky diodes displayed a negative temperature coefficient of −0.15 V·K for reverse breakdown voltage.     

Structure and properties of InGaAs layers grown by low-temperature molecular-beam epitaxy

This paper describes studies of InGaAs layers grown by molecular-beam epitaxy on InP (100) substrates at temperatures of 150–480 °C using various arsenic fluxes. It was found that lowering the epitaxy temperature leads to changes in the growth surface, trapping of excess arsenic, and an increased lattice parameter of the epitaxial layer. When these lowtemperature (LT) grown samples are annealed, the lattice parameter relaxes and excess arsenic clusters form in the InGaAs matrix. For samples grown at 150 °C and annealed at 500 °C, the concentration of these clusters was ∼8×10¹⁶ cm⁻³, with an average cluster size of ∼5 nm. Assuming that all the excess arsenic is initially trapped in the form of antisite defects, the magnitude of the LT-grown InGaAs lattice parameter relaxation caused by annealing implies an excess arsenic concentration (N _As−N _Ga−N _In)/(N _As+N _Ga+N _In)=0.4 at.%. For layers of InGaAs grown at 150 °C, a high concentration of free electrons (∼1×10¹⁷ cm⁻³) is characteristic. Annealing such layers at 500 °C decreases the concentration of electrons to ∼1×10¹⁷ cm⁻³. The results obtained here indicate that this change in the free-electron concentration correlates qualitatively with the change in excess arsenic concentration in the layers. Fiz. Tekh. Poluprovodn. 33, 900–906 (August 1999)     

P-type doping of GaAs by carbon implantation

The electrical properties of C-implanted <100> GaAs have been studied following rapid thermal annealing at temperatures in the range from 750 to 950°C. This includes dopant profiling using differential Hall measurements. The maximum p-type activation efficiency was found to be a function of C-dose and annealing temperature, with the optimum annealing temperature varying from 900°C for C doses of 5 × 10¹³ cm⁻² to 800°C for doses ≥5 × 10¹⁴cm⁻². For low dose implants, the net p-type activation efficiency was as high as 75%; while for the highest dose implants, it dropped to as low as 0.5%. Moreover, for these high-dose samples, 5 × 10¹⁵ cm⁻², the activation efficiency was found to decrease with increasing annealing temperature, for temperatures above ∼800°C, and the net hole concentration fell below that of samples implanted to lower doses. This issue is discussed in terms of the amphoteric doping behavior of C in GaAs. Hole mobilities showed little dependence on annealing temperature but decreased with increasing implant dose, ranging from ∼100 cm²/V·s for low dose implants, to ∼65 cm²/V·s for high dose samples. These mobility values are the same or higher than those for Be-, Zn-, or Cd-implanted GaAs.     

Nitrogen implantation into GaP: Damage and nitrogen location studies

Implantations of 80 and 40 keV nitrogen into GaP have been carried out. The implants were performed at temperatures from 25° to 400°C for total doses in the range 10¹⁵ to 10^l6 cm⁻². Backscattering and channeling techniques have been used to determine the associated damage and its annealing characteristics for temperatures up to 800°C . Also determined, has been the lattice location of the implanted nitrogen. This was performed by implanting with the isotope¹⁵N and using the¹⁵N (p,α_o)¹²C nuclear reaction in conjunction with channeling techniques using a l MeV proton beam. Results indicate that the GaP is totally damaged following 25°C implants with 10¹⁵N⁺cm⁻². After annealing to 800°C , 70% of the damage has recovered. For implants above 100°C the initial damage is ~15%. Lattice location studies on samples implanted above 150°C show that following an 8 × 10¹⁴N⁺cm⁻² implant, ~60% of the nitrogen is located substitutionally. The substitutional content is reduced to 33% for an 8 × 10^l5N⁺cm^-2 implant. For anneals above 600°C the nitrogen is found to move off the substitutional sites. The results show that using ion implantation at elevated temperatures it is possible to obtain high concentrations of substitutional nitrogen in relatively damage free space.     

Role of impurities in the formation of silicyne (long-chain silicon): Theory and experiment

The short-range order structure of amorphous silicon prepared by various methods is investigated by electron diffraction analysis. The influence of impurities in the as-prepared films and those irradiated with neon, oxygen, and carbon ions at doses up to 1×10¹⁶ cm⁻² on the character of structural transformations and the formation of interatomic silicon multiple bonds during annealing are investigated. The structure of films annealed at 500 °C is found to depend on the type of impurities and the nature of their chemical bond with silicon atoms. In particular, oxygen (>0.2 at. %), unlike hydrogen and carbon, acts as an inhibitor for the formation of silicyne. Good agreement is also noted between the experimentally determined short-range order parameters and those calculated by the nonempirical Hartree-Fock method. Fiz. Tekh. Poluprovodn. 33, 1253–1259 (October 1999)     

A high-performance thermal module for computer packaging

A thermal module was designed to transfer heat efficiently from high power dissipation chips to a liquid coolant via forced convection. Turbulent and laminar flow regimes were investigated. Channel geometries for deep channels (1000 μm deep, and used for turbulent flow), and shallow channels (100 μm deep, and used for laminar flow) were optimized for high heat transfer coefficient, ease of fabrication, and better structural rigidity of the module. A 4″ x 4″ module, made out of Cu, was tested using a 4″ Si “thermal” wafer as a heat generating source as well as a temperature sensor. Wafer scale integration and high energy ion implantation were employed to obtain nine l x l cm heat sources, and temperature sensing diodes embedded within the thermal wafer. For the deep channel design, the maximum device temperature rise on the module was 18° C for a power dissipation of 42 W/chip, and a flow rate of 126 cc/sec. For the shallow channel design, the temperature rise was 19° C for a flow rate of 19 cc/sec, and a power dissipation level of 42 W/chip. With all nine chips on the thermal module powered to 42 W/chip, the maximum chip to chip temperature variations were found to be 2 and 8° C for deep and shallow channel designs, respectively.     

Solid boron and antimony doping of Si and SiGe grown by gas source molecular beam epitaxy

Solid boron and antimony doping of silicon and SiGe grown by molecular beam epitaxy using disilane and germane as sources has been studied. Elemental boron is a well behaved p-type dopant. At effusion cell temperatures of 1700–1750°C, hole carrier concentrations in the 10²⁰ cm⁻³ range have been obtained. Elemental antimony doping shows surface segregation problems. For uniformly doped layers, the as-grown materials do not show n-type conductivity. Electron concentrations in the 10¹⁷ cm⁻³ range were obtained by post-growth conventional and rapid thermal annealing at 900 and 1000°C, respectively. The electron Hall mobility improves with optimum annealing time. Delta doping of buried layers exhibits slightly better incorporation behavior including significant surface riding effects.     

Thermomechanical effects in embedded passive materials

Thermomechanical fatigue was measured using electron-beam moiré (EBM) and infrared (IR) microscopy. A specimen was made using a FR-4 printed wiring board (PWB), a silver-filled conductive adhesive, and a carbon-filled conductive paste. Both filled polymeric materials are used for embedded resistor applications. We studied the behavior of these materials and, particularly, the interfaces between these materials as a function of thermal cycling. The EBM gives quantitative information on localized strains, whereas the IR microscopy gives quantitative information on changes in heat flow. The filled polymeric materials showed strains of −0.6% at −55°C and 1.4% at 125°C. The interface between the silver and carbon-filled materials increased in thermal resistance on the order of 10⁻⁶ m²·K·W⁻¹ per thermal cycle.