Emission Ghost Imaging: reconstruction with data augmentation.

Ghost Imaging enables 2D reconstruction of an object even though particles transmitted or emitted by the object of interest are detected with a single pixel detector without spatial resolution. This is possible because for the particular implementation of ghost imaging presented here, the incident beam is spatially modulated with a non-configurable attenuating mask whose orientation is varied (e.g. via transverse displacement or rotation) in the course of the ghost imaging experiment. Each orientation yields a distinct spatial pattern in the attenuated beam. In many cases, ghost imaging reconstructions can be dramatically improved by factoring the measurement matrix which consists of measured attenuated incident radiation for each of many orientations of the mask at each pixel to be reconstructed as the product of an orthonormal matrix Qand an upper triangular matrix R provided that the number of orientations of the mask (N) is greater than or equal to the number of pixels (P) reconstructed. For the N

Mechanism of Bismuth Stimulated Bottom-up Gold Feature Filling.

The mechanism underlying Bi 3+-stimulated bottom-up Au filling and self-passivation in trenches and vias in slightly alkaline Na 3 Au(SO 3)2 + Na 2 SO 3 electrolytes is explored. The impacts of electrolyte components Na 3 Au(SO 3)2, Na 2 SO 3 and Bi 3+ and potential-dependent kinetic factors on the rate of Au electrodeposition are quantified experimentally. Derived parameters are applied within the surfactant conservation Curvature Enhanced Accelerator Coverage model to simulate the filling of high aspect ratio trenches. It is observed that Bi adsorption accelerates the Au deposition rate with a non-linear dependence occurring around a critical coverage. Further, the impact of electrolyte composition is such that gradients of A u ( S O 3 ) 2 3 - and S O 3 2 - derived from reduction of A u ( S O 3 ) 2 3 - during deposition accentuate deposition farther from the feature opening. These factors and surface area reduction at the bottoms of filling features localize active deposition to feature bottoms. Ultimately, weakening of the concentration gradients and associated kinetics as bottom-up feature filling progresses reduces the Bi coverage on the growth front below the critical value and bottom-up growth terminates. Good agreement is observed with key experimental features including the incubation period of conformal deposition, transition to bottom-up growth, subsequent bottom-up filling and finally self-passivation as the growth front nears the field.

Microstructure and Texture in Copper Filled Millimeter Scale Through Silicon Vias.

The microstructure and crystallographic texture of copper electrodeposits in millimeter scale through silicon vias are characterized using electron backscatter diffraction. The deposits obtained from additive-containing CuSO4-H2SO4 electrolytes are characteristic of the superconformal deposition process, with growth textures and columnar grains consistent with previous findings in smaller TSV. The microstructure, like the filling evolution it records, changes substantially with chloride concentration for the concentrations of polymer suppressor used. With chloride concentrations of 80 µmol·L-1 and less, columnar grains of Cu capture the linear motion of the local growth front during filling with a strong <110> orientation along the elongated grain axes typical of deposition in chloride-containing Cu electrolytes. In the mid- and upper- via locations these columnar grains are angled upward from the sidewalls toward the center of the v-shaped growth front. In a limited region adjacent to the via bottom they extend vertically from the bottom surface. With millimolar chloride concentration, deposition also exhibits columnar grains with preferred <110> growth orientation in the lower region of the via and adjacent to the sidewalls. However, separation of the central deposit from the sidewalls results in a convex geometry of the growth front and spatially varying texture in most of the deposit.

Robust Bottom-Up Gold Filling of Deep Trenches and Gratings.

This work extends an extreme variant of superconformal Au electrodeposition to deeper device architectures while exploring factors that constrain its function and the robustness of void-free processing. The unconventional bottom-up process is used to fill diffraction gratings with trenches 94 µm deep and 305 µm deep, with aspect ratios (height/width) of just below 20 and 15, respectively, in near-neutral 0.16 mol·L-1 Na3Au(SO3)2 + 0.64 mol·L-1 Na2SO3 electrolyte containing 50 µmol·L-1 Bi3+. Although the aspect ratios are modest compared to previously demonstrated void-free filling beyond AR = 60, the deepest trenches filled exceed those in previous work by 100 µm - a nearly 50 % increase in depth. Processes that substantially accelerate the start of bottom-up deposition demonstrate a linkage between transport and void-free filling. Final profiles are highly uniform across 65 mm square gratings because of self-passivation inherent in the process. Electron microscopy and electron backscatter diffraction confirm the fully dense Au and void-free filling suggested by the electrochemical measurements. X-ray transmission "fringe visibility" average more than 80 % at 50 kV X-ray tube voltage across the deeper gratings and 70 % at 40 kV across the shallower gratings, also consistent with uniformly dense, void-free fill across the gratings.

Survey of P-Block Metal Additives for Superconformal Cu Deposition in an Alkaline Electrolyte.

Catalysis of Cu deposition from a near-neutral Cu2+ complexed electrolyte is examined using Bi3+, Pb2+ and Tl+ additives that were selected based on their known ability to accelerate Au deposition in near neutral pH gold sulfite electrolytes. Where appropriate, the ability of these electrolytes to yield superconformal filling of recessed features is also briefly examined. Voltammetry reveals strong acceleration of Cu deposition by Bi3+ additions while indication of superconformal filling accompanied by unusual microstructural transitions are evident in cross-sectioned specimens examined by scanning electron microscopy. Results are discussed in the context of behaviors observed for the same heavy metal additives in gold sulfite electrolytes.

Effect of Chloride on Microstructure in Cu Filled Microscale Through Silicon Vias.

The microstructure of copper filled through silicon vias deposited in a CuSO4 + H2SO4 electrolyte containing micromolar concentrations of deposition rate suppressing poloxamine and chloride additives is explored using electron backscatter diffraction. Regions with distinct microstructures are observed in the vias, including conformal deposition and seam formation localized adjacent to the bottom that can transition to bottom-up filling higher in the features. The presence and extent of each microstructure depends on applied potential as well as additive concentration. Deposition in the presence of higher chloride concentration yields a strong (110) growth texture in regions where bottom-up filling exhibits a horizontal growth front profile while (110) textured or untextured growth is observed for different conditions where upward growth proceeds with a v-notch profile.

Additives for Superconformal Gold Feature Filling.

An overview of the effect of additives on Au electrodeposition from Na3Au(SO3)2 based electrolytes is presented with an emphasis on filling of fully metallized recessed surface features such as trenches and vias. The impact of heavy metals additives Tl+, Pb2+, and Bi3+ is reviewed and accompanied by a brief survey of the effects of Sb3+, Te4+, SeCN-, 3-mercapto-1-propanesulfonic acid (MPS) and polyethyleneimine (PEI) additions. The addition of Tl+, Pb2+, Bi3+ or Sb3+ accelerates the kinetics of Au(SO3)2 3- reduction to Au, as manifest in hysteretic voltammetry and rising chronoamperometric transients, and yields bright specular deposits. Gold deposition with Pb2+ addition exhibits superconformal filling in sub-micrometer trenches while Bi3+ addition induces a more extreme bottom-up filling, but Tl+ and Sb3+ additions yield essentially conformal deposition for the conditions examined. Modest acceleration and hysteresis observed with Te4+ addition reflect roughening due to limited nucleation, 3D growth and delayed coalescence, rather than catalysis, and are associated with conformal feature filling. Unlike the other additives, SeCN-, MPS and PEI inhibit the deposition kinetics. Breakdown of suppression during deposition in micrometer size trenches is biased toward recessed surfaces where the flux of suppressor is constrained, yielding localized deposition and superconformal filling.

Bottom-Up Gold Filling in New Geometries and Yet Higher Aspect Ratio Gratings for Hard X-Ray Interferometry.

An extreme bottom-up filling variant of superconformal Au electrodeposition yielding void-free filling of recessed features is demonstrated with diffraction gratings composed of a two-dimensional patterned "chessboard" array of square vias of aspect ratio (depth/width) ≈ 23 as well as one-dimensional arrays of trenches having aspect ratios exceeding 50 and 65. Deposition on planar and patterned substrates is examined in several near-neutral x mol·L-1 Na3Au(SO3)2 + 0.64 mol·L-1 Na2SO3 electrolytes (x = [0.08, 0.16, 0.32]) containing ≈ 50 µmol·L-1 Bi3+ additive. The electrolytes are similar to those used in earlier work, although the upper bound on Au(SO3)2 concentration is twofold greater than previously described. Filling results are complemented by associated current and deposition charge transients whose features, particularly with well controlled pH, exhibit repeatable behaviors and timescales for incubation of passive deposition followed by bottom-up, void-free filling. While incompletely filled features can exhibit substantial via-to-via variation in fill height, self-passivation that follows complete bottom-up filling results in highly uniform filling profiles across the substrates. Visibility measurements capture the quality and uniformity of the as-formed wafer scale gratings. X-ray phase contrast imaging demonstrates their potential for imaging applications.

Simulation of Copper Electrodeposition in Through-Hole Vias.

Copper electrodeposition processes for filling metallized through-hole (TH) and through-silicon vias (TSV) depend on spatially selective breakdown of a co-adsorbed polyether-chloride adlayer within the recessed surface features. In this work, a co-adsorption-dependent suppression model that has previously captured experimental observations of localized Cu deposition in TSV is used to explore filling of TH features. Simulations of potentiodynamic and galvanostatic TH filling are presented. An appropriate applied potential or current localizes deposition to the middle of the TH. Subsequent deposition proceeds most rapidly in the radial direction leading to sidewall impingement at the via center creating two blind vias. The growth front then evolves primarily toward the two via openings to completely fill the TH in a manner analogous to TSV filling. Applied potentials, or currents, that are overly reducing result in metal ion depletion within the via and void formation. Simulations in larger TH features (i.e., diameter = 85 µm instead of 10 µm) indicate that lateral diffusional gradients within the via can lead to fluctuations between active and passive deposition along the metal/electrolyte interface.

Effect of Chloride Concentration on Copper Deposition in Through Silicon Vias.

Bottom-up Cu deposition in metallized through silicon vias (TSV) depends on a co-adsorbed polyether-Cl- suppressor layer that selectively breaks down within recessed surface features. This work explores Cu deposition when formation of the suppressor blocking layer is limited by the flux of Cl-. This constraint leads to a transition from passive surfaces to active deposition partway down the via sidewall due to coupling between suppressor formation and breakdown as well as surface topography. The impact of Cl- concentration and hydrodynamics on the formation of the suppressor surface phase and its potential-dependent breakdown is examined. The onset of suppression breakdown is related to the local Cl- coverage as determined by the adsorption isotherm or transport limited flux. A two-additive co-adsorption model is presented that correlates the voltammetric potential of suppression breakdown with the depth of the passive-active transition during TSV filling under conditions of transport limited flux and incorporation of Cl-. The utility of potential waveforms to optimize the feature filling process is demonstrated. At higher Cl- concentrations (≥80 µmol/L), sidewall breakdown during Cu deposition occurs near the bottom of the via followed by a shift to bottom-up growth like that seen at higher Cl- concentrations.

Bottom-up Filling of Damascene Trenches with Gold in a Sulfite Electrolyte.

Superconformal Au deposition is demonstrated in a Na3Au(SO3)2 + Na2SO3 electrolyte using Bi species to catalyze the reduction of Au(SO3)2 3-. Micromolar additions of Bi3+ to the sulfite-based electrolyte accelerate the reduction of Au(SO3)2 3- as shown by hysteretic voltammetry and rising chronoamperometric transients. Superconformal feature filling is observed over a defined range of Bi3+ concentration, potential and hydrodynamics. Over a more limited parameter range, approximately -0.9 V to -0.95 V, void-free, bottom-up filling of Damascene trenches is achieved. Furthermore, in the presence of significant convection the bottom-up filling is accompanied by passivation of the free surface. Bottom-up feature filling is characterized by a counterintuitive dependence of the free surface reactivity on the available flux of the Bi3+ accelerator species suggestive of an unusual coupling between hydrodynamic transport, shear and interfacial chemistry.

Bottom-Up Copper Filling of Millimeter Size Through Silicon Vias.

This work demonstrates void-free Cu filling of millimeter size Through Silicon Vias (mm-TSV) in an acid copper sulfate electrolyte using a combination of a poloxamine suppressor and chloride, analogous to previous work filling TSV that were an order of magnitude smaller in size. For high chloride concentration (i.e., 1 mmol/L) bottom-up deposition is demonstrated with the growth front being convex in shape. Instabilities in filling profile arise as the growth front approaches the free-surface due to coupling with electrolyte non-uniform hydrodynamics. The reentrant notches at the bottom of the TSVs caused by intentional over-etching during fabrication negatively impact the filling results. In contrast, deposition from low chloride electrolytes (i.e., 80 µmol/L) proceeds with a passive-active transition on the via sidewalls. For a given applied potential the location of the transition is fixed in time and the growth front is concave in nature reflecting the gradient in chloride surface coverage. Application of a suitable potential wave form enables the location of the sidewall transition to be systematically advanced thereby giving rise to void-free filling of the TSV.

Accelerated Bottom-Up Gold Filling of Metallized Trenches.

This work extends previously detailed void-free, bottom-up feature filling in a near-neutral Na3Au(SO3)2 + Na2SO3 electrolyte containing micromolar concentrations of Bi3+. Bottom-up electrodeposition in 17 µm and 45 µm tall trenches with an aspect ratio greater than 10 is demonstrated using potentiostatic, stepped potential and/or stepped current control. Strategies to shorten the incubation period associated with slow deposition on uniformly passivated surfaces, which precedes bottom-up filling at fixed potential, are explored. The first electron backscatter diffraction studies of bottom-up filled Au deposits reveal large grains that span the trench width and often exceed tens of micrometers in length. In contrast, smaller grains are observed near the tops of filled trenches and, under conditions of marginal filling, mid-height within them.

Exploring the Limits of Bottom-Up Gold Filling to Fabricate Diffraction Gratings.

Gold deposition on rotating disk electrodes, Bi3+ adsorption on planar Au films and superconformal Au filling of trenches up to 45 µm deep are examined in Bi3+-containing Na3Au(SO3)2 electrolytes with pH between 9.5 and 11.5. Higher pH is found to increase the potential-dependent rate of Bi3+ adsorption on planar Au surfaces, shortening the incubation period that precedes active Au deposition on planar surfaces and bottom-up filling in patterned features. Decreased contact angles between the Au seeded sidewalls and bottom-up growth front also suggest improved wetting. The bottom-up filling dynamic in trenches is, however, lost at pH 11.5. The impact of Au concentration, 80 mmol/L versus 160 mmol/L Na3Au(SO3)2, on bottom-up filling is examined in trenches up to ≈ 210 µm deep with aspect ratio of depth/width ≈ 30. The microstructures of void-free, bottom-up filled trench arrays used as X-ray diffraction gratings are characterized by scanning electron microscopy (SEM) and Electron Backscatter Diffraction (EBSD), revealing marked spatial variation of the grain size and orientation within the filled features.

Superconformal Nickel Deposition in Through Silicon Vias: Experiment and Prediction.

This work examines the filling of Through Silicon Vias (TSV) by Ni deposition from a NiSO4 + NiCl2 + H3BO3 electrolyte containing a branched polyethyleneimine suppressor. Feature filling occurs due to the interaction of transport limited suppressor adsorption and its consumption by potential dependent metal deposition. The interaction between surface topography and suppressor transport yields a sharp transition from passive to active deposition within the TSV. The transition is associated with significant incorporation of the suppressor, or its components, within the Ni deposit that results in grain refinement evident by electron backscatter diffraction (EBSD). Potential waveforms that progressively shift the location of the passive-active transition upward to optimize feature filling were examined. The evolution of feature filling and deposit microstructure are compared to predictions of a three-dimensional model that reflect critical behavior associated with suppressor-derived, S-shaped negative differential resistance (S-NDR). The model uses adsorption and consumption kinetics obtained from voltammetric measurements of the critical potential associated with suppression breakdown. Good agreement between experiment and simulation is demonstrated.

Superconformal Copper Deposition in Through Silicon Vias by Suppression-Breakdown.

The evolution of superconformal Cu electrodeposition in high aspect ratio through silicon vias (TSVs) is examined as a function of polymer suppressor concentration, applied potential and hydrodynamics. Electroanalytical measurements in a CuSO4-H2SO4-Cl electrolyte are used to explore and quantify the effect of such parameters on the metal deposition process. Hysteretic voltammetry due to suppressor breakdown reveals an S-shaped negative differential resistance that leads to non-linear spatial-temporal patterning during metal deposition. For the given hydrodynamic conditions, cyclic voltammetry reveals the potential regime over which positive-feedback gives rise to the superconformal feature filling dynamic. Breakdown of suppression is primarily related to polymer concentrations in the electrolyte while its reformation is dependent on its transport to the interface. Morphological evolution during the early stages of TSV filling reveals two distinct growth front geometries. For dilute polymer concentrations, an initial bifurcation into passive-active surfaces occurs on the side walls of the TSVs followed by bottom-up fill. The depth of the initial sidewall bifurcation within the via increases with polymer concentration. For higher polymer concentrations, i.e. ≥ 25 µmol/L, active metal deposition is rapidly confined to the bottom surface of the via followed by sustained bottom-up filling.

Bottom-Up Copper Filling of Large Scale Through Silicon Vias for MEMS Technology.

An electrodeposition process for void-free bottom-up filling of sub-millimeter scale through silicon vias (TSVs) with Cu is detailed. The 600 µm deep and nominally 125 µm diameter metallized vias were filled with Cu in less than 7 hours under potentiostatic control. The electrolyte is comprised of 1.25 mol/L CuSO4 -0.25 mol/L CH3SO3H with polyether and halide additions that selectively suppress metal deposition on the free surface and side walls. A brief qualitative discussion of the procedures used to identify and optimize the bottom-up void-free feature filling is presented.

The Impact of Adsorbates on Metal Deposition Through The Curvature Enhanced Accelerator Coverage Mechanism And Beyond.

Superconformal electrodeposition utilizing additives that adsorb on the deposit surface and either enhance or suppress metal deposition enabled the implementation of Cu damascene interconnects in microelectronics. The filling process is a consequence of the Curvature Enhanced Accelerator Coverage (CEAC) mechanism, which captures the interplay between adsorbate coverage and the metal deposition rate during area change that necessarily accompanies growth on nonplanar surfaces. CEAC-based models have successfully predicted superconformal deposition using a variety of different chemistries that yield void-free filling of fine features, as well as optically smooth, surfaces of metals such as Cu, Ag and Au. Herein a brief review of advances in the superconformal Au deposition will be detailed. Of particular interest are additives whose behavior is non-monotonic with adsorbate coverage that present new challenges for understanding as well as interesting opportunities for application.

Superconformal Bottom-Up Gold Deposition in High Aspect Ratio Through Silicon Vias.

This work presents superconformal, bottom-up Au filling of high aspect ratio through silicon vias (TSVs) along with a predictive framework based on the coupling of suppression breakdown and surface topography. The work extends a previous study of superconformal Au deposition in lower aspect ratio TSVs. Deposition was performed in a Na3AuSO3 electrolyte containing a branched polyethyleneimine (PEI) deposition-rate suppressing additive. Voltammetric measurements using a rotating disk electrode (RDE) were used to assess the impact of the PEI suppressor concentration and transport on the rate of metal deposition, enabling the interplay between metal deposition and suppressor adsorption to be quantified. The positive feedback associated with suppression breakdown gives rise to an S-shaped negative differential resistance (S-NDR). The derived kinetics for suppressor adsorption and consumption were used in a mass conservation model to account for bottom-up filling of patterned features. Predictions, including the impact of deposition potential and additive concentration on feature filling, are shown to match experimental results for filling of TSVs. This further generalizes the utility of the additive derived S-NDR model as a predictive formalism for identifying additives capable of generating localized, void-free filling of TSVs by electrodeposition.

Superconformal Bottom-Up Cobalt Deposition in High Aspect Ratio Through Silicon Vias.

This work demonstrates void-free cobalt filling of 56 µm tall, annular Through Silicon Vias (TSVs) using a mechanism that couples suppression breakdown and surface topography to achieve controlled bottom-up deposition. The chemistry, a Watts electrolyte containing a dilute suppressing additive, and processes are described. This work extends understanding and application of the additive-derived S-shaped Negative Differential Resistance (S-NDR) mechanism, including previous demonstrations of superconformal filling of TSVs with nickel, copper, zinc and gold.