Haiyong Cao

Through silicon via filling by copper electroplating in acidic cupric methanesulfonate bath

Copper electrodeposition in acidic cupric methanesulfonate bath with organic additives is discussed in this paper. The influence of poly(ethylene glycol) (PEG) and bis-(3-sodiumsulfopropyl disulfide) (SPS) on copper deposition were studied by means of linear sweep voltammetry, cyclic voltammetry and chronoamperometry. These electrochemical analysises revealed a competition of PEG and SPS on electrode surface site. The swiftness of SPS chemisorption and the subsequent displacement by the passivating film of PEG exerted an extra wave at small overpotential on the negative-going sweep. The following polarization curve indicated the firmness of the passivating film. All these features of additives in acidic cupric methanesulfonate bath suggested a novel method to achieve superconformal or bottom-up filling which was proved by actual TSV plating.

Collaborative effect between additives and current in TSV via filling process

Deep via filling is one of key technologies of 3D packaging. Vias are commonly filled by electroplating. Since cupric transportation in vias is limited by diffusion, Current density is one of the most influential factors for copper plating in vias. We investigated new additive of accelerator, suppressor and leveler. Simulation of the competitive adsorption ability of accelerator and suppressor at different potential was studied. It was found that accelerator has more powerful adsorption ability than suppressor at high potential. Suppressor would form a passivating layer at the surface in the electroplating process, but the layer is easily disrupted by accelerator at high potential. We also investigated vias filling at different current density to prove our assumption. 0.4ASD was the best condition which got a fulfilled via without voids or seams. Conformal growth performance was attained at low current density and large current density would sealed the opening quickly, leaving seam at the bottom.

Simulation of electric field uniformity in through silicon via filling

There are many factors which will affect the final result of TSV filling, such as ratio of the additives including accelerator, suppresser, leveler and so on. Complicated environment is also hard to control exactly. If we want to form the “bottom up” in the plating, there must be comprehensively consideration of all factors. Many studies have focused on the effect of additives on the plating uniformity. But in industrial production, the macro-uniformity is the primary consideration. In this paper, we are talking about the most important factor-the electric field. ANSYS is used to simulate the electric field distribution in order to find the most Optimal parameters. Considering some major factors in electroplating, an optimization model is suggested by simulation. And the non-uniformity is reduced to below 10%. Based on this, we have simulated the electroplating of microholes in the beginning of TSV filling. The uniformity of the electric field inside the hole is mainly determined by the adsorption of chemical additives, especially for the suppressor. Deeper of the adsorbed position of suppressor, better “bottom up” trend in the process of electroplating.

Simulation of TSV copper electrodeposition process with additives

The kinetics which explained the Copper Super-filling by electrodepostition was investigated since the copper deposition became the standard technique for TSV. A simple numerical model is built to explain the copper deposition process. Consider with the effect of the accelerator, a linear equation is built to explain the relationship between the exchange current density and the coverage of accelerator as well as the relationship of the potential and the coverage of accelerator. The parameter of the equation is investigated in the article. The results show that the linear equation indicates process of copper electrodeposition sufficiently with properly parameters.

Investigation of competitive adsorption between accelerator and suppressor in TSV copper electroplating

In TSV copper electroplating, the goal is to achieve superfilling deposition. In order to reach the bottom-up in TSV copper electroplating, some additives (accelerator suppressor and leveler) are added into the electroplating bath. To know the relationship between additives in the plating solution is very important. In the present work, by means of linear sweep voltammetry (LSV) and cyclic voltammogram (CV), it is found that there is a competition between the accelerator and the suppressor. At the less negative potential, the accelerator shows a strong adsorption property, and the suppressor plays its role in the relatively more negative potential area, which shows a competitive adsorption relationship between the accelerator and the suppressor during the process of bottom-up filling.

Study of internal stress on electroplating copper used in through silicon via filling

Through silicon via (TSV) is a high performance technique to create 3D packages and 3Dintegrated circuits, compared to alternatives such as package-on-package, because the density of the via is substantially higher. The effect of current density and baths organic additives on internal stress of copper electrodeposited from TSV filling methanesulfonate bath has been studied by Surface Profiler, X-ray diffraction (XRD) and Scanning electron microscope (SEM). We show that addition of the organic additives and current density can affect the internal stress of electrodeposited cupper films. The XRD patterns of copper films reveal the presence of (1 1 1), (2 0 0), (2 2 0), and (3 1 1) peaks, intensity of which is connect with the concentration of the organic additives in the plating bath and the current density of electrodeposition.

Influence of leveler concentration on copper electrodeposition for through silicon via filling

Through silicon via technology is one of the critical and enabling technologies for 3D packaging. 300μm deep vias with a diameter of 50μm were filled by copper electroplating with CuSO4 and H2SO4 as base electrolyte. Chloride ions, accelerator and leveler were added. The effect of leveler concentration on filling performance was studied. Electrochemical measurements were used to investigate the cathode process and the action of additives. It was found that mass transportation of copper in via became the slowest step in deep vias, small current is necessary to obtain void free deposit and only conformal growth was obtained. And with increasing of leveler concentration filling performance became better.

Effect of different pretreatments on through silicon via copper filling

Pretreatments have a great effect on the through silicon via (TSV) copper electroplating filling process. In this study, we compared the wetting effect by observing cross-sectional images of samples pretreated with ultrasound and vacuuming method. And the electrochemical test was used to verify the effect of acid plating solution on the oxidation of the Cu seed layer. Without any pretreatment, vias showed a poor wettability that large voids existed at the bottom. The TSV pretreated by ultrasound and vacuuming method both were coated well with copper deposition layer, which showed a good wettability. However, when the Cu seed layer of the TSV was not well sputtered, ultrasound vibration may cause the exfoliation of the Cu seed layer resulting in the formation of void at the bottom of the vias. Diluted additives as prewetting solution made no much difference on filling effect. The acidic plating solution could dissolve the oxide layer at a high speed and react with the Cu seed layer as well.

Analysis of accelerator consumption in TSV copper electroplating

Accelerator consumption was analyzed by electrochemical method. As the presence of SPS and PEG in the acidic cupric sulfate solution with chloride ions, the cathodic polarization curves become have peaks and valleys. The peak and valley have a strong association with the additive concentration and are associated with the electroplating process. The effect of electroplating time on the cathodic polarization curves is similar to the effect of SPS concentration. The linear relation between parameter Q with the concentration of SPS is Q = 0.007624 + 0.150992CSPS. The parameter Q is useful for the analysis of SPS consumption in TSV copper electroplating. The real deposition in TSV verifies that SPS consumption in the process of plating can be obtained by analyzing cathodic polarization curves.

The effect of different TSV electroplating levelers on the copper residual stress

Levelers have great influence on residual stress of electrodeposited film used in TSV. In this study, 2-MP (2-mercaptopyridine), JGB (Janus green B) and 2-ABT (2-aminobenzothiazole) are used as levelers to study the effects of different levelers on residual stress. The film stress deposited with different levelers decreases in the order of 2-MP, JGB and 2-ABT and closely related to grain size. Grain size is 15nm, 80nm, 150nm with 2-MP, JGB, 2-ABT respectively. So the larger the grain size, the smaller the tensile stress. Levelers inhibitory decreases in the order of 2-MP, JGB and 2-ABT and higher inhibitory of levelers induce finer grains. Preferred planes change from (200), (111) to (220) with increasing 2-MP and JGB in the solution. The texture coefficient of (220) plane keeps increasing with increasing of 2-ABT concentration. The film surface is rough with 2-MP and JGB as levelers while the surface is smooth with 2-ABT as levelers.