Electrodeposition for 3D Integration (Elektrodepositiemethodes voor 3D integratie)

In dit proefschrift werd de koperelektrodepositie in caviteiten voor thr ough-silicon vias toepassingen bestudeerd. De belangrijkste factor daarb ij zijn additieven toegevoegd aan electrolysebaden en die de polarisatie tijdens elektrolyse sterk beïnvloeden. Elektrochemische metingen werden uitgevoerd om het polarisatiegedrag van een vlakke elektrode te bepalen tijdens koperlektrodepositie. De invloed van additieven op de polarisat ie, werd onderzocht naar het gehalte ingebouwde onzuiverheden in het gedeponeerde koper, naar hun invloed op de elektrische weerstand van dun ne filmen en op de microstructuur van het materiaal. Caviteiten met diameters van 5 tot 100 μm en diepten tot 700&n bsp;μm werden aangemaakt in silicium-substraten door middel van pla sma etsen. Depositiemethoden, zoals sputteren, sputteren met een geïonis eerd plasma, atomaire laag depositie (ALD) en nat-chemische depositie we rden onderzocht om een continue barrière laag van voldoende dikte te bek omen. Op deze laag werd vervolgens een elektrisch geleidende koper film aangebracht. De limieten van deze verschillende methoden werden vergelek en. De evolutie van de (over)potentiaal tijdens koperelektrodepositie in cav iteiten werd vergeleken met elektrochemische metingen uitgevoerd op vlak ke kopersubstraten. Elektrochemische metingen uitgevoerd tijdens het opv ullen werden vergeleken met doorsneden aangemaakt na het deponeren van k oper. Deze vergelijking toonde aan dat de lokale elektrodepositievoorwaarden van gr oot belang zijn voor het effect van additieven tijdens superconformale d epositie. Caviteiten zijn vrijwel volledig vrij van levellers tijdens de superconf ormale aangroei alhoewel die stoffen aanwezig zijn in het elektrolysebad . Hoe de verhouding van leveller en accelerator concentraties en een var iatie van de lokale stroomdichtheid, de koperafzetting beïnvloeden, werd besproken. Door gebruik te maken van een gesputterde laag op het oppervlak van de wafer, kon een me thode ontwikkeld worden om de vooruitgang van het opvulproces op te volg en. Het gedeponeerde koper werd nader onderzocht om te bepalen welke fac toren van belang zijn bij toepassingen als interconnecties. Een thermisc he behandeling bleek essentieel te zijn om de microstructuur te verander en zodat een stabiel materiaal werd bekomen. De vergaarde wetenschappelijke kennis omtrent elektrochemische processen werd gebruikt om een gecontroleerde depositie te realiseren in structur en met afmetingen die over een groot bereik variëren. Caviteiten met dia meters van 5 tot 100 μm en diepten variërend van 25 tot 115&nb sp;μm werden opgevuld met koper zonder poriën in het materiaal. Er is aangetoond dat de wijze van aangroei van elektrolytisch koper kan beïnvloed worden voor structuren met een identieke geometrie. Differenti ële inhibitie door diffusie/adsorptie is het dominante mechanisme wannee r een leveller aanwezig is in het elektrolysebad. Differentiële accelera tie door curvature-enhanced-accelerator coverage (CEAC) is dominant in e en leveller-vrije situatie en in het geval dat het substraat passief is ten opzichte van elektrodepositie. Voor structuren met verschillende geometrie is differentiële inhibitie m eer effectief in caviteiten met een hoge aspect ratio en kleine diameter , in vergelijking met caviteiten met een lage aspect ratio en grote diam eter. De hydrodynamische vloeistofstroming ververst de elektrolyt met le veller in het geval de caviteit voldoende groot is, waardoor de verschillen in lokale elektrodepositie condities minder uitgesproken zijn. De depositie proces sen die zijn ontwikkeld in het kader van het doctoraatsonderzoek werden recentelijk geïntegreerd in het 3D interconnect onderzoek bij IMEC te Le uven.Scientific Summary 12 Wetenschappelijke Samenvatting 14 1 Introduction to 3D Integration 17 1.1 Technologies and applications for 3D integrated circuits 17 1.2 Filling of blind holes with copper as key-technology 21 1.3 Technological and scientific challenges on copper electrodeposition 22 1.4 Structure of this thesis 24 1.5 References Chapter 1 26 2 Copper electrodeposition processes into recessed features 31 2.1 The damascene process for integrated circuit manufacturing 32 2.2 Levelling process as surface finishing 38 2.3 Through-mask patterns 44 2.4 Filling processes for through-silicon vias 46 2.5 The concept of overpotential in electrodeposition 50 2.6 Structures and Properties of Electrodeposits 55 2.7 Conclusions 57 2.8 References Chapter 2 58 3 Objectives 71 4 Investigation of the copper electrodeposition on flat surfaces 75 4.1 Electrodeposition in absence of addition agents 75 4.1.1 Fundamentals of copper electrodeposition 75 4.1.2 Lifetime of Cu-(I) species during copper electrodeposition 79 4.2 Polarization and depolarization effects induced by additives 81 4.2.1 The effect of chloride ions 81 4.2.2 Suppression of copper deposition by polymeric glycols 83 4.2.3 Acceleration of copper deposition by disulfides 85 4.2.4 Inhibition of copper deposition by levellers 87 4.3 Characteristics of copper electrodeposits on flat surfaces 89 4.4 Conclusions 93 4.5 References Chapter 4 94 5 Synthesis of metallized recessed features with high aspect ratio 99 5.1 Etching structures in silicon 99 5.2 Full substrate metallization by barrier and copper seed layer deposition 102 5.3 From discontinuous towards continuous metallized substrates 103 5.4 Top surface capping of fully metallized substrates with tantalum 105 5.5 Limits of technologies for synthesis of recessed features 106 5.6 Current density evolution during filling of recessed metallized blind holes 107 5.7 Current density distribution for electrodeposition of metals 109 5.8 Conclusions 115 5.9 References Chapter 5 116 6 Filling blind holes with aspect ratio 5 by electrodeposited copper 119 6.1 Blind holes in fully metallized substrates 119 6.1.1 Evolution of cathode potential during galvanostatic polarization 119 6.1.2 Origin of cathode potential evolution during fill-up process 120 6.1.3 Growth of copper deposits in fully metallized blind holes 124 6.1.4 Dependence of filling performance on applied cathodic current 126 6.1.5 Effect of current waveform on filling performance 127 6.1.6 Mapping of additives by changing leveller-to-accelerator ratio 128 6.2 Blind holes in Ta capped substrates 130 6.2.1 Evolution of cathode potential during galvanostatic polarization 130 6.2.2 Evolution of copper growth for blind hole filling 132 6.2.3 Analysis of potential evolution during filling 134 6.3 Growth mode for copper electrodeposition for blind hole filling 137 6.4 Conclusions 140 6.5 References Chapter 6 141 7 Structural and mechanical characteristics of copper fillings 143 7.1 Copper microstructure in a blind hole 143 7.2 Mechanical properties of copper in blind hole 148 7.3 Adaptability of electrodeposited copper for 3D interconnection 150 7.4 Conclusions 151 7.5 References Chapter 7 152 8 The extension of the filling process to various dimensions 155 8.1 Filling of blind holes at micrometer scale with moderate aspect ratios 155 8.2 Filling process for 3D Wafer-Level Packaging on 200 mm wafers 158 8.3 Filling of blind holes with aspect ratio 10 161 8.4 Comparison of the filling behaviour 165 8.5 Conclusions 169 8.6 References Chapter 8 170 9 Achievements of this work and future perspectives 173 Appendices 179 A1 List of abbreviations 179 A2 Deep reactive ion etching of trenches 183 A3 Filling evolution of electrodeposited copper in blind holes 184 A4 List of publications 185nrpages: 188status: publishe

Filling of microvia with an aspect ratio of 5 by copper electrodeposition

The filling of microvias with a diameter of 5 µm and a depth of 25 µm (aspect ratio of 5) by copper electroplating was investigated. Filling experiments were evaluated by analyzing cross-sections of filled vias with scanning electron microscopy and focused ion beam. The fill-up evolution shows a bottom-up mechanism, also known as superfilling mechanism. The evolution of potential with time (chronopotentiometric measurements) was recorded during the fill-up process of vias and is interpreted based on potentiodynamic polarization measurements. The bottom-up fill mode is affected by the concentration of leveler inside the vias. A differential plating rate that is responsible for bottom-up plating, develops along the profile of the via on depletion of the leveler inside the vias. Since the depleted via is less inhibited, the local electrodeposition rate increases in the via. At the top part and outside the via, the electrodeposition rate is strongly inhibited due to a higher leveler concentration comparable to the one in the bulk electrolyte, what results in a low local electrodeposition rate. In this paper, the contribution of levelers to the bottom-up mechanism during the electrodeposition of copper in microvias is investigated. The observed microstructure supports the superfilling mechanism.status: publishe

Leveling of microvias by electroplating for wafer-level-packaging

The filling of microvias with diameters between 30 and 100 μm and aspect ratios up to 2.5 in silicon wafers, was investigated to determine the performance of copper electroplating. An electrolyte with a low leveler concentration was only suitable for filling microvias with aspect ratios below 1. An increase of the leveler concentration enabled the filling of microvias with higher aspect ratios. The fill-up evolution shows a bottom-up plating behavior. Electrochemical measurements show that for conditions prevailing at the bottom of the via, the electrodeposition rate is enhanced while for conditions prevailing at the top, it is inhibited. This difference originates from a concentration gradient of leveler inside the via and different convection conditions inside and outside the via.status: publishe

Influence of annealing conditions on the mechanical and microstructural behavior of electroplated Cu-TSV

In this paper, the effect of annealing condition on the microstructural and mechanical behavior of copper through-silicon via (Cu-TSV) is studied. The hardness of Cu-TSV scaled with the Hall-Petch relation, with the average hardness values of 1.9 GPa, 2.2 GPa and 2.3-2.8 GPa, respectively for the annealed, room temperature (RT) aged and the as-deposited samples. The increase in hardness toward the top of the as-deposited sample is related to the decrease in grain size. The annealed and the as-deposited samples showed a constant elastic modulus (E-modulus) value across the length of Cu-TSV of 140 GPa and 125 GPa respectively, while the RT aged sample showed a degradation in E-modulus from the bottom of the TSV (140 GPa) to the top (110 GPa). These differences in E-modulus values and trends under the different test conditions were found to be unrelated with the crystallographic texture of the samples, but could be related to the presence of residual stresses. No correlation is found between the hardness and E-modulus data. This is attributed to the coupling and competitive effects of grain size and residual stresses, with the grain size effect having a dominant influence on hardness, while the presence of residual stresses dominated the E-modulus result.status: publishe

Changing superfilling mode for copper electrodeposition in blind holes from differential inhibition to differential acceleration

Blind holes 5 mu m diam and 25 mu m in depth were filled with electrodeposited copper. Before electrodeposition, the blind holes were fully metallized with a copper seed layer or with a thin sputtered Ta layer deposited on top of the copper seed layer. Filling of partially Ta-capped features was three times faster than filling fully metallized features. For fully metallized features, the growth mode for copper electrodeposition is dominated by diffusion adsorption of a leveling agent. For partially Ta-capped surfaces, the growth mode was modified to differential acceleration through accumulation of an accelerating species at the bottom of features.status: publishe

Reducing the electrodeposition time for filling microvias with copper for 3D technology

We present two approaches to reduce the process time needed for filling vias of 5 µm diameter and 25 µm depth with copper by electrodeposition. In the first approach, the effect of model additives on the filling of vias with electroplated copper was investigated as well as the influence of the applied current density on the filling process. The variation of the concentration of leveler and accelerator additives was investigated. Their influence on the void-free filling of such vias was determined. A high leveler concentration allowed to achieve a void-free fill. The copper deposited on the top surface was in the range of 2.5 µm. The filling was completed within 45 minutes. The filling time could even be further reduced to 25 minutes by introducing a waveform with two galvanostatic steps. The second approach demonstrates void-free via filling with copper electrodeposition at the top of the waver surface blocked with self-assembled monolayers of octadecanethiol. With thin Ta-films deposited at the top surface before electrodeposition is started, almost no copper was deposited at the top surface as well. The vias were filled within 30 minutes when the top surface was completely blocked.status: publishe