Fig.7-1. PE of pulse-polishing as a function of frequency for various trench widths.
The positive voltage is 1.75V, and the negative voltage is 1.3V. The chemical recipe is H3PO4 + CH3COOH 10000ppm + Glycerol1/100………..…………76 Fig.7-2. PE of constant voltage polishing and Pulse Cu-EP as function of line width The pulse frequency is 0.033 and duty cycle is 75%...76
Chapter 1: Review of Electrochemical Deposition (ECD) in Multilevel Interconnection and Experimental Procedure
1.1 Motivation
Cu has been adopted in deep submicron ULSI metallization due to its lower resistivity and better electromigration performance compared to conventional Al alloys,1-2 as shown in Fig.1-1. Recently, the challenge of Damascene processinig is thus considered and has been studied for several years; this integration approach requires Cu to be deposited void-free in trench and via structures with high aspect ratios recently.
Electrochemical deposition (ECD) is an important technology for constructing damascene Cu schemes for interconnects3-4 or even for three-dimensional metal photonic crystals5. The performances of organic additives (suppressors, accelerators, levelers) in electrolytes have effect on gap-filling ability, uniformity of plated surface. Many groups demonstrated their research results about these additives.
Cu electrodeposition usually takes place at atmospheric pressure, room temperature and in the presence of an aqueous electrolyte in a non-conducting cell. Figure 1-2 display a schematic diagram of a electroplating cell.6 In the cell, the wafer, which has a thin copper conductive layer (seed layer) deposited by either chemical vapor deposition (CVD) or physical vapor deposition (PVD), acts as a cathode. A consumable Cu anode at the another side of the cell completes the electrochemical circuit.6
However, it would be of interest to examine depletion of various organic additives after aging of plating bath now. Very little literature has been published about the aging influence of these additives. In this study, we concentrate on the depletion of suppressors (wetting agents) and accelerators after aging.
Fig. 1-1. Procedure for Cu Damascene process
Fig. 1-2. Schematic Diagram of a electrodeposition cell 6
Via PR Metal
(a). IMD & etch stopper layer deposition+via lithography.
(b). Via etch + metal trench lithography.
(c). Damascene trench etch.
(d). Barrier layer deposition. (e). Cu deposition. (f). Cu CMP.
Cu Barrier
Metal Line Metal Line
Metal Line Metal Line Metal Line
Cu
(a). IMD & etch stopper layer deposition+via lithography.
(b). Via etch + metal trench lithography.
(c). Damascene trench etch.
(d). Barrier layer deposition. (e). Cu deposition. (f). Cu CMP.
Cu Barrier
Metal Line Metal Line
Metal Line Metal Line Metal Line
Cu
1.2 Introduction of Cu Electroplating Bath
1.2.1 Basic solution of Cu Electrodeposition
Cu ECD baths are normally formulated using a highly stable acid electrolyte solution containing copper sulfate and sulfuric acid. The basic kinetics and properties of these solutions have been investigated for more than 80 years and well understood 7.
In integrated circuit damascene applications, the unique important criterion for copper sulfate concentration is to avoid the depletion of cupric ion during gap-filling in features processes. Typical cupric ion concentrations in use are in the range of 17.5-60 g/L and sulfuric acid is usually added into the ECD electrolyte (45-325 g/L) to maintain solution conductivity and improve wetting or oxide dissolution on seed surfaces.
Generally, more conductivity solutions result in a system where plating thickness distribution is less dependent on plating cell geometry, while low acid electrolytes result in a system with less dependence on seed layer resistivity.
1.2.2 Organic Additives of Plating Baths
Copper Electroplating can provide bottom-up filling or superfilling behavior by adding chemical additives such as poly (ethylene glycol) (PEG) suppressors together with chloride ions, inhibitors, and accelerators, into H2SO4-based electrolytes thus resulting in the void-free or super filling of narrow Cu trenches and vias in damascene process 8-9. This is commonly assigned to the action of organic additives and chlorides ions added in small amounts to the electrodeposition (ECD) bath10. The mechanisms by which these additives lead to super-fill have been proposed in many investigations. The basic solution and followed by the additives will be introduced in next section.
At present, organic additives added to the electrolyte of Cu ECD lead to three basic categories.
Accelerator which can be called brightener or anti-suppressor, catalyzes and accelerates the conformal overfilling of vias and trenches by locally accelerating current at a given voltage where they are adsorbed. It contains sulfur-containing molecules, typically sulfonic acid groups or disulfides such as SPS (Bis- (sodium
sulfopropyl)-disulfide) with the chemical formula of NaSO3(CH2)3S-S(CH2)3SO3Na.
Accelerators are usually adopted in the plating bath in the concentration range of 1-25ppm 8-9, 11-12.
Suppressors is a kind of surfactant or wetting agent which can be called the carrier, that suppresses Cu growth at the top edges of vias and trenches, thus slowing the deposition rate by competing with copper for electron transfer sites or crystal lattice sites on the metal surface. Surfactants containing long chain polymers such as polyethylene glycol (PEG) or co-polymers of polyoxyethylene and polyoxypropylene have average molecular weights more than 1000. 13-17
Levelers (also called a grain refiner or over-plate inhibitor) are usually high-molecular weight polymers with amine (-NH3) or amide (-NH2) functional groups.
They are second class of current-suppressing molecules, which are usually added to the plating bath at a low concentration. Hence, unlike suppressors, the concentration of levelers at the interface is mass-transfer-dependent. Therefore, isolated locations such as the inside of a via (where mass transfer is limited) are less suppressed, while protruding surfaces or corners (where mass transfer by diffusion or migration is more efficient) are more suppressed 2-3, 8-9, 12, 17.
1.2.3 Relationship between PEG and SPS
Many researchers suggested one reaction equations for the (PEG, SPS)-containing baths to achieve void-free filling. 9,15, 18-21.
In the statement of Moffat et al, the I-V curves revealed that the competition between the effects of PEG and MPS during electrodeposition21. The linear sweep voltammetry (LSV) indicates the competition between inhibition provided by Cl-PEG-Cu2+/Cu+/Cu interaction and the catalytic effects of Cl-MPS-Cu2+/Cu+/Cu interaction. A possible reason of the LSV effect entails the following sequence of events.
Potential-driven desorption or disruption of the blocking Cl-PEG-based layer allows thiolate, or a derivative thereof, then to adsorb on the surface. This further disrupts the inhibiting function of the Cl-PEG-based monolayer. N. Kovarsky et. al agreed with this
Furthermore, M. Tan proposed that a transition time of the system to reach steady state was observed under both galvanostatic and potentiostatic conditions and found to be a strong function of SPS concentration 8. These experimental results provide evidence for slow adsorption and desorption of the accelerator and SPS incorporation into the deposit. Linear sweep voltammetry (LSV) indicates that behavior of the accelerator is potential dependent.
In the respect for the interaction of SPS in plating bath, the reaction was established in the system containing SPS only 19
4Cu(I) + SPS Æ 2Cu(I)MPS + Cu(II) [1]
In conclusively of the mentioned above, we would like to introduce the kinetics in SPS system to our two-component-additive system, in agreement with the assumption from Moffat.
The both effects of PEG and SPS on the deposition of Cu from dilute acid sulphate solutions have been studied for several years. When were PEG added alone in the plating bath, a polymer film adsorbed at a metal surface is expected to slow down deposition currents by imposing a dense barrier at the surface and thus lowering the concentration of cupric ion at inner Helmholtz plane. Cupric ions compete with adsorbed PEG and adsorbed cuprous ions for free adsorption site, the adsorbed PEG molecules do not further affect electrode kinetics. The proposed mechanism is :13
Cu2+ + e- ÅÆ Cu+ads [2]
Cu+ads + e- ÅÆ Cu [3]
Where Cu+ads denotes an adsorbed cuprous ion, which competes with PEGads on Cu substrate.
When PEG-containing bath combine with SPS, the v-t curves will be different, as shown in Fig.1-3. During copper ECD, PEGs immediately adsorb on the interface in the potential range and then SPS gradually substitutes the sites of PEGs on the Cu surface.
18-19, 23 Ultimately, equilibrium between the adsorption of these two additives is accomplished. Possible reactions are:
SPS Æ (driven by ECD current) Æ 2MPS [4]
4MPS + 2Cu2+ Æ 2Cu(I)MPS + SPS [5]
Thus the complex Cu(I)MPSs gradually replace the adsorbed PEGs, and then reduce the inhibition as well as enhances depolarization. This model is called slow adsorption/desorption mechanism. A schematic diagram is shown below in the Fig. 1-4.
The relative information to SPS and MPS 17are listed in Table Ⅰ.
Fig.1-3. Evolution of v-t curves of PEG bath and SPS bath during ECD, respectively.
3-6 Illustration of the slow adsorption/desorption mechanism
Fig.1-4. Illustration of the slow adsorption/desorption mechanism17
0 100 200 300 400 500 600
0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28
After SPS 6ppm added
Cell voltage (V)
Deposition time (s) PEG-containing bath
Table Ⅰ. Relative information to SPS and MPS
1.2.4 Filling Model of Cu Line/Via
Accelerated bottom-up deposition has been explained by a local accumulation of the accelerator species at the deep of a feature, as the surface area within the feature decreases during deposition process 11, 24. Recently, the bottom-up gap filling of Cu ECD with plating additives has been clarified by growth-accelerator species such as SPS and its byproduct such as MPS at the bottom of small features and slow diffusion of free MPS out of these features. The reduction of SPS to MPS provides a possible catalytic path for copper deposition through the formation of cuprous thiolate 10. The model of overfilling is described step-by-step in Fig. 1-5 10. As current flow was applied in electrolyte, it is assumed that all additive species have reached an equilibrium level on all surfaces of the wafer. In the initial stage, applied currents should be expected are approximately equivalent on all surfaces. This effect is corresponded with the observing relatively small amount of bottom-up growth seen in the initial 5-10 sec of a filling process.
After a period of plating time, two effects might begin to contribute filling. First, the accumulation of accelerating mercapto species (or their more accelerating derivatives) within the features takes place on the surfaces. This accumulation results in surface area within feature and sidewall decreases and adsorbed mercapto species (which are neither
Name abbreviation Chemical formula
bis-(3-sodiumsulfopropyl
3-mercapto-1-propanesulfonate MPS -SO3(CH2)3SH 3-mercapto-1-propane sulfonic
acid, or mercaptopropyl sulfonic acid
MPSA HSO3(CH2)3SH
incorporated in the deposit nor desorbed into solution) are thereby increased. Current increases in the areas of geometric concentration (bottom’s corners) as chloride and suppressing polymer is displaced.
Too much accelerator in plating bath disrupts filling because the accumulation of accelerating species also take place outside the feature, and differentiation of the deposition rate from the via base is lost. The accumulation of catalytic species on the growing surfaces within features is strongly supported by the continued Cu growth above features in the absence of leveler. Discontinuing current flow to allow polymer re-equilibration does not interrupt this behavior. However, this disrupted by reversal of interfacial potential to a value causing oxidation or desorption of the adsorbed catalytic material, or by addition of a leveling additive that suppresses current at protruding geometries.10, 17, 24
Fig.1-5. Scheme of the mechanism of the overfilling.17 suppressor.
accelerator
1.2.5 Aging Influence of Electrodeposition Bath for Gaps Filling (Degradation of Electrodeposition Bath)
K.H. Dietz proposed that PEG is quite stable in acidic solution 12. However, it is interesting to learn that these organic additive or organic-copper complexes degrading in acid Cu plating baths. Once the kind of polyglycol chain is cleaved, thus can yielding shorter chain polyglycol fractions. A solvent extract of a fresh plating bath gives a high performance liquid chromatography (HPLC) scan with a narrow peak indicating a narrow molecular weight distribution of the poly (ethyleneglycol), as shown in Fig.1-6. After a couple of time, the original peak gets smaller, and wide distributions of lower molecular weight PEG species appears in the HPLC of the extract.25
However, Koh et al. found that a large MW PEG polymer cleaved randomly into smaller molecules is activated by catalyzed oxidation or hydrolysis25.
On the other hand, the following is a summary of the proposed mechanism about the effect of SPS in ECD bath. Accelerator SPS will crack into two MPS when driven by the overpotential of plating, and MPS is more active depolarization than SPS.19, 26 Moreover, the more and more existed MPS will make the electrolyte more ineffective for filling capability during aging process. As illustrated in Fig.1-7, after aging of plating cycle, it can be assumed that the entire reaction between the MPS/SPS and Cu2+/Cu+ are circulating and autocatalytic reaction systems, where the reaction products involve the initial reaction again and the reaction rate is slow initially.19, 26-27 In other words, two molecular MPS tend to oxidize to one SPS after more aging periods of plating, thus the relationship are listed below 18-19, 26:
SPS Æ (oxidation) Æ 2MPS [6]
2MPS Æ (reduction) Æ SPS [7]
Furthermore, detail reactions with Cu ions involved is shown as followed 4Cu+ + SPS Æ 2Cu2+ + 2Cu (I)MPS [8]
4MPS + 2Cu2+ Æ SPS + 2Cu (I)MPS [9]
Therefore, the key depolarization effect of SPS is the oxidation of MPS, and then accelerate the cupric ions reduce to cuprous ions. The interfacial-adsorbed cuprous-thiolate complex allowed to move by surface diffusion, and also to break apart
into an adsorbed cuprous ion and an adsorbed disulfide product, and thus acted to accelerate the rate-limiting step9. However, T.O. Drews et al. provided another destructive oxidation reaction of SPS.
SPS Æ (further oxidation) Æ S-product [10]
The electrolyte will lose all the accelerating ability after SPS become S product which is some forms of derivatives of aged SPS. Koh confirmed the dissolved-oxygen damage to SPS and observed a similar phenomenon to the above results 25, as shown in Fig. 1-8. The effect of accelerator will be depleted at atmosphere. We suggest that the most amounts of SPS in electrolyte will be decomposed to S-product and loss depolarization ability after many plating cycles faster at atmosphere.
Furthermore, K.H. Dietz also described the possible chemical formulas for that two reactions, as shown in Fig. 1-9. Since the formed compounds contains no divalent sulfur, the oxidation will result in a loss of depolarization.12
Fig.1-6. The HPLC scan of electrolytes before and after aged respectively. The carrier component reduced after aging process.25
Fig.1-7. Schematic of assumed mechanisms for different filling aspects between MPS and SPS/aged MPS. (stoichiometry is ignored in this illustration). 27
Fig.1-8. Comparison of the effect of ambient on the accelerator decomposition rate.25
Fig.1-9. Possible Chemical formula of S-products 17