quartz crystal microbalance studies of leveling effects of thiourea

Confocal Laser Scanning Microscopy, Electrochemistry, andQuartz Crystal Microbalance Studies of Leveling Effects of

Thiourea on Copper DepositionMaher A. Alodan and William H. Smyrl*

Corrosion Research Center, University of Minnesota, Minneapolis, Minnesota, 55455, USA

ABSTRACT

Three techniques were used to evaluate and study the effect of additives in the electrodeposition process. The evolu-tion of surface profiles on model copper surface topography was monitored by confocal laser scanning microscopy(CLSM) in the presence of different concentrations of thiourea in copper sulfate solutions. The model surfaces were fab-ricated by photolithography. Along with the CLSM, concurrent measurements of electrochemistry and quartz crystalmicrobalance (QCM) were made. It was found that thiourea acted as a mediator in the electrochemical reactions thatoccurred at the electrode surface as revealed by current efficiency measurements. The cathodic consumption of thioureawas found to be negligible and it was strongly adsorbed on the surface. From the CLSM and QCM, it was found thatthiourea acted as a brightening agent and did not exhibit any leveling ability.

IntroductionElectrodeposition processes depend on the coupled inter-

action of several variables (i) substrate conditions, (ii) solu-tion constituents, (iii) current density and electrode poten-tial, (iv) hydrodynamic conditions at the deposition site.Characterizing and monitoring the effect of these vari-ables in the electrodeposition of metals is important inunderstanding and developing better electrodepositionprocesses. For many functional applications, it is desirableto have uniform and smooth metal deposits, even on roughsubstrate surfaces. A smooth surface must be either main-tained or achieved by manipulating deposition conditionsor electrolyte chemistry. The latter category includes lev-eling additives or chemical species that cause the surfaceroughness to decrease as deposition proceeds. Here, a rel-atively new technique, confocal laser scanning microscopy(CLSM), was used to monitor the evolution of the topogra-phy of copper deposition from solutions containing a can-didate leveling agent. CLSM offers significant advantagesfor studying surface topography, since it can be used toview surfaces in air or immersed under relatively thicksolution layers in an in situ mode. The microscope can beused for monitoring electrodeposition and surface rough-ness as well as for applications such as corrosion process-es and local reactivities over surfaces.1-5

The leveling power of various additives has been studiedfor various electrodeposition processes,6-9 and more recent-ly in Ref. 10 and 11. The studies have usually been based ontwo steps. The first step in the evaluation of an additive isthe fabrication of the desired surface profile on an elec-trode surface. The second step involves monitoring theevolution of the surface morphology during deposition. Inthis work a general method involving these two steps hasbeen developed to test the leveling power of additives. Theprocedure is aimed at studies of leveling of surfaces hav-ing roughness in the micrometer range. The thiourea-cop-per system is used for demonstrating the procedure. Wealso seek to distinguish between additives that producemacroscopic leveling, and those that cause "brightening"on a microscopic scale.

A widely accepted theory of the leveling mechanism isthe diffusion-adsorption theory.12-'4 The theory presumesthat it is necessary to have nonuniform accessibility of theleveling agent to the electrode surface, and that the acces-sibility changes because of variations in the diffusion layerthickness on a microprofile electrode. Areas on the elec-trode that receive a high flux of the leveling agent sufferstrong inhibition, and the deposition rate is reduced. Theareas where the inhibition is strongest would then be thepeaks and high points on the surface. In this case, the cur-

* Electrochemical Society Active Member.

rent distribution on the electrode favors surface leveling.The incorporation or consumption rate of a leveling agent,which has to be under diffusion control, is a major impor-tant requirement for the validity of the diffusion-adsorp-tion theory of leveling. However, in the case of brighteners,the later assumption is relaxed, and the main characteris-tic of a brightening agent is the strong adsorption alongthe electrode surface. The adsorption of brighteners on theelectrode surface inhibits deposition and changes the mor-phology and crystallographic properties of the deposit toyield bright and smooth films, i.e., "microscopic smooth-ing" of the deposit.

Here, we demonstrate the CLSM, and apply it to thecopper-thiourea system to reveal the role of thiourea as anadditive in a copper plating bath. Along with CLSM,cyclic voltammetry and galvanostatic measurements wereperformed. To investigate and understand the function ofthiourea and its consumption rate, quartz crystalmicrobalance experiments were performed. The rate ofincorporation of thiourea into copper deposits has beenstudied previously as a function of the bulk concentrationof thiourea and the current density, but no strong conclu-sions were reached.'5'16

ExperimentalThe CLSM.—Because of its ability to obtain 3D images

the confocal laser scanning microscope has been usedrecently to study reactions on heterogeneous surfaces toobtain information about the electrolyte layer next to theheterogeneous surface. Figure 1 shows a schematic diagramof the CLSM (Molecular Dynamics). CLSM offers twomain advantages over a conventional optical microscope.The first is the capability to scan the sample surface pointby point, and the second is the ability to reject any reflect-ed light not in the focal plane. The first advantage isobtained by the use of two mirrors in the optical chamberthat scan a focused laser spot in the x-y plane to cover thefield of view. The image then is collected point by point.The intensity of the reflected light from each point on thesample is recorded by a photomultiplier tube and assigneda value between 1 and 255. Point illumination provides theadvantage of a digitally stored image that can be readilysubjected to image processing and enhancementY' Thesecond advantage of the CLSM is the ability to use a pin-hole aperture in front of the detector (Fig. 1) in conjunc-tion with a focused laser light source to increase the later-al and vertical resolution. A lateral resolution of the orderof 0.5 p.m and a vertical resolution of 0.2 p.m, dependingon the objective lens used, can be obtained.

The collected image over the scanned area from a singlefocal plane is called an optical section. The object stagecan be moved in the z direction and other images can be

J. Electrochem. Soc., Vol. 145, No. 3, March 1998 The Electrochemical Society, Inc. 957

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958 J. Electrochem. Soc., Vol. 145, No. 3, March 1998 The Electrochemical Society, Inc.

Fig. 1. Schematic diagram of the confocal laser scanning micro-scope. The two mirrors scan the laser beam over the x-y plane.

acquired in the x-y plane to get other optical sections. Thecollected optical sections can be stacked together to per-form and manipulate a three-dimensional analysis of thesurface. The sample can be scanned in the vertical z direc-tion also (optical slicing), at any predetermined plane. Amajor advantage of CLSM use is obtaining optical crosssections of surfaces under solutions without destroying thesample. Further details can be found in Ref. 18.

Leveling experiments—The starting profile of the elec-trode surface plays an important role in defining the lev-eling power of additives. One has to start with a well-con-trolled dimension and shape of the desired profile in orderto test additives as proper leveling agents. In this section,a method that is based on photolithography, which wasused to create a patterned surface for subsequent testing,is described.

The photolithography method used was based on thefollowing steps. (i) Spin coat an acid resistant photoresistto the desired thickness on top of the copper substrates. Inthe present studies, the thickness was 20 p.m at a rotationrate of 3000 rpm using a photoresist for thick films (AZ4000). The thickness of the photoresist can be controlledby changing the rotation rate; for very thick layers of pho-toresist, one can deposit multilayers of the photoresistuntil the desired thickness is reached. The photoresist wasbaked at around 90°C for about 20 mm. (ii) Expose thephotoresist to an ultraviolet light through a mask with thedesired pattern. The dimensions of the mask control thefinal lateral dimensions of the surface pattern. (hi) Immersethe exposed sample in a developer. Only the regions exposedto the IJV light in step (iii) were dissolved into the develop-er, a solution of sodium or potassium hydroxide (1 M). (iv)Place the sample in 0.5 M Cu504 and 1 M H2S04 solutionand deposit copper on top of the masked surface to createthe desired pattern. The areas where the photoresist wasremoved by the developer were coated with copper. Thethickness of the deposit was controlled by the amount ofthe charge passed; in all cases, the thickness was smallerthan the thickness of the surrounding photoresist. (v)

44 flflfl un40 200 40 50ll

20050 1000 200

All dimensions are in micrometer

Fig. 2. Illustration of the cross section of the patterned surface ofthe copper electrode.

Finally the sample was immersed in acetone to dissolve allthe remaining photoresist in order to leave the surfacewith the deposited pattern. Figure 2 illustrates the profileof the created pattern.

The dimensions of the lines and grooves were variedbetween 5 and 80 p.m by changing the mask dimensions.The thickness of the deposited copper was kept constant inmost of the cases at aroi.md 20 p.m, in order to have amicroprofile surface in stagnant solutions. After creating apattern on the copper disks, the sample was mounted onthe bottom of the electrochemical cell and the evolution ofthe deposit topography was followed by the CLSM.

Cyclic voltammetry and quartz crystal nsicrobalance.—Cyclic voltammetry was carried out on a quartz crystalmicrobalance in order to monitor mass changes of the elec-trode concurrently with the electrochemical measurements.On top of a gold coated QCM, copper was plated from 0.5 Mcopper sulfate and 1 M sulfuric acid at 10 mA/cm2, until athickness of °°2 p.m was reached. During the deposition ofthe initial copper film, the frequency of the QCM wasmonitored and plotted vs the charge passed to obtain theexperimental sensitivity factor for the QCM for subse-quent measurements. A saturated calomel reference elec-trode and a platinum mesh counter electrode were mount-ed in the cell. Different concentrations of thiourea andcopper sulfate were used. In most of the solutions, 1 M sul-furic acid was used. Nitrogen gas (99.999%) was bubbledthrough the cell in order to reduce the concentration ofdissolved oxygen. All chemicals were reagent grade.

A potentiostat from EG&G PAR model 273 was used toobtain polarization curves along with a universal pro-grammer 371 from EG&G. Data was collected and ana-lyzed by a computer. Charge passed to the electrode wasobtained by integrating the current.

Resu'ts and DiscussionThe evolution of surface topography—The CLSM was

used to follow the changes of surface topography duringthe deposition of copper on photolithographicafly patternedsurfaces from the desired solution and current densities. Allsolutions contained 0.5 M copper sulfate and 1 M sulfuricacid in addition to one of the following concentrations ofthiourea: 0, 0.005, 0.01, 0.05, 0.1, and 0.5 mM. The deposi-tion current densities were selected to be 15, 30, 45, 60,and 75 mA/cm2, respectively, in order to cover the rangebetween kinetic control and mass-transfer control of thecopper deposition process.

After a certain deposition time, the current was inter-rupted and the samples were removed from the cell. Thenthe samples were imaged by the CLSM. The collectedimages were optical cross sections from a z direction scan.Images of the surface were taken at time intervals whichdepended on the deposition rate.

Figure 3 shows an optical slice of the initial surface ofthe copper disk before any electrodeposition, the dimen-

zControl in z

direction Object stage

Scannit mirrorin x direction

cross section of the

1

copperelectrode

Mask layoot

Ilichroic

ComputerII—-——

lens

Pin hole

Photomultiplierdetector



a. ciecrrocnem. ,oc., vol. I 'to, No. i, iviarcn I ø I ne tiecirocnemicai aociety, Inc.

sions of the starting profile may be seen. The starting depthof the rectangifiar groove and width were 15 and 50 p.m,respectively, and the profile had a relatively large aspectratio. An example of the profile of the groove after platingcopper from 0.5 M copper sulfate and 1 M sulfuric acid at—30 mA/cm2 for 35 mm is shown in Fig. 4. The profilechanged from a rectangular to a trapezoidal shape. Fig-ure 5 shows another image of the surface after 58 mm at—30 mA/cm2 deposition current density. One can see thatthe shape of the profile became triangular, and the surfaceprofile continued to have this triangular shape until com-plete leveling was achieved. From the above images, notethat some leveling of the surface of copper was achieved,even in the absence of thiourea. This case involves geo-metric leveling.14 The images may be compared withimages from thiourea solutions in Fig. 6a and b. Figure 6ais the initial profile and Fig. 6b is the profile after 70 mmof plating. The plating solution in this case was 0.5 MCuSO4, 1 M H2S04, and 0.01 mM thiourea. The depositioncurrent density was —60 mA/cm2.

The starting dimension for the grooves varied from sam-ple to sample due to difficulties in controlling the exactetching rate of the developer, and this caused a variationof final thickness of the photoresist. In order to unify thebasis of comparison between each experiment, the depthand width of the groove were divided by their initial val-ues. In the following discussion, only the dimensionlessvariables are discussed.

The dimensions of the groove such as the depth andwidth at the bottom and top of the groove were recordedvs time for each run, and plotted vs time, to indicate theleveling speed of the specific solution used. Figures 7—12show the development in the surface profile for selectedconcentrations of thiourea in 0.5 M CuSO4 and 1 M H,S04at different currents as indicated by each figure. The depthof the groove and width at the bottom are reported in thesegraphs. The top width of the groove did not show any

Fig. 4. Vertical image of the copper surface in Fig. 3 after 35 mmof plating at —30 mA/cm2 from a thiourea-free solution.

strong dependence on the concentration of thiourea or thedeposition current. Also, the rate of decrease in the bottomwidth, Wb, of the groove was larger than that at the top.The width at the bottom of the groove continued todecrease until the profile turned into a triangular shape,i.e., the width at the bottom of the groove was very small.At this point, the depth of the profile started to decrease athigher rates. This observation may seem to be unusualsince peaks or edges of the surface are expected to havehigher current densities than at more remote locations atthe bottom of the valleys. The apparent increase in thedeposition rate at the bottom of the groove was due to geo-metrical factors. Once can see that the local surfacegrowth at the bottom corner was in the direction of reduc-ing the surface area. Since the current entering the open-ing of the groove was constant, the local current densitytends to increase during deposition, indicating an increasein the local thickness of the deposit. The above statementremains true when the profile dimension is such that theohmic drop is small along the groove depth compared tothe resistance of polarization. The latter comparison can

Fig. i. vertical image of the inihal copper surtace protile. I hescale bar is in micrometers.

Fig. 5. Vertical image of the copper surface in Fig. 3 after 58 mmof plating at —30 mA/cm2 from a thiourea-free solution.

Fig. 6. (a, top) Vertical image of the initial copper surface profile.The scale bar is in micrometers. (b, bottom) Vertical image of the cop-per surface in Fig. 6a after a plating time of 72 mm at —60 mA/cm2from a solution containing 0.01 mM thiaurea.



960 J. Electrochem. Soc., Vol. 145, No.3, March 1998 The Electrochemical Society, Inc.

40 50 60 0 20 40 60 80 100 120

Time, mm.

0 20 40

Time, mm.

1.2

1.0

10.80.6

0.4

0.20 10 20 30

Time, mm.

Fig. 7. Change of the width of the bottom of the groove vs plat-ing time at different concentrations of thiourea. Electrodepositioncurrent was —'15 mA/cm2.

LI

1.0

0.9

0.8

0.7

O 0.6

0.5

0.4

0.360 80 100

Fig. 8. Change of the groove depth vs plating time at different con-centrations of thiourea. Electrodeposition current was —15 mA/cm2

be estimated by evaluating the Wagner number which is ameasure of the uniformity of the primary current distrib-ution. The Wagner number (Wa) can be expressed as

Wa = K(31111/di) [1J

Where K is the electrolyte conductivity, 1 is the cathodicoverpotential, i is the current density, and 1 is a character-istic dimension of the cathode topography. The lower the

1.2

1.0

0.8

40.6

0.40.2

0.00 10 20 30 40 50

Time, mth.

Fig. 9. Change of the width of the bottom of the groove vs plat-ing time at different concentrations of thiourea. Electrodepositioncurrent was —45 mA/cm2.

1.6

1.4

1.2

1 1.0

1080.6

0.4

0.2

Fig. 10. Change of the groove depth vs plating time at different con-centrations of thiourea. Elecfrodeposition current was —45 mA/cm2.

1.2

1.0

0.8

40.60A

0.2

0.00 10 20 30 40 50 60 70 80

Time, mitt.

Fig. 11. Change of the width of the bottom of the groove vs plat-ing time at different concentrations of thiourea. Electrodepositioncurrent was —60 mA/cm2.

Wa, the more dominant the ohmic effects which result in anonuniform primary current distribution.'8'25 In the regionof Tafel kinetics, one can write the Wa as

[2]

Where i, = RT/Fa is the cathodic Tafel slope and cx,, is thecathodic transfer coefficient. For copper electrodepositionfrom a sulfate bath containing 0.5 IV! copper sulfate and 1 Msulfuric acid, the parameters in Eq. 2 are25 K = 0.26 1/fl cm,

2

1.8

1.6

1.4I41.2

1

0.8

0.6

0.4

Fig. '12. Changed the groove depth vs plating time at different con-centrations of thiourea. Electrodeposition current was —60 mA/cm2.

60 70 800 10 20 30 40 50 60 70 80

Time, mm.



J. Electrochem. Soc., Vol. 145, No. 3, March 1998 The Electrochemical Society, Inc. 961

and 13, = 46 mV. For an average current density of 40 mA/cm2and a characteristic dimension of 10 p.m, the Wagner numbervalue is 299, indicating that the ohmic contribution to theuniformity of the primary current distribution is smallcompared to the kinetic contribution.

The above observations are general for most thioureaconcentrations and applied current densities. At low cur-rent densities, thiourea had a larger effect on W than athigh current densities. The depth of the profile decreasedwith time at low current densities, i < —15 mA/cm2. Athigher current densities, i> —45 mA/cm; the depth of theprofile increased with time until the shape of the profilebecame triangular, and then the depth of the profiledecreased and surface leveling was observed.

Generally, thiourea had a weak effect on the true level-ing of copper deposition as can be seen from the above fig-ures. The effect of the current magnitude on the currentdistribution was stronger than any effect of thiourea con-centration. At low current densities (i < —20 mA/cm2), thedeposition was uniform and followed the surface profile.At higher current densities (i > —30 mA/cm2), the deposi-tion was higher at edges and at sharp corners.

The brightening effect on the copper surface by thioureawas more pronounced than the leveling. Images of the sur-face were collected after electrodeposition along with thesurface optical cross sections. Figure 13 shows the surfacemorphology of the copper surface after electrodepositionfor 58 mm at —30 mA/cm2 from a thiourea-free solution.The dark region across the surface images represents thecreated groove because it was below the focal plane. Thesurface was relatively smooth and appeared dull to thenaked eye. Figure 14 shows the surface morphology afterelectrodeposition from 0.01 mM thiourea solutions at acurrent density of —60 mA/cm2 for 72 mm. The surfacewas brighter in the presence of thiourea and almost had amirror-like finish. Nodulation of the deposit was enhancedin the presence of higher concentrations of thiourea (0.1 mlvi)as shown in Fig. 15. We propose that the existence of nodu-lation during copper deposition was due to the adsorptionof thiourea-copper complexes at high concentrations ofthiourea. The adsorbed layer is not uniform and hasdefects, so that deposition was accelerated at the siteswhere the adsorbed film thickness was small, and this ledto the formation of particles or nodules. The occurrence ofnodulation was more pronounced at high deposition cur-rent densities (above —30 mA/cm2).

Cyclic voltammetry and QCM.—.Cyclic voltammetry wasperformed on the copper electrode at varying concentra-tions of thiourea (0.0, 0.01, 0.05, 0.1, 0.15, 0.2, and 0.4mM),

0.5 M copper sulfate, and 1 M sulfuric acid. Figure 16shows the cathodic branch of the cyclic voltammogram at5 mV/s scan rate for selected concentrations of thiourea. Astrong peak appeared in the presence of thiourea when thepotential was scanned in the negative direction. The peakwas less pronounced when the potential was scanned inthe positive direction, and the magnitude of the peak dur-rent lessened as the concentration of thiourea increased.This behavior was an indication of the strong adsorptionof species containing thiourea on the electrode.19 At lowconcentrations of thiourea (less than 0.1 mM), there was anaccelerating effect especially at low overvoltages, but asthe concentration of thiourea was increased, inhibitioneffects dominated the copper deposition. Acceleration ofcopper deposition by thiourea was observed also by previ-ous investigators.20-22

The relationship between the frequency change of theQCM and the charge was used to explore the net electro-chemical reaction and the instantaneous current efficien-cy. From the QCM results, the current efficiency was about100% with respect to cupric ion deposition, even when theacceleration by thiourea was at a maximum as shown inFig. 17. The slope of the line gave an equivalent weight of

Fig. 14. Surface image of the copper surface after deposition from asolution containing 0.01 mM thiourea for 72 mm at —60 mA/cm2.

Fig. 13. Surface image of the copper surface after depositionfrom a thiourea-free solution for 58 mm at —30 mA/cm2.

Fig. 15. Surface image of the copper surface after deposition from asolution containing 0.1 mM thiourea for 52 mm at —30 mA/cm2.



962 1J. Electrochem. Soc., Vol. 145, No. 3, March 1998 The Electrochemical Society, Inc.

-15

Fig. 16. Cyclic voltammogram of copper deposition in the pres-ence of different concentrations of thiourea. The reference electrodewas SCE and the scan rate was 5 mV/s. The area of the electrodewas 0.33 cm2.

the deposited material of 31.5 g eq which is exactly theequivalent weight of cupric ions. This result precludes anysignificant simultaneous consumption of thiourea at theelectrode.

The current density-potential curve was strongly affect-ed by the presence of thiourea in the solution. Figure 16shows that the current density at 0.01 mM thiourea ishigher than that in the absence of thiourea. On the otherhand, strong inhibition of the deposition was seen at larg-er concentrations. The acceleration action of thiourea isunlikely to be due to complexation in the bulk since theTh/Cu*2 ratio was quite small at iiY to 10. The actionwas instead at the solution-electrode interface due to thestrong adsorption of thiourea on the copper electrode. Thenet process that occurred at the electrode was the deposi-tion of cupric ions with an equivalent weight of 31.5 asshown by the QCM results and there was negligible elec-trochemical reaction of thiourea at the electrode. Thisfinding by the QCM does not support the accelerationmechanism proposed by Suarez,2223 because there was noevidence that cuprous ions contribute separately to theelectrode reaction. If the latter had occurred, there wouldhave been an increase of the equivalent weight of thedeposit. The mechanism of the thiourea effect is more like-ly through the mediation of electron transfer from theelectrode surface to the metal ions in the solution. Themediation reduces the activation energy for the electro-chemical reaction and increases the reaction rate (the cur-

p

-1000 -800 -600 -400 -200 0

Charge passed, inC

Fig. 17. Plot of frequency change vs charge passed to the elec-trode of cyclic voltammogram shown in Fig. 16 (for the whole ofthiourea concentrations).

rent density). This effect is known for sulfur-containingcompounds in which the reduction reaction is acceleratedfor metal ions such as zinc and cadmium, and is known asthe cap-pair effect.23 There is a similar effect of halide ionmediation on electrode processes as well.19

At higher concentrations of thiourea, the effect on theelectron transfer is mitigated by the presence of multilay-er adsorption of thiourea and its complexes at the copperelectrode. The multilayer adsorption increases the dis-tance that electrons have to travel, retards surface diff u-sion and blocks the active sites for the deposition of cop-per. The net result is a reduction of the current density ata constant overpotential.

If there were any cathodic consumption reactionsinvolving thiourea during copper deposition, it wouldhave been manifested by its incorporation into the depositsince there was no evidence for reduction of thiourea atthe electrode. However, from the above QCM measure-ments, any thiourea incorporation must have been verysmall relative to the deposition rate of copper; it probablydid not occur at all. The flux of cupric ions to the electrodesurface was 7.8 x lO mol/(cm2/s) at an average deposi-tion rate corresponding to 15 mA/cm2, whereas the maxi-mum diffusion limited flux of thiourea from the bulk (at0.5 mM concentrations) to the electrode surface can beestimated for a thiourea diffusion coefficient of 0.9 x iO(cm2/s) and a stagnant diffusion layer thickness of 100.im.24 The diffusion limited flux calculated from the abovevalues was 4.5 )< lObo mol/(cm2/s). Incorporation of.thiourea at this rate would cause a deviation of less than1% in the calculated equivalent weight of the deposit. TheQCM measurements were not sensitive enough to measuresuch small deviations in the deposited mass under theseconditions.

In order to detect small deviations in the deposited massthat could have been due to codeposition or electrochemi-cal consumption of thiourea, the copper sulfate concentra-tion was reduced to either 50 mM or 5 mM in 1 M sulfuricacid and cyclic voltammetry was performed for concentra-tions of thiourea, of 0.01, 0.05, and 0.1 mM, respectivelyThe result for 1 mM thiourea concentration compared tothe thiourea-free solution are shown in Fig. 18 for thecase of 5 mM copper sulfate. The QCM response shown inFig. 19, indicates that the electrode mass change was a lin-ear function of the charge passed to the electrode. Sincethere were no differences between the electrode masschange for thiourea free solution and solutions with I mMthiourea, and one concludes that the deposited materialrepresents only the reduction of cupric ions and that theincorporation of thiourea in the deposit was negligible. Atsmall concentrations of copper sulfate, the current densityis small because of mass transfer limitations, yet no incor-poration of thiourea was observed.

The results from the QCM and cyclic voltammetry stud-ies show that thiourea is strongly adsorbed along the cop-per surface, but its consumption is negligible. The pres-ence of thiourea changes the microscopic morphology andcrystallography of the deposit, and this leads to brightdeposits. This confirms the above results from the CLSMwhich revealed that thiourea is more of a brightening thana leveling agent.

ConclusionsTo evaluate the influence of thiourea on the deposition

of Cu, three techniques were used. In the first, CLSM mon-itored the evolution of surface profiles on model coppersurfaces. The model surfaces were prepared by microlith-ography techniques. The CLSM technique was found towork extremely well on surfaces that had shallow rough-ness features, surfaces that are typically encounteredwhere leveling additives are desired. The vertical range ofthe surface profile that can be resolved by the technique isfrom submicron to tens of microns. The microscopic tech-nique showed that only geometric leveling occurred in thepresence of thiourea. The microscope was used here in theex situ mode, but in situ morphology monitoring is a

0

-to

-300 -250 -200 -150 -100 -50 0

Potential, mV

0

-l l0-2 l0-3 l0-4 l0

Nc in4

-6 l0

-7 l0

-8 l0



J. Electrochem. Soc., Vol 145, No. 3, March 1998 The Electrochemical Society, Inc. 963

I

50

0

-50

-100

-iso

-200

Fig. 18. Cyclic voltammogrom of copper deposition from 5 mMCuSO4 and I M H2S04 in the presence and absence of thiourea. Thearea of the electrode was 0.33 cm2.

0

-100

-200

-300N

-400

-500

-600

-700

-800

Fig. 19. Plot of the QCM frequency vs charge for the cyclicvoltammogram shown in Fig. 18 for 5 mM CuSO4 and 1 M H2S04at different concentrations of thiourea.

straightforward extension of the present study. It is conclud-ed that CLSM has considerable potential to monitor the evo-lution of surface morphology during electrodeposition.

Thiourea has only a small effect on surface leveling dur-ing copper deposition, and as noted above, geometric lev-eling was found to be dominant. At small current densi-ties, geometrical leveling was most pronounced. At highcurrent densities, the surface irregularities began to growwhich increased the surface roughness. A strong brighten-ing effect on the copper surface occurred at a thioureaconcentration of about 0.01 mM. At higher concentrationsof thiourea, a strong inhibition of the current density andnodulation of the deposit was observed.

In the second set of studies, the electrochemistry of thecopper-thiourea system was studied. Thiourea had astrong effect on the cyclic voltammogram of copper depo-sition from sulfate solutions due to its strong adsorptionalong the surface of the electrode. In the low concentrationregion thiourea accelerated copper deposition and theacceleration was found to be a function of the electrodepotential. At higher concentrations of thiourea, inhibitionof the deposition started to appear, and at high concentra-tions (0.8 niM) the inhibition by thiourea reached a plateau.

The third technique to monitor the effectiveness of can-didates for leveling was the combination of electrochem-istry and the quartz crystal microbalance in concurrentmeasurements. Thiourea acted only as a mediator in theelectrochemical reactions that occurred at the electrode

surface in the range of the studied concentrations andpotentials as revealed by the current efficiency measure-ments. Thiourea changed the rate of deposition, but thenet process remained unchanged. Also, there was no evi-dence of thiourea incorporation into the deposit, whichimplies that thiourea has a negligible cathodic consump-tion rate in acidic sulfate solutions during copper deposi-tion. The results support the findings from the CLSM exper-iments and one concludes that thiourea is a brighteningagent rather than leveling agent in copper deposition.

AcknowledgmentPortions of this work were supported by NSF under

Grant DMR-9509766.

Manuscript submitted July 21, 1997; revised manuscriptreceived November 3, 1997.

University of Minnesota assisted in meeting the publica-tion costs of this article.

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Charge, tC



quartz crystal microbalance studies of leveling effects of thiourea

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