The influence of organic additives on the thickness distribution of tubular metallized through-holes

Ehrtroch ftca Acts, vol 39, No 8/9, pp 1133-1137,1994 Copyngbt 0 1994 Ehe,m 9aeam Ltd

Pergamon

Pnowd m Greet Brim . AN nghu re*nnd 0013-4686144 $7m+000

00134686(94)E0027-W

THE INFLUENCE OF ORGANIC ADDITIVES ON THE THICKNESS DISTRIBUTION OF TUBULAR METALLIZED THROUGH-HOLES M. WuNsclle, W DAHMS, H MEYER and R SctiumAcmm* Atotech Deutschland GmbH, Erasmusstralle 20-24, D-10553 Berlin, Germany (Receuced

30 September 1993, in revised form 6 January 1994)

Abstract-Through-hole plating capability was examined for process electrolytes made up with various organic additives Tafel plots were obtained from cyclic voltammetry using the rotating-disc technique The levelling capability of the electrolytes was obtained from couloinetry and in situ microgravimetry in combination with optical microscopy, the throwing power for through-hole plating from the thickness ratio center of hole-plane surface outside the bole . It was shown that a high throwing power correlates with a low levelling capability The results shed some light on the inhibiting character of the process electrolytes used Key words tubular electrode, thickness distribution of metallic deposits, organic additives, electrocrystallization, m situ microgravimetry, in situ laser-surface-reflexion

INTRODUCTION Besides the physical and mechanical properties of a metallic deposit obtained from process electrolytes, the capability of these electrolytes to generate umforroly distributed metal layers inside of a tubular through-hole is a major requirement for their practical use in printed circuit teclmology[l-3] This is one of the quality requirements for a high capacity process electrolyte It has been stated that the influence of organic additives upon the thickness distribution inside a tubular metallized through-hole is associated with the quantity and the chemical and physical nature of these additives[4] It is common knowledge that in high capacity electrolytes a variety of acting organic agents are used which were added to the electrolytes in various concentrations Typically, the overall concentration for organic additives is in the order of several ppm . Continuously operating process electrolytes may be enriched with decomposition products of these additives which might operate themselves as acting agents to support a specific process feature Because of the complexity of process electrolytes and surface processes which occur during metal deposition it is difficult to obtain specific mechanistic details of the individual electrode processes for these electrolytic systems Theoretical simulations to obtain information on the thickness distribution of metahzed tubular electrodes have been performed on the basis of the current distribution along the tube and are reported in the literature The results definitely show that several parameters do effect the current distribution inside the tube The calculations considered hole geometry[2, 5, 6], conductivity of the electrolyte[2, 5, * Author to whom correspondence should be addressed

6], deposition kmetics[1, 5, 6] and hydrodynamic effects[2, 5, 6-8] This includes mass and charge transfer, Tafel parameters as well as laminar flow, periodic flow reversal and turbulent flow of the electrolyte In addition experiments were performed with a stagnant fluid[3] It is demonstrated here that for a number of process electrolytes there is a correlation between kinetic and topographic features on one hand and the thickness distribution of metallized throughholes on the other hand These experimental facts were evaluated from cyclic voltammetry expressed as Tafel kinetics and from surface topography of growing electrodes The thickness distribution was obtained from a microsection of the metalhzed through-hole All experiments were performed under controlled hydrodynamic and mass transport conditions EXPERIMENTAL TECHNIQUES, SAMPLE PREPARATION AND PROCESS ELECTROLYTES The experimental techniques used in this study were cyclic voltammetry performed with a rotatingring electrode and surface microscopy using a conventional Axioplan light microscope from Zeiss Some of the experiments were earned out with a combined in situ microgravimetric (QMB) and lasersurface-reflexion (LSR) set-up which allowed a simultaneous registration of changes in mass and the relative intensity of the reflected light 31/I Details of the technique are described elsewhere[9] All potentials referred to in this paper are measured with respect to the saturated calomel electrode (see) . The electrodes (a) and (b) (cf. Fig 1) were used for the quantification of the levelling capability and

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1 13 4

M WUNSCHE

el al

Fig 1 Surface pat em of rough (a) and fla (b) Pt electrodes after being coated wi h a 1 um thick Cu deposit recording of cyclic voltammograms, respectively They were prepared by polishing the Pt ring either with a diamond paste of 1 ,urn (electrode b) or 10 Am (electrode a) before coating them electrochemically with a 1 pm thick Cu film The surface topography of these samples are shown in Fig 1 The surface area of the rough electrode (a) is about 30% larger than sample (b) In situ microgravimetry and lasersurface-reflexion were performed on rough 5MHz quartz oscillators The used Cu matrices of conventional acid process electrolytes (A), (B), (C) and (D) are basic make up solutions consisting of 0 3 M Cu dissolved in 2 M sulfuric acid and additives of CI - (ca 50 ppm) and various organic components (a), (b) and (c) (a) HO,S-(CH,). S-S-(CH,),SO,H with n between 2 and 4, (b) HO-(CH,-CH,-0) goH, (c) H 2 N-(CH2 -CH iNH), aH Electrolyte A Basic make up solution, I ppm component (a), 300 ppm component (b) Electrolyte B Basic make up solution, 1ppm component (a), 300 ppm component (b), 1 ppm component (c) Electrolyte C Electrolyte A, 100 ppm crystal violet Electrolyte D Electrolyte B, 100 ppm crystal violet

phenomena The cyclic voltammograms were found to be independent of the scan rate, which was varied from 1 to 200mV s - ' The cv curves obtained with 1 mV s - ' are given in Fig 2 In the presence of 100 ppm crystal violet a significant inhibition of the cathodic current is observed (cf curves C and D of Fig 2) This observation is consistent with an earlier

0 -20

r1OA 4 &

-20

RESULTS Kinetic investigations Copper deposition from the process electrolytes (A-D) was investigated by means of cyclic voltammetry (cv) in the potential range from 0 to - 300 mV by using smooth Cu working electrodes of type (b) The electrode was rotated with a speed of 2000 cycles per minute (cpm) to minimize mass transport

-40

-60 -300

-200

-100

0

U,~/mV

Fig 2 Cyclic voltammograms obtained from the process electrolytes (AD) with working electrode (b)

The influence of organic additives study in which 3-ammo-5-heptyl-a4tnazole was used as an inhibitor[10] The evaluation of the current-potential data in terms of Tafel kinetics is shown in Fig 3 The Tafel plot (E) represents Cu deposition from a 05M aqueous CuSO, electrolyte and is given for comparison The plot agrees well with those reported in literature[l1, 12] For example, with the equilibrium potential of Uc,,, . /~ = 90mV,,- [11] the exchange current density l 0 = 18 mAcm gwas evaluated The Tafel slopes obtained for Cu deposition from the process electrolytes (A-D) decrease from 165 mV for electrolyte (A) to 115 mV for electrolyte (D) The latter value corresponds well to 118 mV evaluated from 05M CuSO4 solution The observed shifts of the Tafel slopes (A-D) toward negative potentials were quantified in terms of the actual working potentials U,. which is needed to maintain a typical deposition current of 25mAcm - ' U, shifts from -185mV,,, for electrolyte (A) to -283mV,,, for electrolyte (D) U,. and Tafel slopes are given in Table 1 Levelling capability

The levelling capability is defined as the potential of a process electrolyte to smooth an initially rough surface during metallization In earlier studies, this property has been related to capacitance data evaluated in terms of geometric assumptions associated with the surface pattern of a vertically growing electrode[13] The technique developed here measures the critical mass m„ needed to smooth an initially rough surface The final point of this titration is reached if a surface pattern comparable to that shown in picture (b) of Fig 1 is obtained m„ was evaluated either from chronoamperometry or from in situ microgravimetry with oscillating quartz crystals The surface characterization was performed ex situ with light microscopy or in situ with lasersurface-reflexion As reported earlier[9, 14], the

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latter technique has uniquely proved its capability of sensing topographic changes of vertically growing electrodes For example, the increase of the reflected light intensity 81/I during deposition from -0 .09 to -001 and the simultaneously registrated total mass gain of ca 3mgcm - ' are demonstrated in Fig 4 These data were obtained by using electrolyte (C) The observed bumps in the 61/I curve are related to intensity changes of the used He-Ne laser The observed 61/1 plateau above 850s resembles the curve at zero which proves that the initially rough surface has been smoothed out during electroplating From the onset of the SI/1 plateau a m„ of 19 mg cm - ' is evaluated (cf Fig 4) This value is in agreement with the one obtained ex situ with light microscopy For visualization the observed data are summarized in Table 1 They sharply decrease from electrolyte (A) to (B), (C) and (D) proving that m„ is inherently associated with the composition of the process electrolyte used

Thickness distribution of metallized tubular through-holes The electrochemical metallization of tubular through-holes results in a thickness distribution d(1) along the length I of the tube as demonstrated in Fig 5 The expression (d,/d,) x 100 is a practical measure for the uniformity of d(l) inside the tubular through-hole and is defined as throwing power A throwing power of 100% corresponds to a uniform thickness distribution The thickness of the layers d, and dz were determined from microsections of through-holes According to the upper illustration of Fig 5, d, corresponds to the metal layer outside the hole, while d2 represents the thickness of the metal layer at 1/2 A typical microsection of a through-hole electroplated from process electrolyte (C) is shown in the lower part of Fig 5 The layer thickness at d,

too to

-05 001

-300

0 090 -200 -100 .,/mV U

0

1m

Fig 3 Tafel plots of the deposition process for the electrolytes (A-D) by using working electrode (b) Curve (E) was obtained from a 0 5 M aqueous copper sulfate solution

400

500

I 1200

1600

TUDe/s Fig 4 simultaneously registrated increase of mass m and the intensity of the reflected light 81/1 during copper deposition From the onset of the 81/1 plateau the critical mass m„ needed to smooth the initially rough electrode was evaluated

Table 1 Correlation of throwing power with Tafel slope, U, and m„ Process electrolyte

A

B

C

D

Throwing power/% Tafel slope/mV ./mV U . mjmgcm - i

60 165 -185 112

65 150 -226

71 125 -260

72 115 -283 070

065

19

1 13 6

M WuNSCtm et

al

Cu - Deposit

////, %ii dz

11111-

/:%//////

Cu - Laminate

Fig 5 Illustration (upper part) of a tubular through-hole of a printed circuit board The quotient length I over diameter of the hole is defined as the aspect ratio The thickness of the metal layer outside and inside of the hole at 1/2 is designated d, and d„ respectively Evaluation of d, and d i from a microsection of a through-hole electroplated with process electrolyte (C) is shown in the lower part The data correspond to an aspect ratio of 5 and d 2 is determined with light microscopy and yield 34 and 24 pm, respectively These data correspond to a throwing power of 71 % Mean values of the throwing power obtained from electrolytes (A-D) are presented in Table I They increase from 60 to 72% Each value bases on a set of at least five independently evaluated microsections of through-holes which were prepared with a constant current density of 25mAcm -r The aspect ratio A, of the used through-holes was 5 A, is defined as the ratio of length over diameter of the tubular through-hole

DISCUSSION An improvement of the uniformity of dQ) is obtained if the adjusted current density of 25mAcm - ' needs a potential significantly below O V., Actually, the best uniformity is obtained if metal deposition is carried out with electrolyte (C) at U w =-283mV,,, In addition, the corresponding Tafel slope should be in the order of about 120mV

This suggests a similarity to the well known consecutive type of deposition mechanism for Cu" including a slow reduction kinetic to form Cu' and a fast Cu' reduction to form the final deposit This mechanism was suggested to occur in a 0 5 M CuSO, solution which exhibits a Tafel slope of ca 120 mV[l 1, 12] Considering a blocking[13, 15] of tiny convex surface sites, tips, spheres etc which make up the roughness pattern of the Cu substrate the deposition process on these sites is slow On the other hand, metal deposition in surface valleys occurs with an appropriate rate To explain the blocking mechanism several effects have been considered earlier such as field induced adsorption and catalytic actions of the additives[15], diffusion controlled processes of additives toward surface sites with microscopic dimensions etc [16] For large ni t , these sites are ineffectively blocked during the vertical growth of the layer so that metal deposition is almost uniformly distributed over the entire surface relief However, small m ., enhances surface blocking which accelerates metallization in surface valleys rather



The influence of organic additives than on blocked elevated sites The result is an extremely non-uniform metal distribution It can be rationalized, that both types of current distribution lead to different gains in the total layer thickness d, if m„ varies along the length of the through-hole As reported earlier[5], a change in current distribution of a tubular electrode was related to a change in the Ohmic resistance of the electrolyte along the hole The performed calculation showed that a nice uniformity of the metal layer is observed if the applied external current density j is small On the other hand, a non-uniform metal distribution was observed by applying a large external current density This observation is in agreement with the data reported here (cf Fig 3, Table 1) At a given potential the measured current density j drops from electrolyte (A)-(D) Simultaneously, the expected increase of the throwing power is in fact observed

CONCLUSION The most uniformly distributed metal layer is obtained from process electrolytes which are capable of being operated at a large overpotential with a slow deposition rate In addition, the Tafel slope should be in the order of 120 mV These observations are consistent with the idea that an appropriate uniformity of the distribution pattern is obtained for process electrolytes in which the deposition process is kinetically inhibited but still occurs via consecutive type of deposition mechanism An improvement in metal distribution is also observed for electrolytes with an effective levelling capability In this case,

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surface inhibition due to adsorption is kinetically favoured compared to metal deposition A reversed kinetic situation leads to a poor metal distribution

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