UNITS) STATES PARTMENT OF OMMERCE

JBUCATION

*«WIS o«

'

NBS TECHNICAL NOTE 767

Microelectronic

Interconnection Bonding

With Ribbon Wire

U.S. PARTMENT

OF

COMMERCE

National

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of IOO ds

5153

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NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and

technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measure- ment system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology, the Center for Computer Sciences and

Technology, and the Office for Information Programs. THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United States of a complete and consistent system of physical measurement; coordinates

that system with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics —

Linac Radiation - — Nuclear Radiation L> — Applied Radiation = — Quantum

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. THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research lead- ing to improved methods of measurement, standards, and data on the properties of

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2 Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.

• Part of the Center for Building Technology.
l>&

MICROELECTRONIC INTERCONNECTION

BONDING WITH RIBBON WIRE

H. K. Kessler and A. H. Sher

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards Washington, D.C. 20234

This research was supported by the Advanced Research Projects Agency

of the Department of Defense under

ARPA ORDER 1889

M. Chernoff, ARPA Agent/Project Officer Directorate of Technology/SYS

Space and Missile Systems Organization Air Force Unit Post Office Los Angeles, CA 90045

SAMSO MIPR FY 76167100331

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of

transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

7

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Issued April 1973
National Bureau of Standards Technical Note 767

Nat. Bur. Stand. (U.S.), Tech. Note 767, 31 pages (Apr. 1973) CODEN: NBTNAE

Cover photograph. SEM photomicrograph of bonds made with 3- by 0.5-mil aluminum (1% silicon) wire on

5-mil square aluminum pads. The capability of placing ribbon-wire bonds one on top of another is shown. Four

bonds are stacked at the center of the letter "B" (Magnification 60X)

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.46:767).

Price $0.50 domestic postpaid or $0.35 G.P.O. Bookstore.
TABLE OF CONTENTS

PAGE

Foreword vi

1. Introduction 1 1.1. Units 2

2. Fabrication and Testing of Wire Bonds 2

2.1. Fabrication of Bonding Pads 2 2.2. Procedures Used to Measure Bond Pull and

Wire Tensile Strengths 3

3. Exploratory Studies 3 3.1. Ribbon Wire Bonder 3 3.2. Bonding Tools 4 3.3. Ribbon Wire 4 3.4. Hard Wire Bonds 5 3.5. Ribbon Wire Bonds - Initial Results 6

4. Results 8 4.1. Comparison of Ribbon and Round Wire Pull

Strength 8 4.2. Pull Strength as a Function of Loop Height .... 13

4.3. Magnesium-Doped Wire 17

4.4. Gold Wire 17

5. Bonding Equipment 18

5.1. Wire Clamps 18

5.2. Tools 19

6. Conclusions 23

7. Acknowledgements 23

8. References 23

iii
LIST OF FIGURES

PAGE

SEM photomicrograph of bonds made with 3- by

0.5-mil aluminum (1% silicon) wire cover

1. SEM photomicrograph of a rectangular wire-feed

hole in an ultrasonic bonding tool 5

2. SEM photomicrograph of a ribbon wire first bond

made with low power and long time 7

3. SEM photomicrograph of a ribbon wire first

bond made with high power and relatively short

time 7

4. SEM photomicrograph of three bonds made with

aluminum ribbon wire on a bonding pad 5-mils

square 9

5. SEM photomicrograph of three stacked bonds

made with aluminum ribbon wire 9

6. Pull strength of ribbon wire bonds as a function

of time and tool tip displacement 11

7. Pull strength of round wire bonds as a function

of time and tool tip displacement 11

8. Pull strength of ribbon wire bonds as a function

of time and tool tip displacement; bonding force of

35 gf 13

9. Time and tool tip displacement settings that

yield bonds with a pull strength of 7 gf or greater 14

10. Time and tool tip displacement settings that

yield bonds with a pull strength of 10 gf or greater 15

11. Pull strength of ribbon wire bonds as a function

of loop height 16

12. SEM photomicrograph of two views of a bonding

tool showing rough surface finish 20
13. SEM photomicrograph of two views of a bonding

tool with a relatively smooth surface finish .

14. SEM photomicrograph of two views of a bonding

tool showing pin holes in the surface finish .

15. SEM photomicrograph of two views of a tungsten carbide bonding tool showing build-up

of gold at the wire-feed hole

PAGE

. 20

. 21

21

LIST OF TABLES

Specifications of Wire Studied

PAGE

5

2. Comparison of Pull Strengths of Ribbon

and Round Wire Bonds 12
FOREWORD

This technical note is the final report to the Advanced

Research Projects Agency for the period June 1, 1971 to May 31,

1972 on a study designed to show the feasibility of using

aluminum ribbon wire for ultrasonic bonding of semiconductor

microelectronic device interconnections. The work was carried

out under contract number MIPR FY 76167100331 (ARPA Order Number

1889), M. Chernoff ARPA Agent/Project Officer.
MICROELECTRONIC INTERCONNECTION BONDING

WITH RIBBON WIRE

H. K. Kessler and A. H. Sher

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

The feasibility of using aluminum ribbon wire for

ultrasonic bonding of semiconductor microelectronic

interconnections was studied, and several advantages

over the use of round wire of equivalent cross-sectional

area were found. Ribbon wire bonds exhibited little

deformation or heel damage, and a greater percentage

of bonds of a certain quality (as judged by pull strength

and appearance) could be made over much greater ranges

of the bonding machine parameters, time and tool tip

displacement, using ribbon wire than was possible with

round wire. The ease of positioning ribbon wire was

indicated by making multiple ribbon wire bonds side-byside on a 5-mil square ^ad, or by stacking up to four

bonds one on top of another. However, bonding with harder

than normal wire, previously thought to offer certain

advantages with respect to higher bond tensile strength,

yielded inconsistent results.

Key Wovds: Aluminum wire; bonding; fabrication (wire

bonds); microelectronics; ribbon wire; round wire; testing (wire bond); ultrasonic bonding; wire bond.

1. INTRODUCTION

At the start of this study, there was no reported industrial use of

aluminum ribbon wire for microelectronic ultrasonic bonding nor were

ribbon wire bonding machines and suitable aluminum ribbon wire generally

available. Prior exploratory work at NBS had indicated that the use of
ribbon wire offered significant advantages over the conventionally used

round wire [1,2]. The present study was undertaken in order to investigate the use of aluminum ribbon wire for ultrasonic bonding in semiconductor microelectronics, with emphasis on the advantages over round wire

and the problems that might be encountered in converting presently used

round wire bonding equipment to ribbon wire use. The criteria used in evaluating bond quality were, primarily, pull

strength and, secondarily, appearance. The pull strength was measured

using a double bond, destructive pull test. The appearance criterion

was based upon a subjective evaluation of the surface finish of the

resulting bond and the degree of deformation at the heel as observed

under high magnification, usually with a scanning electron microscope

(SEM)

.

1.1. Units

The following units which are in general usage in the bonding

field are used rather than the International System (SI) of units in

this report: (1) mils (1 mil = 25.4 um) for dimensions of the wire,

bond loop, bond spacing, substrate, and tool, and (2) grams-force

(1 gf = 9.8 mN) for the force exerted on the wire or wire bond.

2. FABRICATION AND TESTING OF WIRE BONDS

Except where otherwise noted, wire bonds studied in the present

work were fabricated on a commercial ultrasonic wire bonder equipped

with a tungsten carbide tool. This bonder, which had been modified to improve its mechanical stability and temperature characteristics [3], was further modified to facilitate the use of ribbon wire. Some

exploratory studies were performed using bonds made on a different

bonder, one that was intended for ribbon-wire use but later found not

to give optimum results. Unless otherwise specified, the nominal value

of bond loop height was 10 mils, bond-to-bond spacing was 40 mils, and

bonding force was 25 gf

.

2.1. Fabrication of Bonding Pads

Bonding pads were fabricated on 10-mil thick, 1-inch diameter

silicon wafers coated with a 0.8- to 1.0-um thick film of steam-grown
oxide and a 0.8- to 1.0-ym thick film of evaporated aluminum. The

aluminum was etched to leave 5-mil square bonding pads on 10-mil centers

by means of standard photolithographic techniques. After etching, the

array of square bonding pads was sintered in a helium atmosphere at 550°

C

for 15 min. Such an array is shown in the cover photograph.

2.2. Procedures Used to Measure Bond Pull and Wire Tensile Strengths

In all of the measurements reported here, the pulling force was applied normal to the plane of the single-level substrates. Pulling was accomplished with an electrolytically-etched wire hook driven by a switch-controlled adjustable speed motor at a pull rate of 1 gf/s. The

hook was mounted on a micropositioner so that it could be precisely

aligned with respect to the wire. The pulling force was measured by a gram-gage dynamometer.

The wire was prepared for measurement of tensile strength by

affixing one end of a 10- to 15-mm length of wire to a base plate with

room-temperature curing epoxy and placing a ball of the epoxy on the

other end. A small fork placed under the epoxy ball was used to apply

the test force. Using the pulling apparatus, each of ten wire specimens

was pulled to breaking and the results averaged. Only specimens that

exhibited a break in the wire were counted in calculating the average.

3. EXPLORATORY STUDIES

3.1. Ribbon Wire Bonder

Initial efforts to achieve optimum performance and reliability

with a bonder manufactured for use with ribbon wire were hampered by

a vertical jumping motion of the bonding tool after the first bond had

been made. This tended to cause severe cracking in the bond heel.

Considerable reduction of the undesired motion was achieved by regrinding the appropriate cam surfaces by hand.

Another problem with the bonder was a considerable variability

of bond tail-length that it would produce. This was reduced by a factor

of two by improving the wire-feed mechanism. This tail-length variation,

which is of only cosmetic concern to bonding on a laboratory basis, must

be controlled before the machine can be used in production since long
wire tails in actual devices may cause short circuits.

3.2. Bonding Tools

A feature of the bonding tool that was considered desirable but

not essential for the initial studies was a rectangular wire-feed hole.

At the time these studies were to begin, commercial tools with rectangular

feed holes could not be located. However, one company agreed to make

such tools if supplied with tungsten wire of rectangular cross section

required for the electrical-discharge-machining operation to fabricate

such holes. A simple procedure was developed to form the tungsten ribbon

wire. Lengths of 1-mil and 2-mil diameter round wire were flattened by

placing the tungsten wire in a press between glass plates and applying

a load to 45 to 76 MN/m^ for 1-mil diameter wire, or 103 to 145 MN/m2 for

2-mil diameter wire. The resulting ribbon was not generally uniform, but

pieces could be selected for the desired sizes of rectangular holes.

At a later time, a supplier of tools with rectangular feed holes was

located. An SEM photomicrograph of the rectangular feed hole in one of

the commercially-available tools is shown in figure 1.

3.3. Ribbon Wire

The aluminum ribbon wire used for most of the experimental work

was doped with one percent silicon and had cross-sectional dimensions

of 1.5 by 0.5 mils. This results in about the same cross-sectional

area for the ribbon wire as for 1-mil diameter round wire. Aluminum

wire doped with one percent magnesium, silicon-doped aluminum wire of

larger dimensions, and undoped gold wire were also studied. Table 1 lists the composition, tensile strength, and nominal cross-sectional

dimensions for the various ribbon wire used.

The desired variation of the cross-sectional dimensions was equal

to or less than 10 percent. Difficulty was encountered in obtaining

wire with this degree of control. Contacts with three manufacturers of

ultrasonic bonding wire were required, and nearly half of the period for

this study passed, before satisfactory wire was obtained.

The ribbon wire was obtained on 0.5-in. or 2-in. diameter spools.

Wire wound on the 2-in. spools showed less twisting and binding and
(mils) Range (gf)

A Al (1% SI) 1.5 x 0.5 13.0 - 13.5a

B Al (1% Mg) 1.5 x 0.5 vL4

C Al (1% Mg) 1.5 x 0.5 23.1 - 23.

5

a

D Au 1.5 x 0.8 ^15

E Al (1% Si) 1.0 (dia.) 12.5 - 13.

2

a

Figure 1: SEM photomicrograph of a rectangular wire-feed

hole in an ultrasonic bonding tool. (There is a slight

build up of aluminum in the upper left corner of the hole.

Magnification: about 750X.)

Table 1: Specifications of Wire Studied

Lot Composition Dimensions Tensile Strength Comments

partially annealed

partially annealed

hard wire , partially annealed,

partially annealed

a low and high values of breaking strength as measured in the

laboratory (see text)

b

by manufacturer

exhibited better feeding characteristics through the transducer horn,

wire clamp, and bonding tool. The tail length and loop height were also more uniform in bonds made from wire wound on the larger spools. Only

minor modification of the bonders was needed to accommodate the larger

spools.

3.4. Hard-Wire Bonds

It had been speculated [1] that high tensile strength ("hard")

ribbon wire could be used to achieve considerably higher bond strengths

5
than with round wire. The expectation was that significant damage to the substrate would not occur with the higher ultrasonic power and bonding

force required when using hard wire because less deformation of the

ribbon wire occurred. However, in a preliminary test bonding with hard

wire (Lot C) proved to be very difficult, and inconsistent results were

obtained. The power range in which bonds of high pull strength could be

obtained was narrow, and the other bonding parameters were critical in

order to accomplish a low variability in mean pull strength. On lifted

bonds (those carefully peeled from the substrate) , cratering was observed

in the substrate due to the increased bonding force required to obtain

adherent bonds. Though this experiment was performed using magnesiumdoped wire, no significant difference would be expected using silicondoped wire (see Section 4.3.). Because of the high quality of bonds

later found with stress-relieved wire, no further attempts to bond with

hard wire were made during subsequent phases of the work.

3.5. Ribbon Wire Bonds - Initial Results

About 2000 bonds were made with Lot A wire on several single-level

bonding substrates in order to investigate the pull strength and appearance of bonds made with different bonding power and time. The bond-tobond spacing was 40 mils, similar to that used in typical devices. A

single tool was used to make all the bonds.

A better appearance was obtained with low ultrasonic power and

relatively long bonding time (figure 2) than with the higher ultrasonic

power and shorter bonding time usually employed in round-wire bonding.

In the latter case, the bond surface appearance was rough and irregular,

and a significant heel crack appeared in the first bond (figure 3)

.

The bond shown in figure 2, typical of those made with the low-power,

long time schedule, had a pull strength approximately 1.5 times that of

the bond shown in figure 3, typical of those made with the high-power,

short-time schedule.

A test was conducted to establish that the positioning and

deformation of the bonded ribbon wire could be controlled sufficiently

to permit the use of the same size bonding pad that would be required

for round wire of cross-sectional area equivalent to the ribbon wire.

6
Figure 2: SEM photomicrograph of a ribbon wire first bond

made with low power [20-yin. (0.5-ym) peak-to-peak tip

amplitude] and long time [155 ms] (Magnification ^470X).

Figure 3: SEM photomicrograph of a ribbon wire first

bond made with high power [82-yin. (2.1-pm) peak-to-peak

tool tip amplitude] and relatively short time [40 ms]

(Magnification ^500X)

.
Three bonds were made with 1.5- by 0.5-mil aluminum (1% silicon) ribbon

wire side-by-side on a bonding pad 5 mils square. The result, pictured

in figure 4 , dispelled any concern that larger bonding pads may be

required if ribbon wire is used. Figure 5 shows that ribbon wire bonds

can be stacked directly one on top of another. This, along with the cover photograph, further illustrates the degree of control in bond

placement that the use of ribbon wire offers. This can be an important

consideration in device repair or replacement, especially for hybrid

devices, and for high frequency devices in which lead inductance must be

minimized.

4 . RESULTS

Measurements were carried out using aluminum (1% silicon) wire:

(1) to compare directly the pull strengths of ribbon and round wire

with equivalent cross-sectional areas, (2) to determine the bonding

parameters (time, bonding force, and tool tip displacement) necessary

to obtain optimum bonds, and (3) to examine the effects of changes in

the bonding parameters on bond appearance and deformation. The results

of these measurements are given below. Results of bond pull strength

measurements using aluminum (1% magnesium) ribbon wire and gold ribbon

wire ultrasonically bonded to aluminum pads are also reported.

4.1. Comparison of Ribbon and Round Wire Pull Strengths

Because the exploratory studies showed that high quality bonds

could be made with ribbon wire, the investigation was broadened to compare ribbon and round wire bonds over a wide range of power and

time settings for a constant bonding force of 25 gf . Using the same bonding machine and tool , wire bonds of both types (wire Lots A and

E) were made with bond-to-bond spacings of 40 mils and loop heights of

10 mils side-by-side on the same metallized silicon substrate in order to minimize any influence of differences in bonding conditions or substrate

characteristics

.

* The use of the same tool (with round feed hole) for both round

and ribbon wire results in less than optimum conditions for bonding

ribbon wire (see Section 5.2.).
Figure 4: SEM photomicrograph of three bonds made with

aluminum ribbon wire with cross-sectional dimensions

1.5 by 0.5 mils on a bonding pad 5-mils square. (The

bonding schedule was low power [32-yin. (0.8-ym) peakto-peak tool tip amplitude] and medium time [80 ms].

Magnification ^460X)

.

Figure 5: SEM photomicrograph of three stacked bonds

(shown by arrows) made with aluminum ribbon wire with

cross-sectional dimensions 3 by 0.5 mils on a 5-mil

square bonding pad (Magnification ^200X)
For each combination of displacement and time, 10 bonds were made

and their pull strengths determined by pulling vertically at the midpoint of the loop. The average pull strength for each group is plotted

against both time and tip displacement in figures 6 and 7 for ribbon

and round, wire bonds, respectively. The data points have been connected

by straight lines to define planes each of which includes groups of bonds

made at a constant time but with a range of power (tool tip displacement).

For convenience, different symbols are used for groups with mean pull

strength less than 7, 7 to 10, and greater than 10 gf. For clarity the

variance is not indicated in the figures. Open points and short dashed

lines indicate portions of a plane lying behind another. In general, at the lower displacements, the major mode of the bond failure was bond lift

off while at higher power it was breakage at the heel of the bond.

The results of a statistical analysis of the bond data considered

as a whole are summarized .in table 2. The entries in this table are the

averages of the mean pull strength of bond groups with mean pull strength

in the specified range and the pooled sample standard deviation for the means. The number of groups included in each category is also given.

The mean pull strength of bonds made using ribbon wire was a statistically significant eight percent higher than the mean pull strength of

bonds made with round wire in the range of pull strengths greater than

10 gf . About 90 percent of the ribbon-wire bonds showed pull strengths

greater than 7 gf while only 42 percent of the round-wire bonds did for

the power and time settings studied.

Also shown in table 2 are the results from a comparable series of

ribbon-wire bonds using Lot A wire made on a different substrate using

a bonding force of 35 gf. Figure 8 shows plots of these data in a form

similar to that of figures 6 and 7. In terms of the bond pull strength,

the use of higher force appears to result in more of the bonds having

pull strengths greater than 10 gf . However, microscopic examination of

the substrate used for the series with higher bonding force after removal

of the bonds and metallization indicated a slight amount of damage

beneath the metallization. Such damage was not observed in the substrate

used for the series with the lower force.

10
Figure 6: Pull strength of ribbon wire bonds as a function of time and tool tip displacement. (Bond

pull strength range is shown by symbols: •,Q> 10 gf;

A, A 7-10 gf; andB,D<7 gf.)

o tP Figure 7: Pull strength of round wire bonds as a function of time and tool tip displacement. (Bond

pull strength range is shown as in figure 6.)

11
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o a Figure 8: Pull strength of ribbon wire bonds as a function of time and tool tip displacement; bonding

force of 35 gf. (Bond pull strength range is shown

as in figure 6.)

The experiments reported above show that a major advantage of

using ribbon over round wire is that with ribbon wire less precise

control of bonding machine parameters is required to yield bonds of a certain quality than is required with round wire. This is illustrated

graphically in figures 9 and 10 which delineate the regions of time and

tool tip displacement where bonds of pull strength greater than 7 gf and

10 gf , respectively, were obtained. In both cases, these regions are much smaller for the round wire bonds than for the ribbon wire bonds.

Although ribbon wire bonds with high pull strength were obtained over

a much wider range of power and time than was indicated in the initial

experiments, observations confirmed that, as previously reported, the

best appearance was obtained at low power and long time.

4.2. Pull Strength as a Function of Loop Height

Measurements of ribbon wire bond pull strengths as a function of

bond loop height were made. As shown in figure 11, up to a loop height

of about 10 mils, the variation of pull strength as a function of loop

height is in good agreement with the mechanical resolution of forces

analysis (solid curve) [4]. Above 10 mils, the measured pull strength

13
260

200

E

in

2

100

- ROUND WIRE

RIBBON WIRE

I 1

20 40 60 80

TOOL TIP DISPLACEMENT (filn.)

100

Figure 9: Time and tool tip displacement settings

used in this work that yield bonds with a pull

strength of 7 gf or greater.

14
260

200

lil

100

RIBBON WIRE

ROUND WIRE

1 I 1 1

20 40 60 80

TOOL TIP DISPLACEMENT (/dn.)

100

Figure 10: Time and tool tip displacement settings used

in this work that yield bonds with a pull strength of

10 gf or greater.

15
8 10

LOOP HEIGHT (in.)

12 14 16x10*-3

Figure 11: Pull strength of ribbon wire bonds as a function of loop height. (The data points represent

the mean for up to 10 bonds. Solid circles include

only bonds which ruptured at the heel; open circles

include both bonds which ruptured at the heel and

which failed by lift off. Error bars represent one

sample standard deviation. The solid curve is calculated from resolution-of-forces using the wire

tensile strength indicated by the arrow.)

16
is lower than the predicted value. The solid data points are average

pull strengths for groups of nominally 10 bonds each, excluding those

bonds that failed due to lift off of one of the bond pairs. The error bars represent one sample standard deviation above and below the mean.

To obtain the two open data points at the two highest loop heights used,

pull strengths of bonds which failed by lift off were included in the

average pull strength calculation. As the loop height becomes increasingly higher, more vertical pulling force is exerted on the bonds. The

experimental data, where bond failure known to be due to complete lift

off is included, suggest some peeling of the bond before breakage at the

heel caused by the increased vertical pulling force might account for

the measured pull strength being less than predicted. This conclusion

is in agreement with the results of measurements of pull strength of

round wire bonds as a function of loop height [5]

.

4.3. Magnesium-Doped Wire

The pull strength and appearance of bonds made with Lot B aluminum

(1% magnesium) ribbon wire were investigated using single-level bonding

substrates. A single tool with a round feed hole was used for all bonds.

Measurements of pull strength of bonds made with various values

of ultrasonic power (tool tip displacement) and time showed that, in

agreement with the results obtained on aluminum (1% silicon) ribbon wire,

the use of a low-power, long-time bonding schedule resulted in higher

pull strengths and better appearance than a high-power, short-time bonding schedule. The highest average pull strengths and smallest variations are obtained at tool tip vibration amplitudes of 25 to 35 yin.

(0.6 to 0.9 urn) and times of 110 to 285 ms. These groups of bonds made

at low power and long time show mean pull strengths of from 11.0 to 12.2 gf with an average standard deviation of ±4 percent about the mean.

Thus, there appears to be no significant differences between aluminum

(1% magnesium) and aluminum (1% silicon) ribbon wire of comparable

tensile strength.

4.4. Gold Wire

A series of experiments was undertaken to determine the feasibility

17
of ultrasonically bonding gold ribbon wire (Lot D) to aluminum pads. It was found that bonds with mean pull strengths of 11.0 gf could be obtained by using high power, about 95-pin . (2.4-um) peak-to-peak tool tip

displacement, long time, from 150 and 275 ms, and a bonding force of

35 gf . The largest standard deviation was 10.7 percent for a group of

ten bonds.

5. BONDING EQUIPMENT

The results of the previous section showed that ribbon-wire bonds

with high pull strengths and a good appearance could be obtained. These were made on an existing round-wire bonding machine with only slight

modifications which are discussed in this section.

5.1. Wire Clamps

The major difference between round and ribbon wire ultrasonic

bonders is in the wire-feed and wire-clamp mechanisms. Round wire

clamps usually open and close in the horizontal plane while ribbon wire

clamps usually open and close in the vertical plane in order to grip

the wire across its largest dimension. During the course of the program, three companies developed ribbon wire clamps for evaluation. In

general, two types of relatively simple modifications were found necessary in order to obtain optimum performance from these clamps. They

involved the clamping surfaces and the clamping force on the wire.

The clamping surfaces were lapped plane-parallel and polished to prevent damage to the wire. It was also found advisable to mill away

a portion of the clamp that comes closest to the substrate so that

bonds could be made in areas on the substrate such as those, for example, near the edge of a package not readily accessible otherwise. These

modifications can be performed during manufacture of the clamp.

It was found that ribbon wire clamps should have adjustable clamp

pressures so that the pressure can be set at a point where the bonding

surfaces of the wire are not work hardened by excessive pressure. One

of the clamps examined did not have provision for easily adjusting the

clamping pressure. Another had a pneumatic control that proved satisfactory for this purpose. In one case, the clamp linkage had to be

18
modified so that the clamping surfaces were far enough apart when open

that the wire could move freely between, otherwise unwanted variation

in bond loop height was found to result.

5.2. Tools

In addition to the experiment previously described where a single

tool was used, ultrasonic bonding tools made from different materials

and from different vendors were used for making aluminum and gold ribbon

wire bonds. Bonding tools from four vendors were used with round and

rectangular feed holes in tungsten carbide, titanium carbide, and alloytip tools. Rectangular feed holes proved better than round feed holes

in feeding the ribbon wire through the bonding tool, and yielded more uniform tail lengths, loop heights, and less twisting of the wire under

the tool. In addition, with a rectangular feed hole, positioning the

wire on a small bonding pad is easier to accomplish (see figures 4 and

5).

The surfaces of the bonding tools supplied by a given company

as well as on tools obtained from different sources appeared to vary

significantly. Some tungsten carbide tools were not homogeneous

and had a rough surface finish as shown in figure 12. Another tungsten

carbide tool from the same source had a much smoother surface finish

as shown in figure 13. Both tools were used for bonding both aluminum

and gold ribbon and round wire. Some gold build-up is apparent on the

tool shown in figure 12. None appears on the tool shown in figure 13. Some titanium carbide tools that were obtained had pin holes on the

bonding surface, and the surface finish was poor (figure 14). The

titanium carbide material appeared to be softer and to exhibit more

wear than other materials. Alloy-tip bonding tools showed gold buildup on the bonding surface, and some gold particles at the exit of the

wire feed-hole due to the roughness of the machined hole as shown in

figure 15. During the bonding operation, in many cases, some wire

shavings were observed to come off the round or ribbon wire and were

found as little curls on the bonding substrate. In addition, the tail

length of the ribbon wire varied considerably when using any tool with

a roughly-machined feed-hole. For one tool, after polishing the feed19
SsSJP:

Figure 12: SEM photomicrograph (magnification

^240X) of two views of a bonding tool showing

rough surface finish.

Figure 13: SEM photomicrograph (magnification

^200X) of two views of a bonding tool with a relatively smooth surface finish.

20
Figure 14: SEM photomicrograph (magnification

M.90X) of two views of a bonding tool showing pin

holes in the surface finish.

Figure 15: SEM photomicrograph (magnification

^200X) of two views of a tungsten carbide bonding

tool showing build-up of gold at the wire feed hole.

21
hole with a nylon filament and 3-um diamond polishing compound,

the tail length variation decreased, and the shaving of the wire was virtually eliminated.

Tools with different footlengths, and front and back radii were

examined for use with ribbon wire. In order to achieve optimum reproducible pull strength from ribbon wire bonds it is important to make

those bonds with a tool having the correct foot length. In round wire

bonding, the tool sinks into the wire as it deforms and increases the

length (and area) of contact to include much of the front radius of the

tool which is typically 1.5 mils. This increases the actual weld length

(and area). However, ribbon wire bonds show less deformation than round

wire bonds, and the extra length provided by the front radius over the

foot length does not contribute to the bond length. A tool with a specified foot length of no less than 4.5 mils has been found to give a satisfactory weld length of approximately 3 mils. Tools with shorter

foot lengths tend to produce weaker bonds and bond loops which typically

fail in bond pull tests on the second bond. Studies also revealed that

the tool heel radius should be no longer than approximately 0.2 mils,

otherwise the wire cutoff operation may be impaired.

22
6. CONCLUSIONS

The advantages that are obtained from the use of ribbon wire rather

than round wire can be summarized as follows:

1. Bonds of a desired pull strength can be made with ribbon

wire over a much greater range of bonding time and power (tool tip

displacement) than can comparable round-wire bonds.

2. Ribbon wire bonds can be made one on top of another. This

implies that device repair or replacement, as in hybrid devices, can be simply achieved using ribbon wire bonds.

3. Location of ribbon wire bonds can be readily controlled and

such bonds can easily be placed side-by-side on a small bonding pad

because there is less tendency for ribbon wire to twist or roll out

from under the tool tip during bonding than for round wire.

The results presented in this report have demonstrated the basic

feasibility and principal advantages of using aluminum ribbon wire for

semiconductor microelectronic ultrasonic bonds.

7 . ACKNOWLEDGEMENTS

This report has benefited considerably from discussions with

W. M. Bullis, G. G. Harman, and H. A. Schafft. One of these (GGH)

contributed significantly to the project during the first half of the

contract period. The work of Mrs. S. A. Davis and Mrs. E. Y. Trager

in typing the various manuscripts and L. R. Williams in preparing the

illustrations is gratefully acknowledged.

8. REFERENCES

References 1, 2, 3, and 5 are contained in quarterly reports of

the NBS Semiconductor Technology Program issued in the NBS Technical

Note series under the title, Methods of Measurement for Semiconductor

Materials, Process Control, and Devices, W. M. Bullis, editor.

1. G. G. Harman, H. K. Kessler, and K. 0. Leedy, NBS Technical Note 488,

pp. 24-25 (July 1969).

2. K. 0. Leedy, NBS Technical Note 495, pp. 24-25 (September 1969).

23
3. G. G. Harman and H. K. Kessler, NBS Technical Note 527, p. 39

(May 1970).

4. H. A. Schafft, Testing and Fabrication of Wire-Bond Electrical

Connections - A Comprehensive Survey, NBS Technical Note 726,

pp. 60-68 (September 1972).

5. K. 0. Leedy, A. H. Sher , and C. A. Main, NBS Technical Note 743,

pp. 27-28 (December 1972).

a U. S. GOVERNMENT PRINTING OFFICE : 1973—511-325/225

24
form NBS-114A (1-711 U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN-767

2. Gov't Accession No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE Microelectronic Interconnection Bonding with Ribbon Wire

5. Publication Date April 1973

6. Performing Oiganization Code

7. AUTHOR(S)

H. K. Kessler and A. H. Sher

8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS NATIONAL BUREAU OF STANDARDS DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Address Advanced Research Projects Agency

1400 Wilson Blvd. Arlington, VA. 22209

13. Type of Report & Period Covered Final Report:

6/1/71 to 5/31/72

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

M. Chernoff , ARPA Agent/Project Officer; Directorate of Technology/ SYS; Space and Missile System Organization; Air Force Unit Post Office; Los Angeles, CA 90045

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.) The feasibility of using aluminum ribbon wire for ultrasonic bonding of semiconductor

microelectronic interconnections was studied, and several advantages over the use of round wire of equivalent cross-sectional area were found. Ribbon wire bonds exhibited

little deformation or heel damage, and a greater percentage of bonds of a certain

quality (as judged by pull strength and appearance) could be made over much greater

ranges of the bonding machine parameters, time and tool tip displacement, using

ribbon wire than was possible with round wire. The ease of positioning ribbon wire

was indicated by making multiple ribbon wire bonds side-by-side on a 5-mil square

pad, or by stacking up to four bonds one on top of another. However, bonding with

harder than normal wire, previously thought to offer certain advantages with respect

to higher bond tensile strength, yielded inconsistent results.

17. KEY WORDS (Alphabetical order, separated by semicolons) Aluminum wire; bonding; fabrication (wire bonds) ; microelectronics; ribbon wire;

round wire; testing (wire bond); ultrasonic bonding: wire bond.

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