National Bureau of Standards

Library, E-01 Admin. Bldg

OCT 6 1981 191116

/CO

NBS TECHNICAL NOTE 785

Scanning

Electron Microscope Examination

of Wire Bonds

from High-Reliability

Devices
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards 1 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 measurement 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 Institute 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 — Nuclear

Sciences " — Applied Radiation - — Quantum Electronics 3 — Electromagnetics 3 — Time

and Frequency 3 — Laboratory Astrophysics " — Cryogenics 3

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

materials needed by industry, commerce, educational institutions, and Government; provides

advisory and research services to other Government agencies; and develops, produces, and

distributes standard reference materials. The Institute consists of the Office of Standard

Reference Materials and the following divisions: Analytical Chemistry — Polymers — Metallurgy — Inorganic Materials — Reactor

Radiation — Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote

the use of available technology and to facilitate technological innovation in industry and

Government; cooperates with public and private organizations leading to the development of

technological standards (including mandatory safety standards), codes and methods of test; and provides technical advice and services to Government agencies upon request. The Institute consists of a Center for Building Technology and the following divisions and offices: Engineering and Product Standards — Weights and Measures — Invention and Innovation — Product Evaluation Technology — Electronic Technology — Technical Analysis — Measurement Engineering — Structures, Materials, and Life Safety * .— Building Environment * — Technical Evaluation and Application * — Fire Technology.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effec- tiveness in the conduct of their programs through the selection, acquisition, and effective

utilization of automatic data processing equipment; and serves as the principal focus within

the executive branch for the development of Federal standards for automatic data processing equipment, techniques, and computer languages. The Center consists of the following offices and divisions: Information Processing Standards — Computer Information — Computer Services —

Systems Development — Information Processing Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal Government; promotes the development of the National Standard Reference Data System and

a system of information analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services to ensure that the NBS staff has optimum

accessibility to the scientific information of the world. The Office consists of the following

organizational units: Office of Standard Reference Data — Office of Technical Information and Publications —

Library — Office of International Relations.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

3 Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.

* Part of the Center for Building Technology.
Scanning Electron Microscope Examination

of Wire Bonds from High-Reliability Devices

Kathryn O. Leedy

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards Washington, D.C. 20234

^7-

7<2d/)t>/C£L# nei-c. s)o. 78-S

^"^

£:*. \"enu of

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

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

•Issued August 1973
National Bureau of Standards Technical Note 785

Nat. Bur. Stand. (U.S.), Tech. Note 785, 35 pages (Aug. 1973) CODEN: NBTNAE

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC. 20402 (Order by SD Catalog No. C13.46:785). Price J0.55
CONTENTS

1. Introduction

4. Wire Appearance

7

.

Summary

8. References

iii

Page

1

2. Background ,

3. Bond Appearance 2

10

5. Metallization Appearance -,£

6. Other Observations 20

28

28
LIST OF FIGURES

Page

1. Typical devices examined 3

2. The lift-off pattern of a partially removed bond 3

3. Sketch of the ultrasonic bonding cycle 4

4. A comparison of a first and a second bond 5

5. First and second bonds from another production line 7

6. Examples of different types of first-bond heel cracks .... 8

7. Variation in bond deformation from bond to bond on a single device 9

8. A bond which has suffered from gross unwanted toolto-substrate motion during bonding as evidenced by

the large side-to-side indentations 11

9. A bond which exhibits small unwanted tool-to-substrate

motion during bonding as indicated by the small "wiggle"

marks seen on the crack in the heel 11

10. Aluminum extruded from under the wire during bonding 11

11. Differences in surface texture of the deformed wire 12

12. A bond apparently produced by a malfunctioning machine .... 13

13. An extreme example of marks left on the wire by the

wire-feed clamps 13

14. A bond showing the clamp marks on the top (and therefore

also on the bottom instead of on the sides of the wire) ... 15

15. A probe mark on a bonding pad 15

16

.

A probe mark on a bonding pad 15

17. Examples of probe marks interfering with bonding 16

18. Rebonding on the package leads 17

19. Rebonding on the bonding pads on the circuit chip 18

20. Rebonding following wire break-off at the heel 19

iv
21. Defective package metallization 21

22. Examples of damaged metallization 22

23. Over-etching of a metallization stripe 23

24. Small metal particles which could become loose and cause electrical shorts 24

25. Improper die placement 24

26. Improper placement of wire bonds on transistor chips 25

27. Misalignment caused by poor masking 26

28. Examples of extraneous material 27
SCANNING ELECTRON MICROSCOPE EXAMINATION

OF WIRE BONDS FROM HIGH-RELIABILITY DEVICES

K. 0. Leedy

ABSTRACT

An examination with a scanning electron microscope was made of the wire bonds of over 75 high-reliability microelectronic devices. The device interconnects were ultrasonically

bonded aluminum wires. Of primary interest were the bonds

themselves; their appearance and its significance are described. Also described is the appearance of the metallization and the wire. Comments and explanations are given where

the phenomena are understood. Although the devices studied

had passed preliminary electrical tests and pre-encapsulation

visual examinations, many potential reliability problems were

identified such as weak bonds, electrical shorts and contamination.

Key Words-: Aluminum wire; high reliability; integrated circuit; metallization; scanning electron microscope; transistor;

ultrasonic bonding; wire bonding.

vi
SCANNING ELECTRON MICROSCOPE EXAMINATION

OF WIRE BONDS FROM HIGH-RELIABILITY DEVICES

K. 0. Leedy

1 . INTRODUCTION

Wire bonds in a variety of integrated circuits and transistors from

production lines dedicated to the production of high-reliability devices were examined with a scanning electron microscope (SEM) . The study was undertaken initially to observe differences in appearance of bonds produced by 1A supposedly identical ultrasonic bonding machines on three

controlled production lines. Subsequently, many devices from other highreliability production lines were examined and found to have similar

characteristics

.

A variety of the commonly used package types and many different circuit types were represented among the more than 75 devices examined. All

devices were ultrasonically bonded with 0.001- in. (25-ym) diameter, 99-

percent aluminum 1-percent silicon wire. The bonding pads were metallized with aluminum while the package leads had either aluminum or gold

metallization. The devices examined had all passed preliminary electrical tests and pre-encapsulation visual examinations but were removed

from the production lines before they were capped, sealed, or environmentally tested. Each of the devices examined would probably have functioned as intended in the final tests if allowed to continue down the

production line.

Although this examination was not a study of failed devices, many

potential reliability problems were observed. Comments and explanations

are given where the phenomena are understood; in other areas the observations are merely reported. In addition, even though the study primarily centered on wire bonding, several other device defects were observed;

these are illustrated in the last section of the report.

Of the several hundred SEM photographs made, the examples shown were

selected to be indicative of particular faults. It is not intended to suggest that the faults illustrated are unique to a particular device

tyPe » production line, or circuit configuration. In fact, most of the

faults were observed to some extent in all types of the devices investigated.

2. BACKGROUND

In order to obtain perspective of the problem and to define some

terms, let us look first at the devices and see why the wire bond is

important. In Figure 1 are shown an integrated circuit (a) and a
transistor (b) representative of the device types studied. Wire bonds

are made to connect the bonding pad on the circuit chip electrically to

the external lead: a post or finger of the package in which the chip is

held. The welded interface between the wire and the pad, post, or finger,

under the flattened areas at the ends of the wire, are the bonds. The

bond which is made first is called the first bond. The second bond then

completes the bond loop. The weld is formed by the combination of a force applied by the bonding tool on the wire and by the application of

ultrasonic vibration of the tool in the direction parallel to the wire

axis.* The welding that occurs at the wire-metallization interface can be seen in Figure 2 which shows the lift-off pattern of a partially re- moved bond which was made under laboratory conditions. A similar pattern

can be seen on both the pad and the wire. The center portion of the wiremetallization interface is not welded.

Figure 3 shows a schematic illustration of the bonding operation.

The wire is fed through a hole in the tool and passes under the foot of

the tool. The foot is the part of the tool that exerts a static force

on the wire during- the bonding process. The front edge of the foot is

rounded to prevent damage to the wire, but the back edge is sharp to facilitate the cut-off of the wire after the second bond is produced.

There are many differences in appearance and in structure between

the first and second bonds. Some of these may be seen in Figure 4 where

the first bond is shown in (a) and the second bond in (b) . The most

obvious difference between the two bonds in the figure is in the shape

of the toe or free end of the bonding wire. The second bond exhibits

evidence of the cut-off operation which occurred after it was made. The

first bond shows the tail, the unflattened portion of the free end of the

wire which extended beyond the foot during bonding. A more important

difference is in the heel of the bond, the juncture between the undeformed wire and the bonded portion. The sharp heel of the tool, which

provides for cut-off at the second bond, causes an indentation or crack,

in the heel of the first bond. This will be discussed in more detail

below. In most of the devices examined, the first bond was made on the

external lead (a post) and the second was on the pad. This accounts for

the different appearances of the metallization in Figures 4a and 4b.

3. BOND APPEARANCE

A comparison of the devices showed significant differences in bond

appearance. Bonds differ mainly in the amount of deformation of the

*A11 bonds examined in this study were formed ultrasonically. However

ultrasonic bonding is not the only wire bonding technique. The other common wire bonding technique is thermocompression bonding, which com- bines pressure and heat to form the weld. It is used primarily with

gold wire and, in some special cases, aluminum wire. In addition there are other techniques for making interconnections such as flip-chip and

beam-lead bonding which do not include the use of wires.

2
'mtk^am

Magnification: 22X b Magnification: 22X

Figure 1. Typical devices examined. An integrated circuit is shown in

(a) and a transistor in (b).

Magnification: 540X

Figure 2. The lift-off pattern of a partially removed bond made under

laboratory conditions. A similar pattern can be observed on both the wire and the pad. The center portion of the wire-metallization interface was not welded.

3 .
FOOT

FIRST BOND

Figure 3. Sketch of the ultrasonic bonding cycle. The first bond is formed, as shown in (a), by the combination of a tool force down on the wire and an ultrasonic motion parallel to the wire. A loop is then made, as shown in (b) and the second bond is made in the same manner as the first. The wire is then cut off by a pull of the wire

by the wire clamp so that the machine is ready to repeat the process.
Magnification: 550X

Magnification: 525X

Figure U. A comparison of a first (a) and second bond (b) . They differ in appearance mainly in the nature of the toe

of the bond (since the effect of the cut-off is evidenced

there on the second bond) and in the condition of the heel

of the bond.
wire. The appearance of the heel of the bond depends on the amount of

deformation. For example, Figure 4 shows bonds typical of those examined

from one manufacturer compared with those of another manufacturer shown

in Figures 5a and 5b. The fact that there is a great difference in bond

deformation results from the different combinations of force, ultrasonic

power and time that are used in bonding on these production lines . These

adjustments of the bonding machine (the bonding schedule) involve a compromise. Setting combinations that yield a small deformation result

in a small crack at the heel leaving the wire itself as strong as possible. However, if the deformation is very small, the bond is

frequently weak due to lack of welding at the interface. This is called

underbonding. In the extreme, the wire does not stick at all; it lifts

off. More welding at the interface can be attained by increasing one of

the settings in the bonding schedule. This will also result in increased

deformation so that the heel will be thinner and weaker. In extreme

cases the wire will be nearly severed at the heel. This is called over- bonding. The optimum bonding schedule would be a compromise between

underbonding and overbonding.

Typical examples of the variation in heel cracks with different deformations are shown in Figure 6 which shows two first bonds from devices

from two different production lines. The bond in Figure 6a is less deformed than the one in Figure 6b so that it has a considerably smaller

crack in the heel. It should be noted that the bonding machine which

made the bond in Figure 6a had a malfunction in its cycle (the tool was allowed to bounce on initial impact) which caused the indentation in the

wire above the heel. The indentation in the bond surface below the crack

is probably due to build-up of aluminum in a small region of the bonding

surface of the tool.

There can also exist a variation in tne amount of deformation from

bond to bond on a single device. The amount of this variation i's usually

(but not always) less than the variation described as existing from production line to production line. Two examples are shown in Figure 7. In

Figure 7a, which shows eight of the bonds on a device, there is seen a large difference in deformation particularly visible in the two bonds

marked A and B. These variations are easily observable with ordinary

optical microscopes. Figures 7b and 7c show two consecutively made first

bonds on another device. There is a significant difference in deformation

between the two bonds. These bond-to-bond deformation differences are characteristic of unwanted motion between the tool and the work stage

during bonding. This motion can be caused by many different factors

such as movement caused by the operator, building vibrations, or just the

normal motion or vibration of the bonder itself as it goes through the

bonding cycle. This kind of motion is significant because it can result

in the type of occasional bond lift-off which has not otherwise been

explained. Relative motions of 0.00025 in. (about 6 um ) or smaller

between the tool and the work stage are large enough to be significant.

In addition to deformation differences there are other aspects of the

bond appearance which give indication of unwanted motion during bonding.

Figure 8 shows a bond which suffered from gross motion of the tool as
t MBBl. A

Magnification: 500X

Magnification: 550X

Figure 5. First and second bends from another production line. These first and second bonds, shewn in (a) and (b) respectively, differ from those- of Figure 4 mainly in the amount of deformation

of the bond and in the shape and size of the crack in the heel

of the first bond.
Magnification: 490X

Magnification: 500X

Figure 6. Examples of different types of first-bond heel cracks. The type of crack depends primarily on the amount of deformation

of the wire during bonding. Small deformation yields a small

crack as shown in (a) , while a bond with greater deformation has

a larger crack as shown in (b)

.
B

^

I J

Magnification 95X

Magnification 575X Magnification 650X

Figure 7- Variation in bond deformation from bond to bond on a single device.

In (a) , there is a variation in the width of the bonds that is particularly

noticeable between those marked "A" and "B". Two consecutively made bonds

from a different device are shown in (b) and (c) . While that in (c) is underbonded, the bond in (b) is not.
evidenced by the lateral deformation. An example of a much smaller

motion is shown in Figure 9 . The wiggle marks seen in the heel are due

to motion between the bonding tool and the post as the tool lowers to form the bond.

During the bonding process, aluminum is often extruded from under

the wire to appear as little curls on either side of the bond. This is

illustrated in Figure 10. The extruded material is part of the aluminum

metallization that is pushed up as the bond is made. Extrusions usually

occur when the metallization is too soft. Variation in the amount of

this extrusion and in its relative position on either or both sides of

the bond from bond-to-bond on a device is another indication of unwanted

motion during bonding. The bonds in Figure 7a illustrate these variations. These variations can be seen with an optical microscope.

Differences in surface texture of the deformed wire were also observed. These differences are primarily due to the surface condition of

the foot of the bonding tool. A clean tool will give a smooth bond

appearance as can be seen in Figure 4a. A tool becomes rough as a result

of aluminum build-up on its bonding surface. The bond in Figure 11a is one produced with a generally rough tool while those in Figures lib and

lie were made by a tool that had only small areas of aluminum build-up

similar to that which made the bond in Figure 6a. When these surface

indentations were observed, the same pattern was generally repeated on all the bonds of a particular device. It is not known at this time if

the presence of aluminum on the tool affects the quality of the bond

produced.

Figure 12 illustrates the unusual appearance found on the bonds of one device. The tail has been flattened. Since the surface appearance

of the flattened area is similar to that of the wire-bond surface, it was probably subjected to a force by the tool. One would suspect malfunction

in the bonding cycle of this particular bonder.

4. WIRE APPEARANCE

Large indentations have been observed on the sides of the wire in some bonds. The worst example is shown in Figure 13a with the area indicated shown at higher magnification in Figure 13b. It has been determined that these marks are produced by the jaws of the clamping system

through which the wire is fed from the spool to the bonding tool. In addition to marring the surface finish, these imprints work-harden the

wire and thus affect the bonding conditions. Occasionally the wire twists

90 degrees after passing through the clamping system, and the indented

portion then becomes a part of the actual wire-metallization interface.

Figure 14 shows a bond in which this twisting has taken place. Indentations are formed on opposing surfaces of the wire by the clamp so that

marks similar to those seen on the top also appear on the underside of

the wire. Of particular concern is the possibility of contaminants on

the clamp becoming embedded in the wire and directly interfering with

the bonding process. This combination of events may cause an occasional,

weak bond which can not be detected by the usual inspection.

10
Figure 8. A bond which has suffered from

gross unwanted tool-to-substrate motion during bonding as evidenced by the laree side-to-side indentations.

Magnification: 610X

Figure 9. A bond which exhibits small un- wanted tool-to-substrate motion during bonding as indicated by the small "wiggle" marks

(indicated by the arrow) seen on the crack

in the heel. The marks on the wire above

the bond are due to impressions of the wirefeed clamp

.

Magnification: 1500X

Figure 10. Aluminum extruded from under

the wire during bonding. The extruded material appears as little curls (in- dicated by the arrows) on either side of the bond. Magnification: 300X

W-"^ \^"\ Jib

11
Magnification: 580X

11

<v ^Hij

Hkk -

; ^^|B

L' >

Magnification: 540X

Magnification: 540X

Figure 11. Differences in surface texture of the deformed wire. In (a), the bond was

made with a tool having significant aluminum build-up over its surface. Those in

(b) and (c) had small areas of build-up which produced a pattern of indentations on

the surface of the bond which was repeated for all the bonds on the device.

12
i

Magnification: 510X

Figure 12. A bond presumably produced by a malfunctioning machine. The flattened

toe of the bond is indicated by the arrow.

1

Magnification: 520X b Magnification: 2100X

Figure 13. An extreme example of marks left on the wire by the wirefeed clamps

.

The bond and wire are shown in (a) with the part of the wire indicated by the arrow shown at greater magnification in (b)

.

13
Another defect that has been observed on the surface of the wire is a scrape or scratch mark. An example of this can be seen on the top part

of the wire shown in Figure 13b. These scrapes or scratches can be due

to rubbing of the soft aluminum wire against a hard surface somewhere

along its path from the wire spool to the bond, such as guides on the

wire feedhole of the tool itself. Since there is the possibility that

damage of this type may induce stress or introduce contamination in the

wire and thus affect the bonding process, these marks may be significant.

5. METALLIZATION APPEARANCE

A mark often found on the bonding pad metallization is the indentation due to the probe used during the electrical testing of the circuit.

A typical example of such a mark is shown in Figure 15. It is possible

for the probe to completely scrape away the metallization as is shown in

Figure 16. In any case the metallization may be scraped quite thin by

the probe so that bonding should not be attempted over probe marks.

Since probes are usually aimed at the center of bonding pads, it often

becomes difficult, particularly on small pads, to find enough space for

placing the bond on an unprobed area of the pad. As a result, there are many cases of bonding over probe marks as in Figure 17a or of bonding

partially off the pad as shown in Figure 17b.

Another type of mark found on the metallization which may be similar

to a probe mark is the lift-off mark (the mark left on the metallization

when a bond does not stick, necessitating rebonding of the pad or post). 2 Examples of rebonded posts are shown in Figure 18 and examples of rebonded pads are shown in Figure 19. In Figures 19a and 19b, the marks to the

left of the bonds are probe marks and those to the right are lift-off

marks. In Figure 19a, lift-off was due to underbonding; the combination

of power, time and pressure did not yield enough welding at the interface

for the bond to stick. In fact, the successful bond which was subsequently made is also underbonded. In Figure 19b, lift-off could be due

to unwanted motion during bonding. In Figure 19c, the initial bond attempt was over a probe mark as indicated by the arrow to the left of the

lift-off mark. In this case lift-off might be due to overbonding or to poor metallization. Figure 19c illustrates the bond placement problems

which result from rebonding. In the small area available for bonding,

there is not enough room for two attempts without having one of them be

partially off the pad, over a probe mark, or over a lift-off mark.

Another type of rebonding occurs where the wire breaks at the heel

leaving the bond itself behind. It occurs for very highly deformed bonds

which was usually not the case in the devices studied. In the example

shown in Figure 20 there were at least two attempts before a successful

connection was made.

The main reason for not rebonding on high reliability devices, par-- ticularly if more than two attempts are necessary, is the likelihood that

if additional tries are needed it is indicative of a problem. It might

14
r

Figure 14. A bond showing the clamp marks

on the top (and therefore also on the bottom

instead of on the sides of the wire)

.

Magnification: 580X

Ajwii^irr

Figure 15. A probe mark on a bonding pad indicated by the arrow.

A

Magnification: 510X

1

Figure 16. A probe mark on a bonding pad. Note where the probe has completely scraped

through the metallization as indicated by

the arrow. Magnification: 560X

L

15
r 1 dfiP?: < I

Magnification: 540X

Magnification: 510X

Figure 17. Examples of probe marks interfering with bonding.

In (a) , the bon'd is made over a probe mark and in (b) , the bond was put partially off the pad to avoid bonding over the probe mark.

16
r

L

^

•-*•_ j#fW ~-- #i^ E5

\- Jib J

Magnification: 465X r

Figure 18. Rebonding on the package leads. The bond in (a) was

the second attempt while that in (b) was the fourth.

17
m

pL

i j ii «a J

Magnification: 5A0X

-*»^r

-

Magnification: A75X

* J

Magnification: 520X

Figure 19. Rebonding on the bonding pads on the circuit chip. The probe marks are indicated as (P) and the lift-off marks as (L) . In (a) , lift-off was due to under- bonding while that in (b) seems to have been caused by unwanted motion during bonding.

The bond which lifted-off in (c) was made over a probe mark.

18
be bad metallization on that particular pad or post, bad wire, excessive

unwanted motion during the bonding cycle, or even an incorrect bonding

schedule. The result is that even if there is enough room left on the

pad and a bond can be made to stick, there would be no assurance that it

will be a good bond.

Poor metallization was frequently observed on the package leads of

the devices examined. Figure 21a shows the metallization of a finger on

a flat-pack. The area in which the bond is placed is extremely rough.

There is a wide variation in texture across the finger and in a portion

the metallization has actually peeled off. While roughness in the metallization surface is not necessarily difficult to deal with in bonding,

variation in this roughness from bond to bond or package to package is a problem. Since the proper bonding parameters (power, time, and pressure)

depend strongly on the surface texture, they cannot be at optimum settings for both extremes in surface texture. Therefore as this texture

varies from package to package there will be a resultant variation in

the character of the bond.

SStfSfc. %&£?,}, \\"*,\«V*

\ *

i-/ **"«.

% . .'V A

Magnification: 600X

Figure 20. Rebonding following wire break-off at the heel.

19
The variation in surface quality of the bonding areas of the packages was greater for the transistor-type packages examined than it was

for the flat-packs and other integrated circuit packages. The integrated

circuit frames were normally coated with evaporated aluminum which tends

to be more uniform than the tumble plated gold on the transistor posts.

Figures 21b and 21c show the gold plated post metallization of two transistor packages. In Figure 21b the metallization is very rough and hard.

So hard, in fact, that instead of the bond being formed by the mutual deformation of the wire and the metallization, the wire was merely mashed

flat over the irregular metallization. This can be seen in the surface

texture of the bond which shows cracks with angles which are similar to the angular features of the metallization below. In Figure 21c there are many holes in the surface of the metallization. This was also true for

the other post of this package and, to a lesser degree, for several other

transistors examined. Since these holes affect the texture of the metallization, they would seem to be detrimental to wire bonding. Also, since

they exist to some extent on the rest of the package surfaces, such as under the die, they might also present a problem during die bonding because the holes could entrap gas as the bond was being made.

6. OTHER OBSERVATIONS

The primary purpose of this investigation was to study the wire

bonds of the devices examined. However, occasional examples of other

possible reliability problems were observed. Some of these have an indirect bearing on wire bonding. The metallization faults encountered

were primarily scraping or scratching of metallization stripes. The area marked with an "S" in Figure 22a exhibits evidence of scraping and

that which is marked "E" shows holes in the metallization where material

was etched away. The dielectrically isolated circuit shown in Figure 22b

illustrates the difficulty in running a metallization stripe over a raised area. The damage to the metallization may have occurred when the

wafer was broken into individual circuit dice since it is then that the

die surface often comes into contact with a hard surface. Figure 23a

shows another example of a metallization stripe 0.001 in (25 um) where

some of the material has been etched away, but here the stripe is almost

etched entirely away at one point. It is shown in a higher magnified

view in Figure 23b. Also illustrated in Figure 23 is the thinning in

the metallization placed over a step in the oxide. The obvious problem

with any of such imperfections in metallization interconnections is the

possibility of electrical open-circuits. Another metallization defect,

shown in Figure 24, is the presence of small metal pieces which could

become loose after packaging and could possibly cause an electrical short

between two metallization stripes.

Improper placement of the die during die bonding can cause difficulties when the device is wire bonded. Figure 25 shows a case where the

wires are unnecessarily long and where there exists the possibility of

the wires touching one another. Bad placement of the wire bonds on a transistor could also cause an electrical short. Figure 26a shows a

20
Magnification: 60X

ft \ I •

Magnification: 660X

Magnification: 555X

Figure 21. Defective package metallization. In (a) the metallization is actually peeling

off of the finger of a flat-pack. In addition there is a great variation in the texture

of the metallization across the lead. The examples in (b) and (c) are posts of transistortype packages. The metallization in (b) is very hard and irregular as indicated by the impressions extending through the bond. The metallization in (c) is full of holes.

21
r E* 1

A

Magnification: 210X f : "1

P)W

Magnification: 540X

Figure 22, Examples of damaged metallization. In (a) some metallization seems to have been scraped away (S) while some other seems to have been etched away (E) . In (b) the metallization over the humps

in a dielectrically isolated device has been damaged.

22
Magnification: 550X

Magnification: 1100X

Figure 23. Over-etching of a metallization stripe. A portion of

a circuit is shown in (a) with the region indicated by the arrow

shown at greater magnification in (b)

.

23
J*- .

Magnification: 560X

Figure 24. Small metal particles observed on the metallization.

Magnification: 20X

Figure 25. Improper die placement resulting in wire bonding problems.

24
Magnification: 60X •X

\/

v

Magnification: 660X

Magnification: 555X

Figure 26. Improper placement of wire bonds on transistor chips. The bond indicated

by the arrow in (a) is shown at greater magnification in (b) . It is possible that the placement of this bond could cause an electrical short circuit. Another example of bad placement is shown in (c)

.

25
transistor; and in Figure 26b the emitter wire bond of the transistor is

shown at greater magnification. Because of the way the bond is placed,

the wire crosses very close to the base metallization. Another example

of this is shown in Figure 26c.

An unusual situation observed on one device, shown in Figure 27, was

caused by poor masking. This is shown by the fact that while the alignment was correct in finger A of the transistor, quite near, at finger B,

the alignment was off by nearly one- third of the width of the finger.

The bonding pad in this example is very small, illustrating another type

of difficult bond placement that is often encountered.

L

Magnification: 515X

Figure 27. Misalignment caused by poor mask. Finger "A" is properly

aligned but "B" is not. Note how the small bonding pad makes it difficult to find a suitable location for the bond.

Extraneous material was observed at the surface of some of the

metallization, on a few of the bonds, and on one piece of wire. Figure

28a shows such material on the metallization of one of the bonding pads

.

In Figure 28b , there is a bond with material on its surface that we have

not identified. At the top of the picture, what appear to be corrosion

products cover portions of the wire. This is shown at greater magnification in Figure 28c. No identification was made of this extraneous

material.

26
—^^s*^r*

Magnification: 510X

1

A

Magnification: 530X Magnification: 1075X

Figure 28. Examples of extraneous material. The arrow in (a) indicates unidentified material seen on a bonding pad. In (b) the bonded area of a wire is covered with some foreign material. The material on the undeformed wire indicated hy the arrow is shown

in (c) at higher magnification.

27
7 . SUMMARY

The results of an SEM examination of a large number of microelectronic devices, intended for use in systems demanding high reliability,

have been described. The appearance of the bonds themselves has been

discussed, including differences between first and second bonds, cracks

in the heel of the first bond, variation in bond deformation, indications

of unwanted tool-to-substrate motion and differences in bond surface

texture. The surface condition of the wire above the bond has been

noted. Also included are descriptions of the appearance of the metallization (package and die) on which the bond is made. This includes

probe marks, lift-off marks, other indications of rebonding, as well as poor or damaged metallization. Miscellaneous observations were presented which covered improper die placement, improper bond placement,

and the existance of extraneous material.

Comments and explanations have been given where phenomena are understood. It is not to be construed that all, or indeed any, of the observations noted would cause a device to fail. It is merely noted that a careful examination of a device can reveal many areas of potential difficulty. In addition, even though this examination was made with an SEM

since it can be used to observe details not seen with an optical microscope, many of the defects could have been observed with an optical

microscope.

8. REFERENCES

1. Leedy, K. 0., and Harman, G. G., in Methods of Measurement for Semiconductor Materials, Process Control, and Devices, Quarterly Report,

January 1 to March 31, 1970, Bullis , W. M. , ed., NBS Technical Note 555,

p. 29 (September 1970).

2. Leedy, K. 0., in Methods of Measurement for Semiconductor Materials,

Process Control, and Devices, Quarterly Report, October 1 to December 31,

1969, Bullis, W. M. , ed., NBS Technical Note 527, p. 43 (May 1970).

U. S. GOVERNMENT PRINTING OFFICE : 1973--542-647/10

28
F0RMNBS-114A (1-71) 2. Gov't Accession No.

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN-785

3. Recipient's Accession No.

4. TITLE AND SUBTITLE Scanning Electron Microscope Examination of Wire Bonds from

High-Reliability Devices.

5. Publication Date August 1973

6. Performing Organization Code

7. AUTHOR(S)

K. 0. Leedy

8. Performing Organization

10. Project/Task/Work Unit No. 4259533

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

WASHINGTON, D.C. 20234

11. Contract/Grant No. P0-0-0055

P0-0-0Q36

12. Sponsoring Organization Name and Address Navy Strategic Systems Project Office, NAD, Crane, Indiana

47 522

13. Type of Report & Period Covered Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

14. ABSTRACT (A 200-word or less factual summaty of most significant information. If document includes a significant bibliography or literature survey, mention it here.) An examination with a scanning electron microscope was made of the wire honds

of over 75 high-reliability microelectronic devices. The device interconnects were ultrasonically bonded aluminum wires. Of primary interest were the bonds themselves;

their appearance and its significance are described. Also described is the appearance of the metallization and the wire. Comments and explanations are given where

the phenomena are understood. Although the devices studied had passed preliminary

electrical tests and pre-encapsulation visual examinations, many potential reliability

problems were identified such as weak bonds, electrical shorts and contamination.

'7. KEY WORDS (Alphabetical order, separated by semicolons) Aluminum wire; high reliability; integrated circuit; metallization; scanning electron

microscope; transistor; ultrasonic bonding; wire bonding.

18. AVAILABILITY STATEMENT

|~%! UNLIMITED.

I I FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

35

22. Price 55

USGOMM-OC 66244-P71

NBS TECHNICAL PUBLICATIONS

PERIODICALS JOURNAL OF RESEARCH reports National Bureau of Standards research and development in

physics, mathematics, and chemistry. Comprehensive

scientific papers give complete details of the work,

including laboratory data, experimental procedures, and theoretical and mathematical analyses. Illustrated with photographs, drawings, and charts. Includes

listings of other NBS papers as issued.

j Published in two sections, available separately:

• Physics and Chemistry (Section A)

Papers of interest primarily to scientists working in

I these fields. This section covers a broad -range of

j physical and chemical research, with major emphasis on standards of physical measurement, fundamental

constants, and properties of matter. Issued six times a year. Annual subscription: Domestic, $17.00; Foreign, $21.25.

• Mathematical Sciences (Section B)

• Studies and compilations designed mainly for the

'. mathematician and theoretical physicist. Topics in

j mathematical statistics, theory of experiment design,

j numerical analysis, theoretical physics and chemistry,

logical design and programming of computers and

computer systems. Short numerical tables. Issued quarterly. Annual subscription: Domestic, $9.00; Foreign,

$11.25.TECHNICAL NEWS BULLETIN

The best single source of information concerning the Bureau's measurement, research, developmental, cooperative, and publication activities, this monthly

publication is designed for the industry-oriented

individual whose daily work involves intimate contact with science and technology

for engineers, chemists,

physicists, research managers, product-development

managers, and company executives. Includes listing of

all NBS papers as issued. Annual subscription: Domestic, $6.50; Foreign, $8.25.

NONPERIODICALS Applied Mathematics Series. Mathematical tables, manuals, and studies. Building Science Series. Research results, test methods, and performance criteria of building materials, components, systems, and structures. Handbooks. Recommended codes of engineering

and industrial practice (including safety codes) de- veloped in cooperation with interested industries, professional organizations, and regulatory bodies. Special Publications. Proceedings of NBS conferences, bibliographies, annual reports, wall charts, pamphlets, etc. Monographs. Major contributions to the technical

literature on various subjects related to the Bureau's

scientific and technical activities. National Standard Reference Data Series. NSRDS provides quantitative data on the physical

and chemical properties of materials, compiled from

the world's literature and critically evaluated. Product Standards. Provide requirements for sizes, types, quality, and methods for testing various industrial products. , These standards are developed co- operatively with interested Government and industry groups and provide the basis for common understanding of product characteristics for both buyers and

sellers. Their use is voluntary.

Technical Notes. This series consists of communications and reports (covering both other-agency and

NBS-sponsored work) of limited or transitory interest. Federal Information Processing Standards

Publications. This series is the official publication within the Federal Government for information on

standards adopted and promulgated under the Public Law 89-306, and Bureau of the Budget Circular A—86

entitled, Standardization of Data Elements and Codes

in Data Systems. Consumer Information Series. Practical information, based on NBS research and experience, cover- ing areas of interest to the consumer. Easily understandable language and illustrations provide useful background knowledge for shopping in today's tech- nological marketplace.

BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey bibliographies are issued periodically by the Bureau

:

Cryogenic Data Center Current Awareness Service (Publications and Reports of Interest in Cryogenics). A literature survey issued weekly. Annual subscription: Domestic, $20.00; foreign, $25.00.

Liquefied Natural Gas. A literature survey issued quarterly. Annual subscription: $20.00. Superconducting Devices and Materials. A literature survey issued quarterly. Annual subscription : $20.00. Send subscription orders and remittances for the preceding bibliographic services to the U.S. Department

of Commerce, National Technical Information Service, Springfield, Va. 22151. Electromagnetic Metrology Current Awareness Service (Abstracts of Selected Articles on Measurement

Techniques and Standards of Electromagnetic Quantities from D-C to Millimeter-Wave Frequencies). Issued monthly. Annual subscription: $100.00 (Special rates for multi-subscriptions). Send subscription order and

remittance to the Electromagnetic Metrology Information Center, Electromagnetics Division, National Bureau

of Standards, Boulder, Colo. 80302. Order NBS publications (except Bibliographic Subscription Services) from : Superintendent of Documents, Government Printing Office. Wash-
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington. O.C. 20234

OFFICIAL BUSINESS

Penalty for Private Use. S300

POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE COM—215