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CHAPTER 15

Non-Elemental Characterization of Films

and Coatings

Donald M. Mattox

15.1 Introduction 716

15.2 Characterization 718

15.3 Film Formation 723

15.4 Elemental and Structural Analysis 727

15.5 Some Property Measurements 728

15.5.1 Adhesion 728

15.5.2 Film Thickness 734

15.5.3 Film Stress 736

15.5.4 Coefﬁcient of Thermal Expansion 738

15.5.5 Mechanical Properties 739

15.5.6 Electrical Resistivity 740

15.5.7 Temperature Coefﬁcient of Resistivity (TCR) 740

15.5.8 Electromigration 741

15.5.9 Density 741

15.5.10 Porosity 742

15.5.11 Chemical Etch Rate (Dissolution) 745

15.6 Summary 745

15.1 Introduction

A coating may be deﬁned as a near-surface region having properties differing from the bulk of

the material which is prepared by adding a material to the surface (overlay coating).

A modiﬁed surface is a near-surface region whose properties differ from the bulk of the

material and which is formed from the bulk material by changing the composition, phase, or

properties; the substrate material is detectable in this region. Generally a modiﬁed surface is

also referred to as a coating.

These deﬁnitions imply no thickness limitation but usually involve a functional or property

difference between the coating and substrate. Thus a coating allows the dissociation of the

716

Non-Elemental Characterization of Films and Coatings 717

surface properties from the bulk properties and allows engineering, fabrication, and design

ﬂexibility which can be obtained by separating the surface properties from the structural

requirements.

Disadvantages of coatings are associated with:

1. Presence of an interface and the need for adhesion;

2. A sharp discontinuity in material properties at the interface;

3. Need for fabrication methods, some of which are expensive;

4. Need for process control for a reproducible product;

5. Properties of the coating material may differ signiﬁcantly from the material in bulk form

and the properties may be very process dependent.

Films are thin coatings, and in some instances the ﬁlm properties are inﬂuenced by the

substrate properties. In this chapter, a ﬁlm is deﬁned as a coating with a thickness less than

1 micron (10

nanometers or 40 micro-inches).

Films and coatings may be fabricated in a variety of compositional, morphological, and

microstructural conﬁgurations. These include:

1. Monolithic – one composition throughout;

2. Alloyed or mixed and not reacted;

3. Compound;

4. Graded composition;

5. Layered structures – few to many, alternating;

6. Composite (dispersed phases);

7. Dispersed impurities – possibly to greater than solubility limits;

8. Special conﬁguration, e.g. ﬁne line metallization;

9. On surfaces with properties that inﬂuence the ﬁlm properties, e.g. roughness, hardness.

Films, coatings, and modiﬁed surfaces are often unique materials with properties that differ

from those normally encountered in the same materials prepared in other ways, and these

unique characteristics should be considered when making property, stability/degradation or

compositional measurements. In many instances, these unique properties are derived from the

fabrication techniques and parameters as well as the limited size and thicknesses that are

encountered in ﬁlm structures. Unique conditions, characteristics, and properties of ﬁlms and

coatings include:

718 Chapter 15

1. Substrate inﬂuence on properties;

2. Presence of the interface and interfacial (interphase) material;

3. Graded composition and properties with thickness;

4. Dispersed impurities;

5. Non-stoichiometric compositions;

6. Unique microstructures (bulk, surface), e.g. columnar morphology;

7. High surface/volume ratio;

8. Local property variations, e.g. pinholes, nodules;

9. Non-equilibrium conditions (defects, stress, crystallographic phase, structures,

composition, impurities, etc.).

15.2 Characterization

There are many reasons to characterize a ﬁlm or coating. These include:

1. In development: determining the effect of processing variables on properties of the

material (process sensitivity). Determining degradation modes;

2. Determining functionality and establishing performance limits for a speciﬁc

application;

3. Establishing product acceptance speciﬁcations (functionality, stability);

4. Establishing a baseline for satisfactory composition, structure, or performance so that

subsequent materials may be compared to this ‘standard’;

5. Monitoring reproducibility of processing;

6. Determining the stability of the material under service and degradation conditions;

7. Assisting in failure analysis;

8. Avoiding surprises.

Note: characterization is essentially meaningless unless the formation conditions are

reproducible. This means that the process must be reproducible and this is generally ensured

by using process controls and speciﬁcations.

In many cases, property measurements are used to establish processing reproducibility. For

instance, in the deposition of a metallization ﬁlm, one might make:

Non-Elemental Characterization of Films and Coatings 719

1. A thickness measurement to insure that the right amount of material has been deposited

and that the deposition conditions (contamination in a plasma when sputtering, for

instance) have not changed when using the speciﬁed deposition parameters;

2. An adhesion measurement to insure that the surface preparation was adequate and that the

surface was not recontaminated during processing;

3. An electrical resistivity (or resistance) measurement to insure functionality of the material;

4. Environmental aging to insure stability of adhesion and electrical resistivity during

subsequent processing, storage, and service;

5. Pinhole density measurements to ensure that the likelihood of developing ‘opens’ in

patterned metallization is small.

Characterization may be categorized as: (i) absolute; (ii) relative; (iii) functional;

(iv) behavioral; and (v) stability.

Absolute characterization means obtaining a speciﬁc value such as: (i) speciﬁc elemental

composition (weight percent); (ii) resistivity (ohm-centimeters); (iii) geometrical thickness

(microns, angstroms); (iv) density (grams/cm

), etc. In order to get absolute values it is often

necessary to use accurate measuring techniques and to compare the measured values to

standards for the parameter of interest.

Relative characterization means a comparison to an acceptable value (or known variation

thereto) such as: an Auger peak height, x-ray ﬂuorescence intensity, color, relative hardness,

etc. Often precise, but not necessarily accurate, measurement techniques are used. Relative

evaluations are generally more easily obtained and are less costly than are absolute values.

Functional characterization relates to the ﬁnal use of the material and include such properties

as: adhesion, electrical resistivity, hardness, wear behavior, optical absorption, etc.

Behavioral properties are not directly related to functionality but are a function of processing.

These properties may be important in use or to indicate possible changes in ﬁlm properties. An

example is adsorption of gases or contaminants.

Stability properties refer to the property changes in the product during subsequent processing,

handling/storage, and service. Stability measurements are usually done as a function of

environment (temperature, chemical species, fatigue, etc.). These environments must be

carefully deﬁned and speciﬁed.

Properties may be general, such as ﬁlm thickness, or may vary locally such as the presence of

pinholes, nodules in the ﬁlm, or small areas of high ﬁlm stress. The general properties may not

be uniform over a large surface area or may not be constant from one area to another on the

deposition ﬁxtures (position equivalency). Often variations may be due to substrate conditions,

720 Chapter 15

deposition parameters, etc. This means that some care must be taken in selecting the samples

(or areas) to be characterized and the sampling statistics must take into consideration the

possibility of such variations.

The importance of the property also determines the type of statistics used in property

measurements. For example, one may measure the mean-time-to-failure of a conductor due to

electromigration, but since one failure can cause failure of a circuit, it may be more important

to know the time-to-ﬁrst-failure for reliability calculations. It is often helpful (or necessary) to

interact with a statistician in order to develop a meaningful statistical evaluation program.

In some cases, special substrates (witness plates or monitor plates) may be used to give

properties or conditions that are not generally available on the product to be used. Examples

are: (i) the use of thin substrates that can be deformed by ﬁlm stress, and (ii) smooth surfaces

that may be masked to give ‘steps’ for stylus or interferometric thickness measurements. In

some measurements such as those used for adhesion tests or stress measurements, it is very

important that the witness plates be of the same material as the substrates and processed in the

same manner. In cases where different materials, surface conditions (e.g. smooth vs. rough) or

different processing (e.g. cleaning) is used for the witness plates, the effects of these

differences on the measured parameters must be known.

Some ﬁlm properties may be measured during the deposition process (in situ) and may be used

to control the deposition process. This may be called in situ characterization and includes such

measurements as:

1. Mass deposited (using deposition rate monitors, weight gain measurements);

2. Optical transmission, reﬂectance, and extinction (used with optical coating processes);

3. Film resistivity (using masks and conductor patterns).

Upon opening a deposition system, some characteristics may be determined before the parts

are removed from their ﬁxtures. These characteristics may be called the ﬁrst check

characterization and include:

1. Uniformity of appearance and color over the deposition ﬁxture, i.e. from

sample-to-sample or over a large area;

2. Color (e.g. TiN [1]) and reﬂectivity – is it like other deposition runs?

3. Optical texturing – when viewed from different angles does the reﬂectance look different

from different areas? This is an indication of morphological variation.

If there are a number of samples in the run, or if the area is large, one should determine if all

the positions in the deposition system are equivalent (i.e. position equivalency). It may be

helpful to identify each sample and its position in the ﬁxture for future reference – variations