Introduction

SURFACE AND INTERFACE ANALYSIS TECHNIQUES have been among the paramount characterization methods since the beginning of modern, scientifically based thin film and coatings technology in the seventies, and their use very often has led to substantial progress in new fabrication techniques and devices (Ref 1, 2). This is obvious because the structure and composition of surfaces and interfaces is decisive for many properties of thin films, such as chemical reactivity, friction and wear, film adhesion, and electronic and diffusional properties. Of course, any of the large variety of physical characterization methods is more or less surface sensitive, but this article refers mainly to the "classical" methods of surface chemical analysis, namely electron spectroscopies (Ref 3) and ion spectroscopies (Ref 4), of which the most important techniques are Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS or ESCA, electron spectroscopy for chemical analysis), and secondary ion (and neutral) mass spectroscopy (SIMS and SNMS) (Ref 5, 6). Glow discharge optical emission spectroscopy (GDOES) or glow discharge mass spectroscopy (GDMS), ion scattering spectroscopy (ISS), and Rutherford backscattering spectroscopy (RBS) are becoming of increasing importance, as is total reflection x-ray fluorescence spectroscopy (TRXF) (Ref 7) in special applications. All these techniques were emerging in the late 1960s and early 1970s (Ref 5) and have been developed since then into often sophisticated instruments for local microanalysis down to the nanometer scale (Ref 3, 4, 6). A survey of the most important surface analysis techniques and their relative frequency of application is shown in Fig. 1.

Fig. 1 Survey of the most important surface analysis techniques. AES (SAM), Auger electron spectroscopy (scanning Auger microscopy). XPS (ESCA), x-ray photoelectron spectroscopy (electron spectroscopy for chemical analysis). SIMS (SNMS), secondary ion mass spectroscopy (secondary neutral mass spectroscopy). GDOES (GDMS), glow discharge optical emission spectroscopy (glow discharge mass spectroscopy). RBS (ISS), Rutherford backscattering spectroscopy (ion scattering spectroscopy). Source: Ref 3

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Fig. 1 Survey of the most important surface analysis techniques. AES (SAM), Auger electron spectroscopy (scanning Auger microscopy). XPS (ESCA), x-ray photoelectron spectroscopy (electron spectroscopy for chemical analysis). SIMS (SNMS), secondary ion mass spectroscopy (secondary neutral mass spectroscopy). GDOES (GDMS), glow discharge optical emission spectroscopy (glow discharge mass spectroscopy). RBS (ISS), Rutherford backscattering spectroscopy (ion scattering spectroscopy). Source: Ref 3

Coatings and thin films can be studied with surface analysis methods because their inherently small depth allows characterization of surface composition, interface composition, and the in-depth distribution of composition. This provides an indispensable method for the control of fabrication parameters as well as for the study of property/composition relationships. The full potential of surface analysis is further enhanced if the methods are applied in combination with other characterization methods including phase and structural identification (x-ray diffraction, transmission electron microscopy, and scanning tunneling microscopy) and stereochemistry (infrared and Raman spectroscopy). Recent trends are the development of increased spatial resolution (e.g., toward the 10 nm region and below in AES and SIMS) and improved databases and evaluation software for quantitative analysis.

With the exception of RBS, any of the above-mentioned surface analysis methods involves the use of an ion sputtering facility, which allows destructive depth profiling. Therefore, these methods can also be used for chemical analysis of the "bulk" of thin films and for the characterization of interfaces. Nondestructive depth profiling can be performed with RBS up to a few micrometers in depth, and with angle-resolved XPS up to about 5 nm thickness. Another method of interface analysis is based on brittle fracture or cleavage of the sample along internal interfaces, which exposes two surfaces to be studied by surface analysis. Following are the most important phenomena and structures that can be studied by the above-mentioned methods, the majority of which are relevant for coatings and thin films:

Surfaces, studied directly

• Segregation

• Contamination

• Adsorption

• Friction and wear

Interfaces, studied by fracture and/or profiling

• Segregation

• Embrittlement

• Intercrystalline corrosion

• Composites

Thin films, studied by depth profiling

• Interdiffusion

• Ion implantation

• Reaction layers (oxides, passivation layers, etc.)

• Evaporation layers

• Protective coatings

• Microelectronic devices

From the more than 50 different existing techniques in surface analysis (Ref 6), this article only considers the main methods that have broad applicability and for which commercially available instruments exist (AES, XPS, SIMS, SNMS, GDOES, ISS, and RBS). They are characterized by the ability to provide quantitative analysis of all elements (except hydrogen and helium in AES and XPS) and intrinsic information depth in the nanometer range or below.

The latter point is the important difference to (spatially resolved) bulk analysis methods such as x-ray analysis in the electron microprobe. It should be mentioned that high-resolution, analytical scanning transmission electron microscopy (STEM) can also be used for surface and interface analysis (see the article "Microstructure/Characterization of Coatings and Thin Films" in this Volume).

A survey of the lateral and in-depth dimensions of the analyzed volume of typical microanalytical techniques is given in Fig. 2. For surface analyzing techniques with depth resolution below 5 nm, the depth of information is an intrinsic, physical parameter determined by the mean escape depth of photoelectrons or Auger electrons or that of the sputtered particles (SIMS, SNMS). In contrast, the lateral resolution depends mainly on instrumental capabilities, such as primary beam diameter or optical imaging facility.

Fig. 2 Depth of information (depth resolution) and lateral resolution of surface and microanalysis techniques. AES, Auger electron spectroscopy. EPMA, electron probe microanalysis. ESCA, electron spectroscopy for chemical analysis. FIM-AP, field ion microscopy - atom probe. ISS, ion scattering spectroscopy. SAM, scanning Auger microscopy. SIMS, secondary ion mass spectroscopy. TEELS, transmission electron energy loss spectroscopy. TEM, transmission electron microscopy.

Fig. 2 Depth of information (depth resolution) and lateral resolution of surface and microanalysis techniques. AES, Auger electron spectroscopy. EPMA, electron probe microanalysis. ESCA, electron spectroscopy for chemical analysis. FIM-AP, field ion microscopy - atom probe. ISS, ion scattering spectroscopy. SAM, scanning Auger microscopy. SIMS, secondary ion mass spectroscopy. TEELS, transmission electron energy loss spectroscopy. TEM, transmission electron microscopy.

The principal components of an analytical surface analysis instrument are schematically shown in Fig. 3. They are: an excitation source (electrons, ions, or x-rays), an auxiliary ion gun for depth profiling, and a spectrometer (energy or mass) with particle detector, data acquisition, and processing facility. Together with the sample, the main components are in an ultrahigh vacuum chamber (UHV, < 10-9 mbar). Therefore they can often be used for process control, as in modern molecular beam epitaxy or physical and chemical vapor deposition methods (Ref 7, 8) which work under UHV base pressure conditions (Ref 7). The principles of the main analysis methods are outlined in the following sections of this article. (For a quick survey and comparison of their most important features, see Table 1.)

Table 1 Comparison of surface and thin-film analysis techniques

Parameter

Technique

AES

XPS

ISS

RBS

SIMS

GDOES

Excitation

Electrons

X-rays

Ions

Ions

Ions

Ions

Emission

Electrons

Electrons

Ions (EE)

Ions (E)

Ions (m/e)

hv (optical)

Typical depth of information, nm

1

1

0.3

1-1000

0.6

10

Lateral resolution

15 nm -30 pm

5 pm -10 mm

1 mm

1 mm

50 nm -10 mm

10 mm

Detection limit

0.1 at.%

0.1 at.%

0.1 at.%

0.1 at.%

1 ppb -10 ppm

1 ppm

Detection of:

Elements

All except H, He

All except H, He

All except H

H only with ERD

All

All

Isotopes

No

No

Yes

Yes

Yes

No

Chemical state

Yes

Yes

No

No

Yes

No

Imaging/mapping

Yes

Limited

No

No

Yes

No

Depth profile:

Nondestructive

No

Yes (<5 nm)

No

Yes

No

No

Destructive

+ Sputtering

+ Sputtering

+ Sputtering

No

Yes

Yes

Main usage

Element surface and thin-film analysis

Chemical state analysis

Topmost layer analysis

Nondestructive thin-film analysis

Dopant profiles

Rapid thin-film analysis

AES, Auger electron spectroscopy; ERD, elastic recoil detection; GDOES, glow discharge optical emission spectroscopy; ISS, ion scattering spectroscopy; RBS, Rutherford backscattering spectroscopy; SIMS, secondary ion mass spectroscopy; XPS, x-ray photoelectron spectroscopy

Fig. 3 Principal components of a surface analysis instrument. AES, Auger electron spectroscopy. SIMS, secondary ion mass spectroscopy. UHV, ultrahigh vacuum chamber. XPS, x-ray photoelectron spectroscopy. Source: Ref 3
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