Optical Diagnostics for Thin Film Processing

Irving P. Herman

• Verlag: Academic Press
• Jahr: 1996
• ISBN:
  9780080538082
• Thema: Technology & Engineering
• Thema: Materials Science

This volume describes the increasing role of in situ optical diagnostics in thin film processing for applications ranging from fundamental science studies to process development to control during manufacturing. The key advantage of optical diagnostics in these applications is that they are usually noninvasive and nonintrusive. Optical probes of the surface, film, wafer, and gas above the wafer are described for many processes, including plasma etching, MBE, MOCVD, and rapid thermal processing. For each optical technique, the underlying principles are presented, modes of experimental implementation are described, and applications of the diagnostic in thin film processing are analyzed, with examples drawn from microelectronics and optoelectronics. Special attention is paid to real-time probing of the surface, to the noninvasive measurement of temperature, and to the use of optical probes for process control.

Optical Diagnostics for Thin Film Processing is unique. No other volume explores the real-time application of optical techniques in all modes of thin film processing. The text can be used by students and those new to the topic as an introduction and review of the subject. It also serves as a comprehensive resource for engineers, technicians, researchers, and scientists already working in the field.

• The only volume that comprehensively explores in situ, real-time, optical probes for all types of thin film processing
• Useful as an introduction to the subject or as a resource handbook
• Covers a wide range of thin film processes including plasma etching, MBE, MOCVD, and rapid thermal processing
• Examples emphasize applications in microelectronics and optoelectronics
• Introductory chapter serves as a guide to all optical diagnostics and their applications
• Each chapter presents the underlying principles, experimental implementation, and applications for a specific optical diagnostic

CHAPTER 1

Overview of Optical Diagnostics

The thin film industry, which has grown rapidly in recent decades, now affects many diverse areas of manufacturing. Thin film processing is essential to the success of the massive semiconductor and microelectronics industry and to the growth of the emerging optoelectronics industry. These techniques are widely employed for fabricating coatings to produce optical elements, to improve machine tools, and to form protective and anticorrosion layers on many components. Along with this growth has come the development of improved and increasingly sophisticated processes that require a deeper fundamental understanding of the science behind the thin film process. This increased sophistication of processing methods and the ever-increasing complexity of new materials and devices have placed new demands on process engineers, thin film equipment vendors, and product manufacturers. These demands have placed a premium on developing and employing diagnostics to understand these methods in the laboratory and to monitor and control them in the fabrication line. In particular, optical diagnostics have been increasingly used in these applications because of their versatility and power. This book addresses these issues by collecting into one volume descriptions of optical techniques for the in situ analysis, monitoring, and controlling of thin film processes.

The subject of thin film processing is huge, due in part to the plethora of techniques that are commonly used. Films can be deposited, etched, patterned, doped, oxidized, and annealed. Physical and chemical phenomena involving gases, liquids, surface layers, solids, plasmas, and photons can be significant. Many interactions affect the satisfactory progress of the film process. Complex chemistry can occur in the gas and on the surface, and fluid flow can greatly affect performance, as can the appearance of contaminants and undesirable complications, such as the formation of particles. Moreover, each of these processes must be developed and optimized for each material used in manufacturing. These techniques can be quite complicated whether they are being developed for processing simple elements, such as silicon and diamond, or more complex materials, such as the binary semiconductor gallium arsenide and high-temperature superconductors. Given the stringent fabrication specifications regarding film thickness, composition, purity, and crystallinity, the development and application of optical diagnostics for process elucidation and control are clearly essential.

This chapter explores how optical diagnostics can be used to improve thin film processing science and technology. It also serves as a master guide to the rest of the book, interrelating optical methods and their applications as diagnostics. Sections 1.1–1.3 briefly discuss the attributes and characteristics of diagnostics. Section 1.4 details why in situ diagnostics can be essential in thin film processing procedures ranging from investigations of fundamental science to process development and real-time control during manufacturing. In essence, it motivates this entire book. Sections 1.5 and 1.6 respectively survey optical and nonoptical probes. Section 1.7 presents commonly used thin film processing methods and analyzes their diagnostic needs. The tables in this chapter (Tables 1.1–1.10) interrelate diagnostics techniques and their applications, citing as well the sections in which each diagnostic is described.

Table 1.1

Useful in Situ Sensors for Process Control in Representative Integrated Circuit Fabrication Steps^*

Metal CVD	Wafer temperature (real-time)
	Sheet resistance (endpoint)
	Metal film thickness, surface roughness (post-processing)
Dielectric CVD	Wafer temperature (real-time)
	Film thickness (endpoint)
	Particles (in gas, on wafer; real-time, post-processing)
	Refractive index, film stress (post-processing)
Silicon CVD/epitaxy	Wafer temperature (real-time)
	Film thickness (endpoint)
	Particles (in gas, on wafer; real-time, post-processing)
	Grain size (post-processing) [polycrystalline]
	Sheet resistance, surface reflectance (post-processing)
Plasma etch (anisotropic/isotropic, ashing)	Plasma power, plasma density (real-time)
	Particles (in gas, on wafer; real-time, post-processing)
	Film thickness, plasma emission (endpoint)
	Critical dimension (real-time, post-processing) [anisotropic etch]
	Anisotropy, side-wall angle, overetch (real-time, post-processing)
	Selectivity (post-processing)
Thermal oxidation/nitridation	Wafer temperature (real-time)
	Film thickness, slips, thickness uniformity (post-processing)
Implant activation and drive-in	Wafer temperature, dopant dose and energy (real-time)
	Electrical activation (endpoint)
	Sheet resistance, slips (post-processing)
Resist processing	Resist thickness, uniformity (post-coating, pre-exposure)
	Latent image, image overlay (during exposure)
	Surface condition, plasma emission (dry-develop endpoint)
	Critical dimension (post-processing)
Glass reflow	Wafer temperature (real-time)
	Surface topography (endpoint)
	Film thickness, stress (pre- and post-processing)

^*Adapted from Moslehi et al. (1992), © 1992 IEEE, and Barna et al. (1994).

Table 1.2

Methods for Endpoint Detection

Method	Measuring	Monitoring
Optical
Optical emission spectroscopy^a^b (Section 6.3.1.2)	Spectrum of light emitted from discharge and its intensity	Emission from reactive species and/or etch products
Absorption^b(Chapter 8; Section 8.3 for films)	Transmitted light	Change in concentrations in different molecules.
Optical reflection (Sections 9.4, 9.5, 9.11)	Reflection changes, interference effects,^b^c or ellipsometry parameter monitoring	Changes in film thickness
Scatterometry^b (Section 11.2)	Diffracted light	The profile of the surface or of the refractive index distribution.
Pyrometry (thermal imaging) (Section 13.2)	Infrared emission	Changes in emissivity
Photoemission optogalvanic spectroscopy^a (Section 17.3)	Plasma current	Changes in work function due to changed surface
Nonoptical
Mass spectrometry	Gas composition	Etch products
Impedance monitoring^a	Impedance mismatch	Voltage change
Langmuir probe^a	Changes in electron density or average energy	Current from probe
Pressure	Total pressure	Changes in total pressure

^aUseful only during plasma etching (Marcoux and Foo, 1981).

^bUseful for evaluating resist processing.

^cInterference effects in OES (Section 6.3.1.3), Raman scattering (Section 12.4.2), and pyrometry (Section 13.4) can also be used for endpoint detection.

Table 1.3

Optical Diagnostics of Surfaces

^aAlso known as Brewster-angle reflection spectroscopy (BARS).

Table 1.4

Probes for Gas-Phase Species Identification and Density Measurements^*

Method	Section
OES^a	6.3
LIF	7.2
Infrared diode laser absorption spectroscopy (IR-DLAS)	8.2.1.1
Fourier transform infrared spectroscopy (FTIRS)	8.2.1.2
Ultraviolet/visible absorption	8.2.2
Interference holography	10.1
Raman scattering	12.3
CARS	16.1
Third harmonic generation (THG)	16.3
REMPI	17.1

^*See Table 1.5 for pr...