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4.8
Coating Processes
K.-D. Bouzakis, N. Vidakis, Aristoteles University of Thessaloniki, Thessaloniki, Greece
G. Erkens, CemeCon GmbH, Würselen, Germany
4.8.1
Coating Process Monitoring
4.8.1.1
Introduction
One of the most promising, generally applicable and well-adopted methods to en-
hance the surface performance of materials is the use of coatings. The reason is
that it is extremely difficult and expensive to meet homogeneous materials having
on their surface the ensemble of desired properties, such as high hardness, wear
resistance, adequate stiffness, increased ductility, chemical inertness, stainless-
ness, controllable electrical and thermal conductivity behavior, etc. Some of the
aforementioned properties are contradictory to each other and it is impossible to
achieve them simultaneously [1–3]. Surface coatings nowadays offer a satisfactory
reliable and economical alternative solution to this problem. There are several
methods to deposit surface films, based on a variety of physical, chemical, and
thermal processes and transformations, taking into account also the substrate
material specifications and the duty of the element or tool to be coated.
Sensors in Manufacturing. Edited by H.K. Tönshoff, I. Inasaki
Copyright © 2001 Wiley-VCH Verlag GmbH
ISBNs: 3-527-29558-5 (Hardcover); 3-527-60002-7 (Electronic)
Coating methods have rapidly gained a significant part of material technology
in a variety of application fields. To quantify this fact, the estimated market share

pound, which is deposited on the surface of the product, called the substrate.
Thereby, the deposition species are atoms or molecules or a combination of them.
The deposition is effected by condensation, exothermic, and makes extensive use
of plasmas. The deposition is performed in a high-vacuum chamber, and the de-
position temperatures vary from 150 to 500 8C [1, 8]. The PVD method is very flex-
ible, and offers a variety of single and multilayer coatings (structural or composi-
tional), covering a wide range of physical, chemical, and electrical properties of
the film deposited.
In a typical CVD process, reactant gases at room temperature enter a tight reac-
tion chamber. The gas mixture is heated as it approaches the deposition surface,
heated radiatively, or placed upon a heated substrate. The CVD method is per-
formed at higher temperatures, > 850 8C [1]. Depending on the process and operat-
4 Sensors for Process Monitoring308
ing conditions, the reactant gases may undergo homogeneous chemical reactions
in the vapor phase before striking the surface. Near the surface thermal, momen-
tum, and chemical concentration boundary layers form as the gas stream heats,
slows owing to viscous drag, and the chemical composition changes. Hetero-
geneous reactions of the source gases or reactive intermediate species, formed
from homogeneous pyrolysis, occur at the deposition surface forming the depos-
ited material. Gaseous reaction by-products are then transported out of the reac-
tion chamber. Traditional applications of the CVD method, such as the production
of coated tools, are being progressively replaced by the PVD method, owing to its
flexibility and significantly lower deposition temperatures.
4.8.1.3
Vacuum Coating Process Parameter Monitoring Requirements
As in any other multi-parametric process, there are two main categories of sens-
ing systems that are involved. The first focuses on the run-in phase and includes
testing and calibration of the coating device. In this respect, these control systems
are used by the research and development and also by the maintenance divisions
of coating device producers and research institutes. For these reasons, the equip-

pose, process sensors are protected, when necessary, by special filters or ion traps,
which are considered as process spare parts. On the other hand, there are sen-
sors, such as quartz crystals for deposition rate monitoring, which modify their in-
dications as they are progressively coated. Therefore, they must be replaced at the
termination of each specific coating process, in order to monitor the course of the
consequent deposition process.
Coating process parameters are monitored during the deposition with the aid of
various types of sensors. A distinctive classification of them is with regard to their
output, ie, whether they produce analog or digital signals. Both types are used in
coating technology and for the same measurable parameter analog or digital sen-
sors may be applied. Nowadays, advanced data acquisition systems are computer
supported and may handle either analog or digital signals. In the first case, ana-
log to digital converters (usually A/D cards) are utilized to let the digital hardware
tools evaluate and control the input measurements. These electronic circuits must
also satisfy the demands of the whole sensing system.
To simplify the coating process control further, advanced and powerful software
tools are exploited today in order to visualize the deposition progress. Coating de-
vice producers support their customers with exclusive programs that electronically
and remotely control the deposition process. For this purpose they develop soft-
ware using modern programming languages in Windows
®
environments, or
using existing platforms, such as the LabView
®
code. In every case, these pro-
grams offer visual environments for data acquisition, alerts, parameter adjust-
ment, and reports of the whole process.
4 Sensors for Process Monitoring310
4.8.2
Sensors in Vapor Deposition Processes

special hardware and software tools.
There are several approaches to vacuum monitoring and control during the
PVD process. Measurement of pressure in a vacuum system is done with any of a
variety of gages, which for the most part work through somewhat indirect means,
eg, thermal conductivity of the gas or the electrical properties of the gas when ion-
ized. The former are typically used at higher pressures (1–10
–3
Torr) and the latter
in lower ranges. Such gages are sensitive to the type of gas in the system, requir-
ing that corrections be made. Accidents have occurred when this was not taken
into account. For example, the presence of argon in a system will result in a pres-
sure reading on a thermal conductivity gage (thermocouple or Pirani, for exam-
ple, as described in the next two sections) that is much lower than the true pres-
sure. It is possible to overpressure a system significantly while the gage is still in-
dicating vacuum. Other indirect vacuum measurements are based on partial pres-
sure determination, ie, through mass spectrometry.
Actually, there is no universal gage that can measure from atmospheric to ultra-
high vacuum pressure. Therefore, the instrument chosen depends on the pres-
sure range and the residual gases in the vacuum. In any case, pressure measure-
ments may be either output sensor signals to be further processed by open- or
closed-loop control units or direct indications in special displays. The only solu-
tion for a single pressure gage covering a wide measurement range is by combin-
ing more than three probes in one control system.
4 Sensors for Process Monitoring312
Fig. 4.8-3
Vacuum monitoring initially and during the coating process
4.8.2.2.1
Thermocouples
Thermocouple vacuum gages offer an economical method of vacuum pressure
measurement with accuracy suitable for many monitoring and control applica-

Thermocouple gage for vacuum monitoring (sensor by A-VAC)