Measuring the Process Parameters of the IBAD Method

Chromi,um nitride flms are knoun as good protective layrs for against both corrosi,on and wear. These coatings haae been studied in denil during recent years. Their protectiae capabilitl strongly depends on the deposition conditions. A modern method for preparing chromium nitride is the IBAD (Ion Beam Assisted Deposition) method. The mrtin parameter deterrnining the compositi,on and properties of the f.lms prepared b1 the IBAD method is the arriaal ratio of impinging nitrogen ions to chromiunT atoms. In orfur to calibrate the ion beam XY-mechanical scanner uith a Faradal cup, a detector uas designed and consltucted. By mathematical processing of the data, the flux of the nitrogen atorns was found. fo obtain the llux of the chromiuln atoms the RBS and Talystep methods uere usea. Nou, on the basis of this d,ata, arc can perfonn CrN, coutings uith controLled composition and propeflies.


Introduction
In recent years the hard coatings have play a major role in mechanical engineering.Hard coatings are used to increase resistance against wear and fatigue.
Many kinds of hard coatings are now known.'lhe most widely used is titanium nitride, because of its high hardness [].Chromium nitride is used for the same pr<rpose [2].
In addition to high hardness it has a very goocl corrosion resistance, especially under high temperatures in various atmospheres.The hardness and friction properties of chrrcmi- um nitride coatings depend on the structute and orientation of the crystals in the film [3].The corrosion properties also depend on the porosity/densitv of the coatings [4].The importance of the deposition parameters is critical.
There are many methods for preparing these coatings, which can be classified in various ways.Eflective modern methods are based on plasma and ion beam processing technology.One of these is the IBAD method [5].
A schematic view of the experimental arrangement for the IBAD method is shown in Fig l.Chromium nitride has nvo stoichiometric modifications -CrN and CrrN [6].Howeveq wewill be preparing a customary composition CrN*, where x is in the range of [0+l], and we will investigate the characteristics of the process in depend- ence on the x ratio.To do this, we need to calibrate the deposition subprocesses -the atomic fluxes of chromium and nitrogen in IBAD equipment.
2 Calibration of an ion beam A flow of nitrogen of high purity is fed into the ion source, where ionization of the nitrogen takes place.The ion source was developed from the Franks Jivin Anode design.It consists of a graphite outer tubewith closed ends, mounted in an insu- lator.Inside the tube there are two tungsten electrodes, closely spaced about the centerline ofthe tube.
At a distance of 35 mm away, there is a corresponding wider slit in the extractor electrode.During operation a high tension of 90 keV is supplied to the ion source.
The large potential diflerence between the positive HT in the ion source and the zero voltage of the extractor electrode provides the force to accelerate the positive nitrogen ions to a high velocity and form the ion beam.Once the ions pass through the slit in the extraction electrode they continue at constant velocity until they hit the work piece.The ion beam is a mixture of molecular and atomic ions in proportion 3:1.
As a goal we need to know the quantity of nitrogen atoms that the sample per unit area per unit timethe flux of the nitrogen atoms.The TECVAC 221 Ion Implanter has no suitable measuring instrument for this, so we made an appro- priate device ourselves and used it to perform the calibration of the ion beam.A program was written in C language to conrrol rwo stepping-motors, which moved the detector into selected points of cross-sections of the ion beam.An XY-scanner was applied in ttre region of dimensions 200 x 200 mm with a step of 2 mm.Another program estimated the value of ion beam densiry and plotted the graph that illustrated the profile of ion beam density.The resulting distribution of ion beam density in the TECVAC Ion Implanter is shown in Fig. 2.This is the profile of the ion beam current density, but we need a profile of the flux of the nitrogen atoms.
We therefore need to perform a mathematical procedure to transform ion beam density into atomic nitrogen flux.We calculated the quantity of nitrogen atoms as follows: So the fluxes of the atomic and molecular ions will be. respectively: ^ P 6.2+2.tot3P* =i =14355'lol3 cm-2's-l; This figure shows that the profile of the flux has its maximum in the ellipse region, extended along the X-axis.
Pr o cedur e for S ohrti,on : We know the ion beam density and charge of one ion.
From this data we are able to calculate the total flux of ions: p =! =-g+rr =5.742' 1013 cm-2's-rtotul fl,,* e 1.60221892.10-r of ions; "'* =? -number of molecular ions: number of atomic ions: PNI P=Px,*Porr=) P =l+3=4.So if we locate our samples in this region we will achieve satisfactory homogeneity and will be able to control the quan- tity of the nitrogen atoms that arrive on a unit area Per one second.
3 Calibration of the electron beam evaPoration system To check the composition of chromium nitride we also need to calibrate the flux of the evaporated chromtum atoms.
In the TECVAC Ion Implanter the evaporation is sup- plied by an electron beam grn.When the electron beam gun is running, it produces a powerful beam ofelectrons that are accelerated by negative high-tension up to -10 kV This elec- tron beam melts and evaporates the chromium that deposits on the sample.
Besides depositing chromium on the sample that is mounted on the rotary manipulator, chromium is also depos- ited on the quartz crystal microbalance.This crystal oscillates with an original frequency, of 5 MHz, and the frequency of oscillation decreases with the mass of deposited chromium.
In this way we can measure the thickness of the deposited coatings.
To check the measured thickness of the coatings, the Thlystep m€thod was used.In this method, a thin needle moves along the surface and detects the step between the surface ofthe substrate and the surface ofthe coating.Then the value of this step is converted into an electrical signal and the thickness of the coating is displayed.
To obtain the quantity of the chromium atoms in our coatings, the RBS (Rutherford Backscattering SpectroscoPy) method was used.Our samples were bombarded by alpha particles with energy 2MeV.By measuring the energy of the backscattered particles the exact quantity of chromium By comparing this data with the data measured by our IBAD equipment we calibrated the electron beam evapora- tion system.
For example, when the electron beam currentwas l00mA, the flux of the chromium atoms was 2'1013 at'cm-2.

Conclusions
Our ultimate goal is to prePare coatings of chromium nitride.To check the properties of these coatings we need to calibrate the deposition subprocesses -the atomic fluxes of chromium and nitrogen in the IBAD equipment.These cali- brations have been done.
In order to calibrate the ion beam we designed and constructed an XY-mechanical scanner with a Faraday cuP detector.By mathematical processing of the data we found the flux of the nitrogen atoms.
To obtain the flux of the chromium atoms we used the RBS and Talystep methods.Now, on the basis of this data, we can perform crN* coat-Doc.Ing.Frantisek ierny, csc.
ings with controlled composirion and properties.
phone: 1420 224 zbz $;7 e-mail : cernyf@fsid.cvur.cz involves deposition of a desired material using a vac- uum electron beam evaporation technique and simultaneous bombardment by an energetic ion beam. Fig.

Fig. 3 :
Fig. 3: Profrle monitoring of total flux of nitrogen atoms Another view of the same profile is presented in Frg. 4.

Fig. 2 :
Fig.4: Profile monitoring of total flux of nitrogen atoms(another   view)

Fig. 5 Fig. 5 :
Fig.5shows a view fiom below of the profile monitoring of the total flux of the nitrogen atoms, and gives more information about the distribution the flux in the cross-section of the ion beam.Using this figure we can select a zone approximately 50 mm long and 30 mm wide, where the flux of the nitrogen will vary within a range of 15Vo.