Arthur Sherman - Chemical Vapor Deposition for Microelectronics (779637), страница 34
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Then, if a tangent is constructed as shown, the absorbance isA=log10RSand this is referred to as the " base line density" method. Other methods arebased on the area between the tangent and the spectrum trace.Taking advantage of the speed of a Fourier transform infrared spectrometer(FTI R) and the ability to quantify concentrations, one manufacturer has developed an instrument to be used for qual ity control of CVD filn1 depositions.A schem~tic of the instrument is shown in Figure 20.196Chemical Vapor Deposition for Microelectronicswc.JZ<tlI~(I)Z<ta::s~FREQUENCYFigure 19: Base line density method of quantitative analysis.Moving Mirror~"""" "\SourceBeamsplitterTGS DetectorLaserFigure 20: Schematic of Qualimatic S-100-Digilab Division of Bio-Rad Laboratories.This instrument is able to carry out the following analysis:(1) Carbon/oxygen in silicon wafers.(2) Boron/phosphorus in silicon dioxide films.(3) Epitaxial silicon thickness measurement, n/n+ and p/p+ wafers.Film Evaluation Techniques197The latter capability is valuable for production process control, since a thickness measurement can be completed every 5 seconds.
As noted earlier, thereis no simple way to evaluate the thickness of such films.7.3.5 Surface SpectroscopyIn contrast to the previous section where we considered infrared radiation that passed through a thin film and was partially absorbed, we now consider emission from a thin film when we bombard it with radiation. Therefore, we now consider a variety of excitation sources and an equally largenUlTlber of emitted or backscattered particles or photons.
For excitation sources,we have:(1)Electrons(2) Ions(3) Photons (x-rays, UV, visible, IR)The backscattered or emitted species can be:(1)Elect ro ns(2) Ions(3) Photons (x-rays, UV, visible, IR)(4) NeutralsFor all of the techniques to be discussed, the process has to be carried outunder high vacuum conditions. For this reason, they are not useful for routineprocess control.7.3.5.1 ESCA: In this technique (electron spectroscopy for chemicalanalysis), a beam of low energy x-rays (e.g., the Kal pha line of aluminum atan energy of 1.487 keY) is used to bombard a sample.
The x-ray photon energy removes an outer shell electron from an atom when it is absorbed. Thiselectron is emitted with a kinetic energy characteristic of the difference between the x-ray and the binding energy of the electron. The energy of theemitted electron defines the type of aton1, and the number of electrons atthis energy is related to the number density of atoms present.A typical complete ESCA system including pumps, computer, x-ray powersupply plus the sample chamber is shown in Figure 21 (a). Details of the system are illustrated in Figure 21 (b).In general, photon (x-ray) bombardment of thin films is less damaging tothe surface. For this reason, it is possible to obtain information on the topseveral atomic layers by this process.
In fact, not only can we determine theelements present, but we can also obtain information on their bonding. Forexample, in Figure 22 we show the ESCA spectrum for two polymer samples.One is contaminated with Na and CI. By examining the CI peak, it can be identified as being present as a chloride. As another illustration, in examining analuminum surface, one finds that electrons emitted from aluminum metalatoms have different energies than those emitted from aluminum atoms bonded198Chemical Vapor Deposition for Microelectronics...<llEWc:....:.l<lla..IE<ll.....'">'"«uenLJ.J000LO.J::.a.......NQ),i2'~.E'u.LInear motion aUachmentposItions x-ray source at optimumanalysis position.Dua.
anode x·ray aourceoffers flexIbility and high fluxHemispherical analyzereQuipped with single channelor position sensitive detectoroffers excellent sensitivity andenergy resolution.Four..lement Input lens focuseselectrons or ions Into the input alitof the spectrometer._.....Specimen stage in single. Ofmultiple·sample configurations. - - - - - - - - accepts mount, 'rom theIntroduction syslem and position,them for anatysia.tor maximum ..nsitivity.DrIve mechanIsm can be used. - - - - - - - - to auIomate the specimen 51 agesample advance and tilt motions.Ion gun Is used lor samplecleaning or for depth-compositlon - - - - - analysis.Color or monochrome graphic.- - - - - - . . . . terminal is used lor parametersetup and data presentation.Sample Introduction hard.ar.used to insert samples Into UHVtest chamber also interfaces WIth aPHI sample transter ves....(b)l{L-.-JI\\'~~~~~~:~~ ::::~~~u:S:~~~~laSIOtage.Printer provides hard copy dalaprintout.Gate valve i5Qlales introductjonchamber from UHV test chamber.Desktop console desIgn olfers aconvenient.
comfortable workingarrangement.Auto valve control module isused to direct and monItorautomated evacuation process.Dlgltallonlzallon gauge controlmonItors vacuum perlormanceand provides necessary vacuumInterlocks.Turbopump evacuatesintroduction chamber anddilterentlally pumps the Ion gun.=::~~I~~ste,.. facilitate 5Ysten~ '""'T13m<Q)CQ)X·ray source control operallon I'dtreded by the computer.l I,~! .V7'Oll·fr.e Ion pump provides clean,ultrahigh vacuum pumping.Shock mounts protedsystem against vibration.lJCard cage electronicsarrangement minimIzes spacereqUIrements. simplifies additionof accessOties. and facilitatesservicing.Perkin-Eimer Model 7500Professional Computer used forsystem automation orters 16-b.toperation and many otherimportant features.Figure 21: (continued)X·ray source power supply,rttliable and well regulated.ensures high stabIlity forrepeatable analysis on mulliplesamples.r-to'::J--fCD(")::r::J.0CCDFull key board lor fleXibIlity ininstrument control.(I)coCO200Chemical Vapor Deposition for Microelectronicsto oxygen (aluminum oxide).
A typical ESCA spectrum for an aluminum surface is shown in Figure 23. The AI (0) peaks correspond to aluminum metaland the AI (III) to aluminum oxide.GOODco1000800600400200o200oBINDING ENERGY teVl1000800600400BINDING ENERGY teVlFigure 22: ESCA spectra for clean and contaminated polymer films.Film Evaluation Techniques201AI (0)7570(0)7570(b)Figure 23: ESCA spectrum of aluminum surface showing metallic aluminumand aluminum oxides: (a) freshly cleaned sample, (b) after 5 days air exposure. 13There are several limitations to the use of ESCA analysis. For one, dueto the difficulty of focusing x-rays, ESCA has generally been done over several mm diameter samples.
Recently, several manufacturers have introducednew systems that can examine areas as small as 150 to 200 microns in diameter, but there is no way one can look at micron-size features. In addition,ESCA cannot detect hydrogen or helium. Finally, it is not a very sensitiveprocess with detection Iim its of 0.1 to 1.0 atom ic percent.Compared to the other techniques that will be reviewed later, this technique excels in its ability to examine the top several monolayers of a thinfilm and to offer chemical bonding information, as contrasted to only elemental data.7.3.5.2 Auger: Another popular technique for thin film analysis is calledAuger electron spectroscopy (AES). In this technique, an energetic electronbeam (up to 10 keV) is used to probe a surface. The energetic electron can202Chemical Vapor Deposition for Microelectronicsionize an atom by dislodging an inner core electron.
When an outer core electron falls to the inner core to replace the ejected electron, the atom can giveup its excess energy by emitting an x-ray. Alternately, it can eject a second"AugerI! electron. The energy of this second "AugerI! electron is typical ofthe atom it came from, allowing the elemental composition to be determined.Although the electron beam will penetrate many angstroms into the sample,only Auger electrons produced in the top 10 to 30 A of the film will contribute to the signal. Therefore, it is truly a surface analysis tool.If a sputter ion gun is included in the Auger system, one can do a depthprofile through a thin film by sputtering to a given depth and then doing theAuger analysis. Also, since the probe is a beam of electrons that can be focussed sharply (""350 A diameter), the system can be used to do microanalysis of micron-size structures. In this way, an Auger system can be used toprovide three-dimensional information on thin film structures by operatingit much like a SEM.The data obtained from an Auger spectrometer is generally presentedas the derivative with respect to energy of the number of electrons emitted,N(E), versus energy, as shown in Figure 24.
Using the derivative of N(E) ratherthan N(E) itself helps distinguish the Auger peaks from the significant background. The removal of carbon and oxygen by cleaning the molybdenum silicide surface can be clearly seen.Depth profiles are usually presented as atomic concentrations versus sputter time, assuming we know the rate at which the sample sputters. A typicaldepth profile is shown in Figure 25. It is interesting to see that at the surfacethere is carbon, silicon dioxide and some molybdenum. As soon as the surfacelayer is sputtered off (300 A), the oxygen and carbon impurities drop to constant and small values.
For this CVD film, the molybdenum silicide came outto be very silicon rich. We can also see that the stoichiometry of the silicidechanged with position (depth) in the film.As with ESCA, Auger analysis can detect all elements heavier than helium.The quantitative presentation shown in Figure 25 can only be done whensuitable standards are available. Since the Auger spectra depend secondarilyon how the atom is bound in the thin film, it is not always possible to secureaccurate standards so that absolute determination of elemental concentrationscan be inaccurate. Also, the Auger technique is not very sensitive, and in thisregard, is similar to ESCA.
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