Arthur Sherman - Chemical Vapor Deposition for Microelectronics (779637), страница 33
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What is neededis a quick way to count different types of defects on each wafer processed.For this purpose, a commercial computerized unit has been developed thatcan distinguish between point, line and area defects. The measurement andwafer-handling unit is shown in Figure 13.The system uses a helium-neon laser to scan a wafer surface. Light scatteredby defects is collected and amplified, and the resulting photomultiplier signalsreveal the location and nature of the defect. Particles as small as 0.3 microncan be detected.Film Evaluation Techniques189Figure 13: Surface defect detector-Surfscan 160, Tencor Instruments.7.2.5 Morphology-SEM/TEMThroughout the earlier chapters, we have seen thin film surface morphologyillustrated by scanning (SEM) and transmission (TEM) electron microscopepictures.
The SEM is the only technique available to examine thin film surfaces with submicron size features, because optical microscopes are limitedby the wavelength of light. Even for larger features, the SEM is valuable because its depth of field is much greater (500X) than that of an optical microscope. Resolutions of 25 to 100 A are possible with a well-maintained andcarefully operated system.The SEM operates by scanning a focused electron beam over a surfaceand sensing the secondary electrons emitted from the surface.
A TEM on theother hand operates like an optical microscope by passing an electron beamthrough a sample. The electron beam is then magnified by electromagneticlenses up to 1,000,000 times to form an image. Sample preparation is themost time consuming part of using TEM for studying the structure of CVD190Chemical Vapor Deposition for Microelectronicsthin films. The procedure typically followed is to thin a sample by ion milling the back of the area of interest, until it is thin enough to pass the TEMelectron beam. Film thickness on the order of 1000 A is necessary.7.3 CHEMICAL MEASUREMENTSIn the following sections, we will deal with only some of the more commontechniques used to evaluate the chemical nature of CVD films.
We will beinterested in the composition of the thin films, both as an average over thefilm thickness as well as a function of position in the film. We will also consider the chemical state of the atoms in terms of the bonds they can formwithin the film.7.3.1 Refractive Index-EllipsometryThe ellipsometric technique described earlier has the unique feature thatthe index of refraction can be determined independently of the film thickness. Then, knowledge of this index can be used to infer the chemical composition of a film. For example, thin silicon dioxide films have an index of1.46, while silicon nitride films have a value of 2.0 typically.
Now, when eitherof these films are deposited by PECVD techniques, their stoichiometry canvary depending on deposition conditions. It turns out that this variation instoichiometry can be related to the measured refractive index. Accordingly,measurements of the refractive index can be used as an approximate guide tofilm stoichiometry.7.3.2 X-Ray SpectroscopyWithin this technique, we include EDX (energy dispersive x-ray analysis),WDX (wavelength dispersive x-ray analysis), and X RF (x-ray fluorescenceanalysis).
In all of these, x-rays emitted from a sample are analyzed. In onecase, they are created by bombarding the sample with x-rays (XRF), and inthe others, they are created by high energy electron beam as in an SEM (EDX,WDX).The method involves the absorption of a high-energy x-ray or the energyof an energetic electron by an atom in the sample. This atom then de-excitesby emitting x-rays. These emitted x-rays have energies or wavelengths typicalof the bombarding x-rays or electrons and of the binding energy of the excitedbound electron which relaxes to its original ground state. The x-rays emittedby the sample can be detected individually, and catalogued (by computer)according to energy.
In this way, all elements heavier than fluorine can bedetected simultaneously. Concentrations of 1% are detectable by this approach.If the wave lengths are analyzed, then sensitivity can be increased to the ppmrange, and elements down to boron, carbon and oxygen can be detected.Use of these x-ray techniques is only applicable to films as thick as 1 to3 microns, since the emitted x-rays come from this depth in a sample (as illustrated in Figure 14).
For a 2000 A layer of tungsten on silicon, for example,we would clearly detect tungsten and silicon. However, there would be noFilm Evaluation Techniques191way to know if any oxygen detected came from the substrate or the thin tungsten film.Finally, when an EDX unit is attached to a SEM, it becomes possible toevaluate surface film compositions with high resolution, and produce concentration maps corresponding to the area viewed. Therefore, this is a frequently found addition to a typical SEM installation.ELECTRONBEAMSECONDARYELECTRONS..--10nmEDXDETECTORSEMDETECTORFigure 14: Simultaneous use of SEM and EDX.
87.3.3 Dopant DistributionAs noted earl ier in Chapter 3, epitaxial silicon films deposited by CVDcan be affected by autodoping. If diffusion of the doping species is excessive,the film is not a useful one. Therefore, quite a lot of effort has been spent toaccurately measure the distribution of dopant through the film thickness.One technique is referred to as the "spreading resistance" method. Inthis procedure, a wafer is fractured and the edge containing the film is beveled,as shown in Figure 15.
Then, a two-point probe is used to measure resistivityat a sequence of points traversing the interface between the substrate andthe epi film. By relating the local resistivity to carrier concentration, one isable to deduce the concentration of dopant atoms over the epi layer. Thistechnique is effective for even highly-doped layers.192Chemical Vapor Deposition for MicroelectronicsEPI FILMSUBSTRATEFigure 15: Beveled segment of epi wafer.For less highly-doped epi films, one can use the C-V method.
In this case,use is made of the fact that a Schottky semiconductor diode has a voltagedependent capacitance. In other words, when such a diode is reverse biased,a depletion layer forms which then has a capacitance determined by the depthof this layer (w) as well as the doping (N) at its edge. The doping profile canbe determined from the following relations. 9(8)N(w)(9)wC 3 (v)e€A2dC(v)dvand€AC(v),where € is the permittivity of silicon, A is the junction area, e is the electroniccharge, C is the capacitance, and v is the bias voltage.For some doped layers, such as a low-dose ion implant for a MOS transistor, this procedure does not reveal the entire doping profile. 1o In this case, aMOS structure is examined rather than a Schottky diode.
Typically, a baresilicon wafer is oxidized, and then aluminum dots are sputtered on to formmany MOS structures. When a MOS device is examined, Equations (8) and (9)have to be supplemented by1(10)Cm(v)where C m is the measured capacitance, Cox is the oxide capacitance, and Csis the depletion region capacitance.As an example of the use of this technique, a silicon wafer lightly dopedwith phosphorus is doped with additional phosphorus by ion implantation(dose of 3.5 x 1011cm~). A thermal oxide film of 857 A thickness was initially grown on the wafer.
The variation of dopant concentration with depthfrom the oxide-silicon interface is shown in Figure 16. The rise in dopant closeFilm Evaluation Techniques193to the interface in the non-implanted sample is due to the usual phosphorus3l6segregation in this region. As can be seen, the implant peak is 1.1 x 10 cmat a depth of 1700 A. The doping level returns to that of the original substrateat 7000 A.,.SDEPTHfMICROMETERSI-+Figure 16: Doping profile for silicon wafer implanted with phosphorus, measured by C-V technique. lo7.3.4 Infrared SpectroscopyAnother technique that can be used to determine the chemical nature ofa thin film is infrared spectroscopy.
Some materials will absorb certain frequencies in the infrared (wavelengths 2 to 25 microns) because of the excitation of vibrational energy transitions in molecular species. In the same waythat electronic transitions in atoms can absorb radiation of specific frequencies,the vibration of a molecule (stretching or bending) will have a resonance value,and it will be excited by any radiation of this frequency.
Consider the H 2 0molecule and its three vibrational modes, as shown in Figure 17. Clearly, eachof these vibrational modes has its own resonant frequency, as indicated, andthey are all in the infrared range.Now, when infrared radiation of a particular frequency is passed througha sample containing molecular species, it mayor may not be absorbed. If allfrequencies are passed through, some witl be absorbed to varying degrees, depending on the molecular species involved. For example, a typical spectrumof transmittance (%) versus wave number (cm-l ) (wave number = l/wavelength)1.5Chemical Vapor Deposition for Microelectronics19400H2.661JrnH(a)00(b)cI~'o2.73 IJmHH00(c)6.27 JimFigure 17: Resonant vibrational modes of the water molecule. (a) asymmetricstretching, (b) symmetric stretching, and (c) scissoring deformation.for silicon nitride thin film is illustrated in Figure 18.
For complex moleculeswith many vibrational modes, there are many peak absorption frequencies.There are two types of spectrometers that one can use to generate suchspectra. 12 One uses a monochromator to evaluate each frequency in turn.The second uses a Michelson interferometer to examine all frequencies simultaneously, and then a Fourier transform to display the spectrum. The advantage of the latter approach is its greater sensitivity, and the speed with whichit can produce a spectrum.Regardless of how it is obtained, the spectrum can be used to make quantitative estimates of the concentration of molecular species in thin films. Usingthe Beer-Lambert Law,12 we can write simplyA(11 )==EC Lwhere10glo 10 /1Aabsorbance10incident radiation==ITransmitted radiationEextinction coefficientLpath lengthCconcentrationFilm Evaluation Techniques1~ ~ r"-\ "":..':N:.......t\\..I.....,.II:,,',I•;:,I;~\ (\ I~i,:\~I2400t"-~....,195,~~~.I~VSi-IN2000]600800400\oJave numberFigure 18: Infrared spectrum for silicon nitride film.!!The extinction coefficient is a constant for one substance and one frequency.Then a measurement of A gives a resulting value for C.Consider the portion of a spectrum near a resona nt frequency, as shown inFigure 19.
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