Arthur Sherman - Chemical Vapor Deposition for Microelectronics (779637), страница 32
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For silicon nitride films, muJriply film thickness by 0.75.CD3C=;'-Q)<Q)1J0""'t0CD1J0(I);:;:0:l-n.,0s:C=;'0CDro("),.....0;:,C=;'(I)AHGLE~GU:OII"INCIDENCELIGHT SOUM:t P'Qwtlt SUrf'\.YIUnECTION\)~v-:~~oPHOTOMUL nf'\.IER POWEll$U,'U. AMPliFIER ANDINTENSI" METEIyO~,-~1113m<~r:::Q)r-to'::J.....:tFigure 2: Elements of typical ellipsometer.2etl(")::T"::J.cr:::etl(/).......(0180Chemical Vapor Deposition for Microelectronicsco.;:;ctl"-o0.o"-Uu;;:.;:;cOJ~"OJ...."-COJctlt:)I"OJ'Q;Eoen0.Qj"0....OJctlE....o:::lctl.!!!~1/IOJEEouFilm Evaluation Techniques181Figure 4: Automated spectrophotometer-NanoSpec/AFT 200 Nanometrics, Inc.182Chemical Vapor Deposition for MicroelectronicsThe only disadvantage of this instrument is the requirement that the index of refraction be known.
Particularly for plasma-enhanced films, the index of refraction can vary considerably, depending on deposition conditions.However, for a production process where the index of refraction is well known,and the primary issue is film thickness uniformity, the Nano Spec can provide information very quickly.The thickness of conducting films on semi-conducting substrates can beinferred from sheet resistance measurements provided the resistivity of thethin film is known.
Measurement of the sheet resistance of conducting filmswill be reviewed in Section 7.2.3.7.2.2 StressWhen a thin film of one material is deposited on a stress-free substrate ofanother material, one may find the thin film to be under tensile or compressive stress. The nature of this stress is typically evaluated by depositing thethin film on a silicon wafer and measuring its deflection.
As illustrated in Figure 5, a tensile stress will deflect the substrate upward, and a compressive stresswill deflect it downward.A formula for the stress has been developed assuming the substrate tobe a circular plate. 3 It is(1 )a=(~) (3(~-V)) G:2)where2astress (dynes/cmvsubstrate Poisson's ratioEsubstrate Young's modulus)disc deflection (cm)film thickness (cm)substrate thickness (cm)disc radius (cm)THIN FILMCIRCULAR SUBSTRATE(a)(b)Figure 5: Thin film stress: (a) tensile, (b) compressive.Film Evaluation Techniques183A variety of techniques may be used to measure the wafer center deflection. The simplest is to place the wafer on the stage of an optical microscopeand calibrate the vertical adjustment.
Then compare the in-focus vertical position at the wafer edge to that at the wafer center.One commercial wafer deflection gauge is available, and is sketched inFigure 6. The degree of light reflection is used to indicate the amount of waferdeflection. The only difficulty with this technique occurs when relatively lowstress films are measured. For normal films (i.e., thermal CVD silicon dioxide)and a stress of 10 9 dynes/cm 2 , a typical 100-mm silicon wafer (0.62-mm thick)with a 1-pm thick film will deflect "'10 pm at its center.
The Ionic Systemsgauge claims a 0.03-pm sensitivity, so the typical stress can be measured readily. For smaller stresses "'10 8 dynes/cm 2 , it may be useful to use a thinnedwafer to make deflection measurements.Displacement "d"Stressed or Bowed WaferKnife EdgeUnstressed WaferFiber Optic BundleSupport Diameter "0"Figure 6: Wafer deflection gauge-Ionic Systems, Inc.There are a number of subtle effects that have to be considered whenmaking thin film stress measurements on silicon wafers. 4 First of all, the crystalorientation of the wafer influences the resulting stress. The same thermal CVDsilicon dioxide film thickness on the same substrate indicates larger tensilestresses on (100)-oriented wafers as compared with (111 I-oriented wafers.It has also been shown that the stress in a deposited film will change withtime, depending on how the wafer is stored.
Figure 7 shows the deflection(stress) as a function of time for a wafer stored in a dry box versus a waferstored at 100% humidity.4 When the second wafer was returned to a dry ambient, its deflection returned to its original value. Clearly, stress will dependon the ambient conditions under which wafers are measured.Chem ical Vapor Deposition for Microelectronics184i., 20--...---r----r---,----.,----,..---,.---,--r--nHEATED FOR 24 hr I{f 4500C IN N2PRIOR TO MEASUREMENT~~ 80[-------~;;R~~~~-;R~-~o~----------------------rt- 40~iiLJJ-,~----0aw -40tiex:STORED IN100-/0 RELATIVE HUMIDITYlii -80CD~L.-._.l..-_""'--_-I.-_-L..._--'-_--L._--'"_---'_----"'--_It-J '\.,-_---...04812162024283270TIME AFTER DEPOSITION,Figure 7: Wafer deflection vs.
storage ambient and time. 4There is also evidence that film stress depends on film thickness, althoughthere are conflicting reports. Therefore, it is prudent to measure stress usingfiln1s equal in thickness to those eventually to be used. For very thin filmsand small stresses, quite small deflections may have to be measured.7.2.3 Sheet ResistanceThe electrical resistivity of thin films of conducting material depositedon insulating substrates is an important quantity to measure. Consider a rectangular conductor, as shown in Figure 8. Current flows through this conductor and a potential difference V exists between its ends.
The resistance ofthis macroscopic segment is, according to Ohm IS Law:R(2)V==I.If we are interested in the local resistivity of a material, we express Ohm IS Lawin terms of the local electric field and the local current density. Then:(3)pE== -:- ,Iwhere E is in volts/em, i is in amps/cm 2 , and p is in S1-cm.We then write for resistivity,V/Q(4)p= I/(bd)bd== R .T .If we choose Q = b, then we can define a sheet resistance which is(5)Rs= ~ (ohms per square, or DID) ,Film Evaluation Techniques185which is independent of the size of the square as long as a square is used tomake the measurement. In this way, a measurement of sheet resistance leadsdirectly to the local film resistivity.Figure 8: Thin film conductor.Sheet resistance can be measured with a four-point probe.
The probesmay be in line or in a square pattern, as shown in Figure 9. In either configuration, a constant current I is passed through two of the probes, and thevoltage difference between the other two is read. Provided the conductinglayer is thin (t ~ 0.60 d), the sheet resistance can be calculated fromRSV4.53 Tin-line Probes 5RSV9.06 T(6)Square Probes. 6vt(a)(b)Figure 9: Four-point probes to measure sheet resistance. (a) in-line; (b) square.186Chemical Vapor Deposition for MicroelectronicsSince most conducting films of practical interest will be less than 10,000 Athick, reasonable probe spacings (d > 1.5 tim) allow the use of the above relations. Equation (5) then enables a determination of the average film resistivity.A commercial instrument that automatically measures sheet resistanceis shown in Figure 10, a typical sheet resistance map of a wafer is shown inFigure 11.This instrument uses a four-point in-line probe with spacing between probesof 0.040".
When the probe is near the wafer center, the expression given inEquation (6) is valid. Near the wafer edge, however, this relation can only beused with a geometric correction factor. Instead of this, a "configuration switching" technique is used to automatically compensate for any such geometricfactors. In this case, the voltage is first read on the inner two probes whencurrent is passed through the outer two.
Then, the current is passed throughthe first and third probes, and a voltage read between the second and fourthprobes.Figure 10: Instrument to measure sheet resistance, Prometrix Corp. Omnimap®RS50.Film Evaluation TechniquesPr~~~trix~187O~~iM~pResistivity Mapping SystemSAMPLE 1.0.:DATE:FILE NO.:01-0CT-85708SOURCE:PROCESS:TASI CVDAVE. VALUE:STD. DEV.:CDNT.INT.:TEST DIAM.:7.33 ohms/sqB.26Z1.00Z3.50 in.Figure 11: Sheet resistance map of silicon wafer with 3000 A film of tantalumsilicide.A geometrical factor, k, can then simply be calculated from a ratio ofthese two voltages and the sheet resistance calculated from(7)Rs = 4.53(k)VI'where Vand I are obtained when current is passed through the outer probes.In general, many points are read on a wafer surface, and a contour mapof sheet resistance is produced, as was illustrated in Figure 11.
Here the heavyline represents the average value, and contour lines (+ or -) indicate a percentchange from the average.188Chemical Vapor Deposition for MicroelectronicsFor conducting films of known resistivity, the above information can beused to generate a map of film thickness.7.2.4 Visible DefectsWhen depositing amorphous or polycrystalline films, defects take theform of particles incorporated into the film either from dust particles or gasphase nucleation. For epitaxial films, as discussed in Chapter 3, there can bea variety of crystallographic defects (stacking faults, slip lines, spikes, etc.).In the as-deposited films, these defects are not always readily visible.
Therefore, epi films are frequently etched with a "preferential" chemical etchantto more clearly delineate whatever faults there may be. A frequently usedetch is the "Wright" etch.? It is a mixture of HF, HN0 3, Cr03' Cu(N0 3lz.acetic acid and deionized water. Stacking faults occurring in epi layers witha 5-minute Wright etch are shown clearly in Figure 12.rapmFigure 12: Stacking faults in epi layers: (a) (100) orientation; (b) (111) orientation.? Reprinted by permission of the publisher, The ElectrochemicalSociety, Inc.Obviously, defects can be seen with an optical microscope.
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