Arthur Sherman - Chemical Vapor Deposition for Microelectronics (779637), страница 35
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Concentrations of an element less than 0.5 atomic% cannot be predicted reliably.7.3.5.3 SIMS: When ions are used as the probe beam, and they are energetic enough to sputter ions from the surface being studied (500 eV to 5keV), the secondary ions can be analyzed in a mass spectrometer to indicatethe elemental character of the surface. This technique is then called secondaryion mass spectrometry (SIMS). Although the concept is simple enough, thereare many practical difficulties in implementing such a process.Both oxygen and cesium ion beams are used.
The former is more effective with the electropositive elements (i.e., B, AI, Cr, etc.), and the latter withthe electronegative elements (i.e., C, 0, As, etc.). Since the technique involvessputtering of the surface, depth profiling can be carried out readily.Film Evaluation Techniques203SiMo".11.110flEe TRO H Ef(ERGY •• EI/.(a)SiMoUO,UII1140.00".0. 00~a.D"IDflEe IRON EHERGY. EV.(b)Figure 24: Auger spectrograph of molybdenum silicide film both (a) beforeand (b) after sputter cleaning.204Chemical Vapor Deposition for Microelectronicscr....2.P.J..'_ _~P/.-.LI_ _,-JIO- J t . . . .
. L -_ _~P1 ' - - - _ - - . L• ..1-,_ _~1:1~CIFigure 25: Auger depth profile of molybdenum silicide film.The single most unique characteristic of the SIMS technique is its sensitivity. It can be as good as one part per billion (ppb). For exarnple, if siliconis sputtered at a rate of 10 A/sec over an area of 1aD-pm x 1DO-pm, then 10-11cm 3 /sec of material is removed. Given the density of silicon, this reduces toapproximately 5 x 10 11 atoms/sec.
If 1% of these atoms are ionized (by chargetransfer with the surface) and 10% of those ionized are collected in the massspectrometer, then the measured ion intensity will be 5 x 108 ions/sec. If weassume we can distinguish 5 ions/sec, then a detection sensitivity of 1 partin 10 8 is achievable. This sensitivity is many orders of magnitude better thanother techniques.Although in principal SIMS can detect hydrogen, this evaluation is notvery reliable. The evaluation of hydrogen concentrations will be discussed inthe last section.
Due to the wide variation in sputtering secondary ion yields,detection Iimits on SI MS can vary from 10-9 to 10-3 depending on the speciesbeing studied. The technique is most sensitive to those elements with lowionization potential (i .e., Na, K, etc.).The biggest problem that occurs with the SIMS technique is masking ofthe species of interest by molecular ions of the same molecular weight. Forexample, when studying phosphorus doping of silicon, the mass spectrometerwill see 31p as well as 30Si1H, since there can be hydrogen either in the siliconor in the chamber as an impurity. One way to eliminate the 30Si l H signal isto recognize that actually there is a difference of 0.008 in molecular weightbetween these two species.
Then, a high mass resolution mass spectrometercan be used to eliminate the spurious signal.Film Evaluation Techniques205A similar problem occurs with arsenic-doped silicon. Now, the interferingmolecular ion is 29Si30Si160+, which has a molecular weight of 75, the sameas arsenic. In this case, use is made of the fact that the atomic ion has a broaderenergy spectrum than the molecular ion, as shown in Figure 26.
We can thenbias the sample (i.e., --30 to -50V) so that only higher energy ions are detected.Although this reduces the ion intensity of the atomic ions, it has the beneficial effect of eliminating interference from the molecular ions.tc"U5c:(l)Ec:.Qo204060Secondary 'on Energy (eV)Figure 26: Secondary ion energy distribution.In the semiconductor industry, SIMS has been particularly useful forthe depth profiling of dopants that are present in silicon in very low conc"entrations.
As an example, a SIMS depth profile for boron implanted into silicon is shown in Figure 27. One of the significant features is that we can detect about 10 15 boron atoms/cm 3 in a silicon matrix of 5 x 1022 atoms/cm 3 •This illustrates an ability to detect 20 ppb. Also, the method spans 5 ordersof magnitude in boron concentration. No other technique can span such alarge range accurately.Similar depth profiles are routinely done for phosphorus and arsenicdoped silicon films.206Chemical Vapor Deposition for Microelectronics1os;.------,----,-----...,1020"s+ Implant1 x 10'5/em 2 50keV10'91()4UQ)10'8~cf;1E:::J0.9u~E~c103.QZ.·iiicQ)10 11 1:§£cc:Q)u0c.QU10210 '610'10'51ao L-_ _~-=-------±I---~---'.255.75Depth (pm)Figure 27: SIMS profile of boron implanted into silicon.Finally, since it is possible to readily focus the probe ion beam, it hasproven feasible to scan with SIMS and display images showing the elementaldistributions.
One example of such an analysis is shown in Figure 28.Figure 28: SIMS images showing, left to right, AI, Si, Na in a good device,and Na in a bad device. 14Film Evaluation Techniques2077.3.5.4 RBS: There is one surface analysis technique that can give quantitative information on elemental composition and depth profiling withoutthe use of standards. This technique is Rutherford backscattering (RBS).
Thephysical concept behind RBS is quite straightforward. When an energetic ionrecoils from an elastic collision with an atom (no angular deflection), its energy depends on the mass of the atom it hit. Such a collision is shown schematically in Figure 29. Typically a doubly-ionized helium ion is used in theprobe beam.mO~E...MI _.....~~OFigure 29: Rutherford backscattering.The backscattered energy is simplyM-mEf = E j M+m(12)When using the He++ ion accelerated to 2 MeV, the RBS procedure is mostsensitive to the heavier atoms. A typical detector will have an energy resolution of ---20 keV, regardless of energy detected.
Then, when the backscatteredion has an energy close to 2 MeV, the experimental accuracy will be approximately 1%. An example showing several RBS spectra is shown in Figure 30. 15Here we can clearly see the greater sensitivity of the process for heavier atoms.Also, the probability of a scattering collision occurring is larger for heavieratoms.
So the number of counts detected, for the same dose, is substantiallyhigher for platinum than for titanium.A second feature of RSS is its ability to give concentration versus depth.As the He++ ion traverses the sample, it loses energy because it undergoes manygrazing collisions before it hits an atom head-on and recoils. Similarly, on itsway back through the sample, it again loses energy. This energy loss can beused to evaluate the depth at which the collision occurred. An example showing this depth profiling capability is illustrated in Figure 31. Here, antimonywas implanted into aluminum at room temperature.
It had a maximum concentration at 750 A. The second curve shows that it took a 500°C anneal tocause diffusion of the antimony.Returning to Equation (12), we recognize that the backscattered energiesof heavy atoms will be quite close to each other, and it will be difficult toseparate them. For example, consider a beam of He++ with 2-MeV ions.
ThenM50 mEf0.961 EjM46 mEfD.957E jM2mEf0.333E;M4mEf0.60DE;208Chemical Vapor Deposition for Microelectronics--iGJJ·r()IQ14-iXVlC::>--i10o~2000AoGJJ---·-W><..?Z1~2800A~_._--120u0-.J~2000A81'--2800I ... ~Wtt<tUen~UPdoa:::6A Pd ~0:*~~)(~~0!H0p,JC \JC4200011 Ti]<t•I· -CD2,~__~0.9Ti.!... , . . .0oQ0~_I_l..•.i~1.01.21.31.4I W_~tD--lI1.51.61.71.81.9ENERGY (MeV)Figure 30: Spectra of 2.0 MeV He++ ions with constant exposure to heliumbeam for each film. Is59820001000DEPTH4.aAunannf~\Y 3iiI[399 CLD 2&isag c//li91.41.51.61.71.8ENERGY (MfU)Figure 31: RSS spectra and depth profile for aluminum with implanted antimony.Film Evaluation Techniques209Clearly, one can distinguish easily between two light atoms, but it will be almost impossible to distinguish between two heavy atoms.This fact provides guidance as to what systems can best be analyzed byRSS.
In general, it will be reasonable to see heavy atoms in a light atom background (i.e., as in Si). Two lighter atoms can generally be distinguished provided there is enough sensitivity to detect them. For example, silicon andoxygen can be seen readily. Finally, detecting a light atom in a heavy atom background is generally possible.Finally, we must note that RSS gives no information on chemical bonding,so it may be necessary to use another technique to establish the nature of anycompound being studied. Also, the beam diameter is typically 1 mm, so onecannot use this procedure to examine small structure in the same way thatwe can with Auger and SIMS.Nonetheless, RSS is unique in that it can provide quantitative elementalcomposition data without resorting to the use of standards.7.3.6 Hydrogen Concentration EvaluationIn all of our earlier discussions, we have not found any satisfactory technique available to evaluate hydrogen concentration in a thin film.
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