CHAPTER 2: Crystal Growth and Wafer Preparation

Transcription

1 hapter 2 1 HAPTER 2: rystal Grwth and Wafer Preparatin Silicn is the mst imprtant semicnductr fr the micrelectrnics industry. When cmpared t germanium, silicn excels fr the fllwing reasns: (1) Si has a larger bandgap (1.1 ev fr Si versus 0.66 ev fr Ge). (2) Si devices can perate at a higher temperature (150 vs 100 ). (3) Intrinsic resistivity is higher (2.3 x 10 5 Ω-cm vs 47 Ω-cm). (4) SiO 2 is mre stable than GeO 2 which is als water sluble. (5) Si is less cstly. 2.1 rystal Structure The prcessing characteristics and sme material prperties f silicn wafers depend n its rientatin. The <111> planes have the highest density f atms n the surface, s crystals grw mst easily n these planes and xidatin ccurs at a higher pace when cmpared t ther crystal planes. Traditinally, biplar devices are fabricated in <111> riented crystals whereas <100> materials are preferred fr OS devices. Real crystals are imperfect and cntain pint defects, line defects r dislcatins, area r plane defects, and vlume defects. Pint defects manifest themselves in several frms, as shwn in Figure 2.1. Any nn-silicn atms incrprated int the lattice at either a substitutinal r interstitial site are cnsidered pint defects. Pint defects are imprtant in the inetics f diffusin and xidatin. rever, t be electrically active, dpants must ccupy substitutinal sites in rder t intrduce an energy level in the bandgap. Dislcatins are line defects. Figure 2.2 is a schematic representatin f a line dislcatin in a cubic lattice. Dislcatins in a lattice are dynamic defects. That is, they can diffuse under applied stress, dissciate int tw r mre dislcatins, r cmbine with ther dislcatins. Dislcatins in devices are generally undesirable, because they act as sins fr metallic impurities and alter diffusin prfiles.

2 hapter 2 2 Figure 2.1: Lcatin and types f pint defects in a simple lattice. Figure 2.2: An edge dislcatin in a cubic lattice created by an extra plane f atms. The line f the dislcatin is perpendicular t the page.

3 hapter 2 3 Tw typical area r planar defects are twins and grain bundaries. Twinning represents a change in the crystal rientatin acrss a twin plane, such that a mirrr image exists acrss that plane. Grain bundaries are mre disrdered than twins and separate grains f single crystals in plycrystalline silicn. Planar defects appear during crystal grwth, and crystals having such defects are nt cnsidered usable fr I manufacture and are discarded. Precipitates f impurity r dpant atms cnstitute the furth class f defects. The slubility f dpants varies with temperature, as exhibited in Figure 2.3. Thus, if an impurity is intrduced at the maximum cncentratin allwed by its slubility, a supersaturated cnditin will exist upn cling. The crystal achieves an equilibrium state by precipitating the impurity atms in excess f the slubility level as a secnd phase. Precipitates are generally undesirable as they act as sites fr dislcatin generatin. Dislcatins result frm the vlume mismatch between the precipitate and the lattice, inducing a strain that is relieved by the frmatin f dislcatins. Figure 2.3: Slid slubility f impurity elements in silicn.

4 hapter Electrnic-Grade Silicn Electrnic-grade silicn (EGS), a plycrystalline material f high purity, is the starting material fr the preparatin f single crystal silicn. EGS is made frm metallurgical-grade silicn (GS) which in turn is made frm quartzite, which is a relatively pure frm f sand (Table 2.1). GS is purified by the fllwing reactin: Si (slid) + 3Hl (gas) SiHl 3 (gas) + H 2 (gas) + heat The biling pint f trichlrsilane (SiHl 3 ) is 32 and can be readily purified using fractinal distillatin. EGS is frmed by reacting trichlrsilane with hydrgen: 2SiHl 3 (gas) + 2H 2 (gas) 2Si (slid) + 6Hl (gas)

5 hapter 2 5 Table 2.1: mparisn f typical impurity cntents in varius materials (values in ppm except as nted). Impurity Quartzite arbn GS EGS rucible quartz Al B <1 ppb... u < Au ppb... Fe P <2 ppb... a r n Sb Ni As Ti La ppb... V W O Na * etallurgical-grade silicn Electrnic-grade silicn

6 hapter zchralsi rystal Grwth rystal grwth typically invlves a phase change frm a slid, liquid, r gas phase t a crystalline slid phase. The zchralsi (Z) prcess, which accunts fr 80% t 90% f wrldwide silicn cnsumptin, cnsists f dipping a small single-crystal seed int mlten silicn and slwly withdrawing the seed while rtating it simultaneusly. Figure 2.4 is a schematic f a zchralsi crystal grwing apparatus. The crucible is usually made f quartz r graphite with a fused silica lining. After the seed is dipped int the EGS melt, the crystal is pulled at a rate that minimizes defects and yields a cnstant ingt diameter. Figure 2.4: The zchralsi crystal grwth apparatus. Impurities, bth intentinal and unintentinal, are intrduced int the silicn ingt. Intentinal dpants are mixed int the melt during crystal grwth, while unintentinal impurities riginate frm the crucible, ambient, etc. All cmmn impurities have different slubilities in the slid and in the melt. An equilibrium segregatin cefficient can be defined t be the rati f the equilibrium

7 hapter 2 7 cncentratin f the impurity in the slid t that in the liquid at the interface, i.e. = s / l. Table 2.2 lists the equilibrium segregatin cefficients fr sme cmmn impurities and dpants. Nte that all are belw unity, implying that the impurities preferentially segregate t the melt and the melt becmes prgressively enriched with these impurities as the crystal is being pulled. Table 2.2: Segregatin cefficients fr cmmn impurities in silicn. Impurity Al As B u Fe O P Sb x10-6 8x The distributin f an impurity in the grwn crystal can be described mathematically by the nrmal freezing relatin: 1 (1 ) s X (Equatin 2.1) where X is the fractin f the melt slidified, is the initial melt cncentratin, s is the slid cncentratin, and is the segregatin cefficient. Figure 2.5 illustrates the segregatin behavir fr three segregatin cefficients. Z-Si crystals are grwn frm a silicn melt cntained in a fused silica (SiO 2 ) crucible. Fused silica reacts with ht silicn and releases xygen int the melt. Hence, Z-Si has an indigenus xygen cncentratin f apprximately atms/cm 3. Althugh the segregatin cefficient f xygen is <1, the axial distributin f xygen is gverned by the amunt f xygen in the melt. Less disslutin f the crucible material ccurs as the melt vlume diminishes, and less xygen is available fr incrpratin. Figure 2.6 shws the variatin in the xygen cntent alng a typical silicn ingt. Oxygen is an imprtant element in Z-Si n accunt f the fllwing reasns: (1) Oxygen frms a thermal dnr in silicn. (2) Oxygen increases the mechanical strength f silicn. (3) Oxygen precipitates prvide gettering sites fr unintentinal impurities. Thermal dnrs are frmed by the plymerizatin f Si and O int cmplexes such as SiO 4 in interstitial sites at 400 t 500. areful quenching f the crystal annihilates these dnrs.

8 hapter 2 8 Under certain annealing cycles, xygen atms in the bul f the crystal can be precipitated as SiO x clusters that act as trapping sites t impurities. This prcess, which is illustrated in Figure 2.7, is called internal gettering and is ne f the mst effective means t remve unintentinal impurities frm the near surface regin where devices are fabricated. Figure 2.5: Impurity cncentratin prfiles fr different with = 1.

9 hapter 2 9 Figure 2.6: Axial distributin f xygen in a typical Z-Si ingt. Figure 2.7: Schematic f a denuded zne in a wafer crss sectin illustrating the gettering sites (a and b are znes denuded f defects whereas c represents the regin f intrinsic gettering).

10 hapter 2 10 Derivatin f the nrmal freezing relatin (Equatin 2.1): Weight = Ingt Weight = d Dpant cnc. = s elt S = dpant remaining in melt nsider a crystal being grwn frm a melt having an initial weight with an initial dping cncentratin in the melt (i.e., the weight f the dpant per 1 gram melt). At a given pint f grwth when a crystal f weight has been grwn, the amunt f the dpant remaining in the melt (by weight) is S. Fr an incremental amunt f the crystal with weight d, the crrespnding reductin f the dpant (-ds) frm the melt is s d, where s is the dping cncentratin in the crystal (by weight): ds d (1) s Nw, the remaining weight f the melt is -, and the dping cncentratin in the liquid (by weight), l, is given by l S (2) mbining (1) and (2) and substituting : s l ds S d (3) Given the initial weight f the dpant,, we can integrate (3):

11 hapter 2 11 ds S d S (4) Slving (4) and cmbing with (2) give s 1 1 (5) Details n hw t slve Eq. (4) S ds S (1) Left hand side f Eq. (4): ln S (2) Right hand side f Eq. (4): Let m d dm ln 0 m, dm d ; Eq. (4) becmes: S ln ln and s S 0 Frm Eq. (2), S ( ) l Therefre, l ( ) 0 s ( ) 0 r s Finally, 1 (5) s

12 hapter 2 12 Example 2.1 Using Equatin 2.1 and the segregatin cefficient fr xygen frm Table 2.2, predict the cncentratin f xygen in the crystal at a fractin slidified f 0.4 in Figure 2.6. Slutin First, find the cncentratin f xygen in the melt at the tp f the crystal (x = 0.05) using Equatin 2.1: 1.3 x = 0.25 x (1-0.05) = 5.0 x atms/cm 3 Secnd, calculate the expected value at a fractin slidified f 0.4: s = 0.25 x 5.0 x (1-0.4) s = 1.83 x atms/cm 3 Since this is higher than the bserved value f 1.0 x atms/cm 3, it indicates a lss f xygen frm the melt during grwth and shws that Equatin 2.1 is nt strictly applicable fr this case.

13 hapter Flat-Zne Prcess The flat-zne prcess has sme advantages ver the zchralsi prcess fr the grwth f certain types f silicn crystals. The mlten silicn in the flat-zne apparatus (Figure 2.8) is nt cntained in a crucible, and is thus nt subject t the xygen cntaminatin present in Z-Si crystals. The flat-zne prcess is als necessary t btain crystals with a high resistivity (>> 25 Ω-cm). Figure 2.8: Flat-zne crystal grwth apparatus.

14 hapter 2 14 Example 2.2 Shw that in the flat-zne prcess, the impurity cncentratin at a distance x, s (x), is given by the fllwing relatinship: ( x) s x (1 )exp[ ] l where is the riginal impurity cncentratin in the ingt, is the impurity segregatin cefficient, and l is the length f the mlten zne. Slutin Assume that the zne in the abve figure mves a distance x t the right. Write the balance that the quantity f impurity in the zne after the shift f x is equal t that befre the shift, minus the quantity slidified at the left, and plus that melted int the zne at the right. Obtain the standard derivative frm and let x apprach zer. Slve the differential equatin fr L (x) using the bundary cnditin that l (x = 0) =. Therefre, la L Since ( x) ( x) ( x x) la ( x) ( x) Ax Ax s L, L s L Let x 0, ( x x) x L ( x) l l L ( x)

15 hapter ) ( ) ( 0 x l l dx x d L L The slutin f this differential equatin is: l x L Be x / ) ( At x = 0, L (0) =, and B = (1-1/ ). Thus, l x L e x ) 1 (1 ) ( ] )exp[ (1 ) ( 0 l x x s

16 hapter haracterizatin Rutine evaluatin f ingts r bules invlves measuring the resistivity, evaluating their crystal perfectin, and examining their mechanical prperties, such as size and mass. Other less rutine tests include the measurement f xygen, carbn, and heavy metal cntents. Resistivity measurements are made n the flat ends f the crystal by the furpint prbe technique. As depicted in Figure 2.9, a current I passes thrugh the uter prbes and the vltage V is measured between the inner prbes. The measured resistance (V/I) is cnverted t resistivity (Ω-cm) using the relatinship ρ = (V/I)2πS (Equatin 2.2) where S is the prbe spacing in centimeters. The calculated resistivity can be crrelated with dpant cncentratin using a chart such as the ne displayed in Figure Grss crystalline imperfectins are detected visually and defective crystals are cut frm the bule. re subtle defects such as dislcatins can be disclsed by preferential chemical etching. hemical infrmatin can be acquired emplying wet analytical techniques r mre sphisticated slid-state and surface analytical methds. Figure 2.9: Fur-pint prbe measurement n crystal end.

17 hapter 2 17 Figure 2.10: nversin between resistivity and dpant density in silicn.

18 hapter 2 18 Example 2.3 A brn-dped crystal is measured at its seed end with a fur-pint prbe f spacing 1 mm. The (V/I) reading is 10 hms. What is the seed end dping and the expected reading at 0.95 fractin slidified? Slutin First use Equatin 2.2 t find the seed end resistivity. = 10 x 2 x x 0.1 = 6.3 -cm Figure 2.10 shws that this equates t a dping f 2 x atms/cm 3. Next use Equatin 2.1 and Table 2.2 t find the melt cncentratin: 2 x = 0.8 (1-0) = 2.5 x atms/cm 3 At 0.95 fractin slidified, the slid cncentratin is: s = 0.8 x 2.5 x (1-0.95) s = 3.6 x atms/cm 3 r abut 5 -cm

19 hapter 2 19 Example 2.4 Resistivity is usually inversely prprtinal t dpant cncentratin in a silicn wafer. Suppse that in a zchralsi prcess, a chun f undped plysilicn is mixed with a small amunt f heavily dped silicn with resistivity f 0.01 Ω-cm and the crystal is subsequently pulled. Fr ne run, the resistivity f the grwn crystal at the midsectin is 1 Ω-cm. The amunt f heavily dped silicn is t be changed in the next run t btain a resistivity f 0.5 Ω-cm at the midsectin f the new ingt. Determine the increase in the amunt f heavily dped silicn that is needed. Assume that all perating cnditins remain the same and the amunt f the added heavily-dped plysilicn is small cmpared t the undped plysilicn charge. Discuss whether yur answer is crrect nly at the midpint f the ingt r is universally true fr all ther axial psitins n the ingt. Slutin Based n the nrmal freezing relatinship: s (1 X ) 1 At the midsectin f the first ingt: At the midsectin f the secnd ingt: 1 0.5,1 1(0.5) 1 0.5,2 2(0.5) Therefre, 0.5,1 0.5, The cncentratin in the midsectin depends nly n the initial dpant cncentratin in the melt. If the vlume change is neglected, ne can simply duble the amunt f the heavily-dped plysilicn t achieve a tw-fld decrease in the resistivity everywhere.

20 hapter Wafer Preparatin Silicn, albeit brittle, is a hard material. The mst suitable material fr shaping and cutting silicn is industrial-grade diamnd. nversin f silicn ingts int plished wafers requires several machining, chemical, and plishing peratins. After a grinding peratin t fix the diameter f the material (Figure 2.11), ne r mre flats are grunded alng the length f the ingt. The largest flat, called the "majr" r "primary" flat, is usually relative t a specific crystal rientatin. The flat is lcated by x-ray diffractin techniques. The primary flat serves as a mechanical lcatr in autmated prcessing equipment t psitin the wafer, and als serves t rient the I device relative t the crystal. Other smaller flats are called "secndary" flats that serve t identify the rientatin and cnductivity type f the wafer. Secndary flats thus prvide a means t quicly srt and identify wafers shuld mixing ccur. The flat lcatins fr the fur cmmn types f silicn wafers are exhibited in Figure Figure 2.11: Schematic f the grinding prcess.

21 hapter 2 21 Figure 2.12: Identifying flats n silicn wafers. The drawbac f these flats is that the usable area n the wafer, i.e. the area n which micrelectrnic devices can be fabricated, is reduced. Fr sme 200 mm and 300 mm diameter wafers, nly a small ntch is cut frm the wafer t enable lithgraphic alignment but n dpant type r crystal rientatin infrmatin is cnveyed. Once these peratins have been cmpleted, the ingt is ready t be sliced by diamnd saw int wafers. Slicing determines fur wafer parameters: surface rientatin (e.g., <111> r <100>); thicness (e.g., mm, depending n wafer diameter); taper, which is the wafer thicness variatins frm ne end t anther; and bw, which is the surface curvature f the wafer measured frm the center f the wafer t its edge. After slicing, the wafers underg lapping peratin that is perfrmed under pressure using a mixture f Al 2 O 3 and glycerine. Subsequent chemical etching remves any remaining damaged and cntaminated regins. Histrically, mixtures f hydrfluric, nitric, and acetic acids have been emplyed, but alaline etching, using ptassium r sdium hydrxide, is als cmmn. Plishing is the final step. Its purpse is t prvide a smth, specular surface n which device features can be phtengraved. Figure 2.13 depicts the schematic f a typical plishing machine and the prcess. Figure 2.14 displays finished

22 hapter 2 22 silicn wafers f varius dimensins. A finished wafer is subject t a myriad f physical tlerances, and examples f the specificatins are shwn in Table 2.3. Figure 2.13: Schematic f the plishing prcess. Figure 2.14: Finished silicn wafers f varius sizes.

23 hapter 2 23 Table 2.3: Typical specificatins fr mncrystalline silicn wafers. Parameter 125 mm 150 mm 200 mm 300 mm Diameter (mm) Thicness (mm) Bw (μm) <30 Ttal thicness variatin <10 (μm) Surface rientatin