Surface chemical electronics at the semiconductor surface

a ~ surface science ELSEVIER

Applied Surface Science 121/122 (1997) 44-62

Surface chemical electronics at the semiconductor surface Tadahiro Ohmi * Department of Electronics, Faculty of Engineering, Tohoku University, Sendal 980, Japan

Abstract Thin film disposition, etching, and cleaning, all of which constitute core of the semiconductor manufacturing process, are governed by surface phenomena that the semiconductor surface features in the gaseous phase and the liquid phase. Essential factors to reveal phenomena of the semiconductor surface are dielectric constant and energy level (Fermi level, redox potential) of media which come in contact with the surface, and energy level (electronegativity) of atoms to terminate dangling bonds of the surface. The energy state of molecules adsorbed on the surface of the solid material is greatly different from that of molecules isolated in the gaseous phase as it is affected by the dielectric constant of the solid material. What determines the surface phenomena is the transfer of electrons through the semiconductor surface, between semiconductor/terminating atoms and adsorbed molecules/contacting media. The energy level concept is popular in the solid state physics. On the other hand, electronegativity of atoms and the redox potential of solutions are among the concept which is popular in the chemical field. The author has selected the title 'Surface Chemical Electronics of Semiconductor' for this article as it discusses these two different concepts in an integrated manner. © 1997 Elsevier Science B.V. Keywords: Surface electronicchemistry; Dielectric constant;Bond energy; Energy level; Redox potential;Electronegativity

1. Hydrogen atomic model and molecular bond The author describes molecular behavior in many aspect by means of the hydrogen atomic model as there is no general theoretical equation of molecular bond energy. It is well known that the energy of an electron ( G ) and the Bohr radius of the first orbit ( a B) in this model are expressed, when the principal quantum number of n is used, as: me 4

G = - 2(4~.e0)2h2

1

13.6

n2

n2

(eV)

(1)

(4-n'e0) h 2 aB

me 2

0.053 nm

(2)

where m, e, e 0 and h = 27rh are the mass of the electron, unit charge, the dielectric constant of vacuum and Planck's constant. The energy level is determined by the Coulomb force between positive and negative charge. This energy is in inverse proportion to the squared dielectric constant, and in proportion to the mass. Si is a tetravalent monoatomic crystal. When phosphor P or arsenic As, either of which is a pentavalent atom, is doped into Si, one electron of P or As does not contribute to the covalent bond of Si. The energy ( G * ) to excite this electron to the conduction band, which is called impurity level (energy level from the bottom of conduction band), is given by the hydrogen atomic model as follows: m*

Tel.: +81-22-2177122; fax: +81-22-2242549; e-mail: [email protected].

G* = - 13.6 X m~2-~ (eV)

0169-4332/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S0 169-4332(97)00259-6

(3)

T. Ohmi / Applied Surface Science 121 / 122 (1997) 44-62

45

Table 1 Bond dissociation energy (homolysis and heterolysis) and typical properties of hydrogen halide (HF, HCI, HBr) and H20

HF HCI HBr H20

Bond energy (homolysis)

Bond energy (heterolysis)

Melting point (°C)

Boiling point (°C)

Dipole moment

Ionization potential

Dielectric constant

5.8 eV 4.4eV 3.8 eV 4.8 eV

16.0 eV 14.4 eV 14.0 eV 16.9 eV

-83 -114 - 89 0

19.5 -85 - 67 100

1.83 DU 1.11DU 0.83 DU 1.94 DU

16.1 eV 12.8 eV 11.7 eV 12.6 eV

83 12 7 81

Dipole moment: 1 DU = 3.3356 × 10 -30 C. m. Homolysis bond dissociation: HF --9 H * + F * Heterolysis bond dissociation: HF ~ H + + F.

where m* and esi are the effective mass of the electron in the conduction band and the relative dielectric constant of Si. When 0.19m and 11.8 are substituted for m* and esi respectively, e~* is calculated to be - 17 meV. This indicates that electrons at impurity level can be easily excited to the conduction band with the thermal energy at room temperature of 26 meV. A radius of circular orbit which the electron at impurity level forms around the doped impurity atom in Si (Bohr radius), a B, is also changed from 0.053 nm to 3.3 nm in accordance with the following equation: m 'ffSi a B = 0.053 × - ' - 7 (nm) m

(4)

As the doping density of an impurity atom such as As, P, and B exceeds 1 × 10 j8 cm -3, the average interatomic distance of neighboring impurity atoms gets smaller than 10 nm. Accordingly the wave function of electrons circulating around the impurity atoms comes to overlap each other. As a result, the impurity level is changed to the impurity band. This is a typical example that the energy level and bond

energy of atoms or molecules show a drastic change due to the dielectric constant of the environment where they exist. HF, HC1, and HBr are often used in the semiconductor manufacturing process both in gas phase and liquid phase. They are hydrides of the halogen-type elements of fluorine, chlorine, and bromine. Table 1 shows their bond energy and typical properties. For reference, Table 1 also shows properties of HeO, hydride of oxygen. HF and H 2 0 featuring high dipole moments are in the liquid state in the wide range of temperature up to 100°C, and, consequently, they feature extremely high dielectric constants. The electronegativity of fluorine and oxygen is extremely high: 4.0 and 3.5 respectively, compared with that of hydrogen: 2.1. HF and H 2 0 are electrically neutral as a molecule, therefore, but electrons locally gather on the side of the fluorine atom and oxygen atom. This raises a dipole moment of HF and H20, which lets them easily form clustered molecules. This is why they stay in the liquid phase during a wide temperature range. Picking up H, O, N, F, C1, Br, and Si, Table 2 shows their electronegativity and

Table 2 Electronegativity (energy level) and typical properties of various atoms such as H, O, N, F, CI, Br and Si which are widely used in semiconductor manufacturing

Atomic number Mass number Group Electronegativity Energy level (eV) Ionization potential (eV)

H

O

N

F

CI

Br

Si

1 1.01 Ia 2.1 - 5.11 13.6

8 16.01 VIb 3.5 -8.29 13.6

7 14.1 Vb 3.0 -7.15 14.5

9 19.00 VIIb 4.0 -9.42 17.4

17 35.45 Vllb 3.0 -7.15 13.0

35 79.90 VIIb 2.8 -6.70 11.8

14 28.09 IVb 1.8 -4.43 8.15

7". Ohmi / Apl?lied Su;jhce Science 121 / 122 (1997) 44-62

46

energy level calculated on the basis of the vacuum level. The relationship between electronegativity ( X ) and energy level (~) is expressed as [1]: = - (2.27X + 0.34) (eV)

(5)

Table 2 also shows the ionization energy of each atom which corresponds to the energy level of electrons at the ground state. As shown in Table 1, HF, HC1, and HBr feature extremely high bond energy: 5.8 eV, 4.4 eV, and 3.8 eV respectively. They, therefore, never get decomposed with a thermal energy of 0.026 eV at room temperature in the gaseous phase. In other words, HF, HCI, and HBr are extremely stable gas molecules in the gaseous phase. As shown in Table 2, the electronegativity of F, C1, and Br is 4.0, 3.0, and 2.8 respectively while that of H is 2.1. When converted from electronegativity by means of Eq. (5), the energy level based on the vacuum level of F, C1, and Br is - 9 . 4 2 eV, - 7 . 1 5 eV, and - 6 . 7 0 eV respectively: much lower than that o f H o f - 5 . 1 1 eV. When a molecular bond is formed between H and F, H and C1, or H and Br, the molecule is electrically neutral as a whole, but electrons get locally concentrated on the side of F, CI or Br. Consequently H is positively charged in practical, which makes the bond like an ionic bond. HF features a high dipole moment as the electronegativity gap of its component atoms is the biggest and the local concentration of electrons is remarkable. When two atoms form a molecule in an ionic-bond-like way, the electric force lines to connect positive and negative charges widely distributed in adjacent region, and the molecule is greatly affected by the dielectric constant of the surrounding medium. If it is assumed, as shown in Eq. (1), that the bond energy of this molecule is in an inverse proportion to the squared dielectric constant, the bond energy of the molecule, which is dissolved in water, is lowered by the high dielectric constant of water (relative dielectric constant of 81). Specifically, the bond energy of HF, HC1, and HBr, when dissolved in water, decreases to 0.88 meV, 0.67 meV, and 0.58 meV respectively. Consequently all of these three molecules are most likely to be well decomposed and dissociated to ions such as H:, F-, C1 and Br even with a thermal energy at room temperature of 26 meV. It is known that HCI and HBr are completely

Table 3 Bond dissociation energy (homolysis) and ionization potential of molecules such as H 2, O:,N,, F2, CI 3 and Br,

Bond energy(eV) lonizalionpotential(eV)

CI~

H.

O,

N,

F,

4.49 15.4

5.13 I2.1

9.51 15.6

1.61 2.49 15.7 11.6

Br, 1.98 10.5

dissociated in water. On the other hand, HF gets only partially dissociated when its concentration is fairly high (over 10 3 mol/kg). Some of the F ions generated in HF dissociation (HF--+ H - + F ) get bound with neutral HF molecules to form HF 2 ions which have been revealed dominant ions to etch SiO, film [2,3]. This fact suggests that the bond energy of the molecule is actually not simply in inverse proportion to the squared dielectric constant. H 2 , N 2, 0 2, F 2, C12, and Br 2 are composed of two exactly identical atoms, which makes them different from these molecules composed of two atoms featuring different electronegativity. In this case, therefore, localization of the electron concentration to result in polarity does not occur. In other words, these molecules feature a covalent bond. External extension of their electric force lines is limited, and they are hardly affected by the dielectric constant of the surrounding medium. Table 3 shows the bond energy and ionization energy of H e, 02, N,. F 2, CI e, and Br 2, negatively-charged electrons attract positively-charged atoms by means of Coulomb force in these molecules. Even when the H~ molecule or N 2, O, molecule gets dissolved into water, however, it is not decomposed or dissociated. The ionization energy of the H, molecule is about 15.4 eV in gaseous phase. A redox potential of 0 V is defined as energy required to ionize H. into 2H + and 2e in ultrapure water. It is equal to an energy level of - 4 . 4 4 eV. Even in the case of molecules with a covalent bond, bond energy and ionization energy get lower when being in media featuring high dielectric constants. Since the relative dielectric constant of water is extremely high at 81, various active species, which are never generated in the gaseous phase, can be easily generated at room temperature in solutions. This is why various reactions can take place in solutions at room temperature. The wet process, therefore, will remain remark-

T. Ohmi /Applied Sz.T/}lce Science 121 / 122 (1997) 44-62

ably important for the Si technology even in the future.

2. Behavior of molecules adsorbed on the Si surface: taking a SiH4-type molecule as an example

If it is assumed that the bond energy of a molecule is in inverse proportion to the squared dielectric constant of the medium where the molecule exists, the binding state of the molecule and energy state of electrons must differ greatly between when the molecule is isolated in the gaseous phase and when the molecule is adsorbed onto solid surface. Let us take the Si surface as an example, where the relative dielectric constant of Si is 11.8. Let us assume, as shown in Fig. 1, that molecule AB composed of atom A and atom B gets adsorbed on the solid surface (the Si surface) from the gaseous phase. Due to interaction between the wave function of electrons in molecule AB and that of electrons in the vicinity of the Si surface, the state of electrons must become very different when molecule AB is adsorbed from when it is isolated in the gaseous phase. Let us limit our discussion to difference in dielectric constant. Molecule AB in the gaseous phase is subjected to a relative dielectric constant of 1. When it is adsorbed onto the Si surface, it is subjected to a apparent relative dielectric constant of ~m expressed as: 1 + Y~'si ,Ueff - - _

_

I+T

where 3' is an influence coefficient of Si on adsorbed

Gas phase

(U) /

" "Solid'i" "/,

Fig. 1. Schematic sketch of a molecule (AB) consisting of atoms A and B adsorbed on the solid surface (Si), where the bond of the molecule is affected by the solid dielectric constant in addition to the gas phase dielectric constant.

47

molecules and ~si is the relative dielectric constant of Si. As a result, the bond energy of molecule AB and the energy level of electrons must get smaller by a factor of

Meanwhile, interaction between various Si surfaces and SiH4-type gas molecules is studied [4,5]. A perfectly-closed reaction system is prepared for this experiment by using a Hastelloy tube reactor of 1,/2 inch in diameter and 40 cm in length. Sill 4 gas is introduced to this reactor. S i l l 4 gas contains residual impurities of moisture of several parts per billion [6] and siloxane of five parts per billion. The gas delivery system is made of all-metal C r : O 3 passivated stainless steel pipes which are characterized by complete chemical stability for various specialty gases [7]. No contamination is introduced from this piping system. Three types of Si surfaces: non-dope surface, n + surface, and p+ surface, are prepared on the inner surface of the tube reactor by means of the thermal CVD method with various gases (Sill 4, S i l l 4 + PH 3, S i l l 4 + B2H~,) introduced. Every deposited Si thin film features (111) orientation. A predetermined gas is continuously introduced into the tube reactor with its inner surface covered with a predetermined thin film while the inner surface is not at all exposed to the air. Gas components produced as a result of interaction between the surface coated with film and the introduced gas are evaluated with the infrared Fourier transform spectrometer (FT-IR) or with the gas chromatography when they come out of the outlet of the reactor tube. The decomposition reaction induced by interaction between Sill 4 gas and various Si surfaces is assumed to be a first-order reaction for the purpose of simplifying discussion. The reaction formula is as follows:

/3

Sill 4 = Si(sol) + ozH(ad) + 2 n 2 ( g a s )

(S)

d[Sin4] d~-

(9)

K[SiH~]

where c~ + / 3 = 4 and K is the rate constant

T. Ohmi /Applied SurJace Science 121 / 122 (1997) 44 62

48

100ppm Sill4 on Various Si Surfaces

The rate constant ( K ) is usually expressed as: K=ae

E,/kT

Activation Energy T( ° C 500400 300 ~00

(10)

[ S i H 4 ] t o = [SiH4]o e - k ' "

( ] 1)

where t o is the residence time of S i l l 4 in the tube reactor. [SiH4] 0 stands for the initial concentration of S i l l 4 which is introduced into the tube reactor. The S i l l 4 concentration decreases in an exponential manner against the residence time (to). Experimental results demonstrate that the first-order reaction, Eqs. (8) and (9), is approximately correct [8,9]. The rate constant ( K ) of the S i l l 4 decomposition obtained from the experiments depends on the S i l l 4 concentration in Ar or N 2 carrier gas: it decreases as the S i l l 4 concentration rises. This is because hydrogen atoms generated in a S i l l 4 decomposition terminate the Si surface. Fig. 2 shows the rate constant ( K ) as a function of an inverse of temperature when an initial S i l l 4 concentration in Ar gas is set at 100 ppm. Surfaces to be interacted with S i l l 4 gas are hydrogen-terminated non-dope Si, n+Si which is scarcely terminated with hydrogen, and p+Si featuring a boron concentration of 1 × 102o cm -3. The p+Si surface is annealed at 600°C in an Ar gas ambience for a long time while the n+Si surface is treated at 850°C in an Ar gas ambience. Terminating hydrogen, therefore, is almost removed from these two surfaces. The S i l l 4 decomposition on the nondope Si surface and n+Si surface features almost the same rate constant. On the p+Si surface, S i l l 4 gas starts to decompose even at extremely low temperatures of 90°C to 100°C. On the non-doped Si surface and the n+Si surface, however, the S i l l 4 decomposition is detected only when the temperature exceeds 330-340°C. The reaction product of S i l l 4 decomposition on the p+Si surface is only H 2, which is also applied to the S i l l 4 decomposition on the non-doped Si surface and the n+Si surface. The activation energy is found to be 0.3 eV which is obtained from an Arrhenius plot to show the rate constant of S i l l 4 decomposition induced by interaction with the p+Si surface in the temperature range of 90°C to 200°C.

100 I

where A is an oscillation factor and E, is an activation energy The S i l l 4 concentration at the outlet of the tube reactor ([SiHa]t 0) can be expressed as:

p+ Si 100 non-dope ----II--

"G" '0

Si

n+ Si 10

C

u0 er

0.1

0.01 i 1

1.5

2

2.5

3

1000/'r(K) Fig. 2. Rate constant of SiH,~ decomposition at the surface of non-dope Si, n + Si and p+ Si is plotted as a function of an inverse of the temperature, where the rate constant is measured by using a tube reactor of 1/ 2" in diameter and 40 cm of length.

Around 400°C, the rate constant of S i l l 4 decomposition on the p+Si surface gradually gets closer to that on the non-dope surface and of the n+Si surface. The following two facts suggest that the S i l l 4 decomposition process on the p+Si surface is not induced by thermal energy but by a kind of catalytic effect of the p+Si surface: - S i l l 4 gets decomposed at extremely low temperature only on the p+Si surface. - Its activation energy is extremely low at 0.3 eV. S i l l 4 decomposition on the p+Si surface at low temperature can be explained in the following way. The electron to bind Si and H atoms in an S i l l 4 molecule, isolated in gaseous phase features an energy level of - 12.8 eV (ionization energy), but this energy level of electrons changes to - 5.45 eV when the S i l l 4 molecule is adsorbed on the Si surface. The electron is recombined with a hole on the top of the valence band ( - 5.15 eV) of the p + Si surface when

T. Ohmi/Applied Surface Science 121 / 122 (1997) 44 62

Distance from surface

49

Sill4 molecule

=_

©

//////~///////

/////////////

Solid surface

Vacuum level

Solid surface . . . . . . . . . . . . .

....................t.......i-

Ii

4.05 eV /

.......

5.45ev

~ 5.15eV

Ec

EA > 1.2 eV

~

Ei

e

Ev

Ea 0.30eV

q

IE > 6.2 6eV Ionization potential 12.8 eV

Fig. 3. Surface reaction model of adsorbed Sill 4 and p+ Si surface, where the energy level of the electron in Sill 4 is changed from - 12.8 eV to - 5.45 eV by receiving an influence of the Si dielectric constant.

100ppm Sill4 Thermal Decomposition V a r i o u s Si Surface(1/2"x40cm), 20cc/min, l ° C / m i n 150

I non-dope •

E O.

Si

p~lSi . . . . . Si-H • . . . . . . . . . . .S~2 ....... I

¢Z

v

~" 100 0

o

~ =-- : "

i- i ÷ i ~ ; ~ ~ . / , ~ o ,

"',

~ ~%

50

0

1 O0

200

300

400

500

Temp.(°C) Fig. 4. Temperature dependence of the Sill 4 concentration in Ar at the outlet of the tube reactor of 1/2" in diameter and 40 cm in length where the gas flow rate is 20 c c / m i n and the temperature is increased with l°C/min. The inner surface of the tube reactor is covered by non-dope Si, hydrogen terminated Si, p+ Si and SiO 2.

T. Ohmi/Applied Surface Science 121 / 122 (1997) 44-62

50

it gains thermal energy of 0.3 eV. As the electron which contributes to the S i - H bond in the Sill 4 molecule disappears due to recombination with the hole of the p+Si surface, the S i - H bond can be dissociated at low temperature: Sill 4 decomposition hardly requires thermal energy in this process. The energy level of an electron to bind Si and H is different by as much as 6.2 eV between when the Sill 4 molecule is isolated in the gaseous phase ( - 1 2 . 8 eV) and when the Sill 4 molecule is absorbed on the Si surface ( - 5 . 4 5 eV) (Fig. 3). In every gas molecule, its bond energy as well as the energy of its electron to contribute to the bond must get lower when it is absorbed onto the solid surface, due to the effect of the dielectric constant of the solid surface, than when it is isolated in the gaseous phase. It is essential to reveal not only, the behavior of molecules of specialty gases in the gaseous phase but also the behavior of those adsorbed on the substrate surface in order to scientifically understand

100ppm Si3H8 on Various Si Surfaces Activation Energy 400 300 T(2~"0/0°"~

i

I n o n - d o p e Si

C

O

t.) •

0.1

÷

---!1--

I

e,, ~3

1

Ea=0.20eV

0.01 t 1

2.5

3

various processes such as CVD (chemical vapor deposition) and RIE (reactive ion etching). This type of investigation is indispensable to develop a perfect computer simulation of the semiconductor manufacturing process. The Sill 4 molecule decomposition characteristics are plotted as a function of the tube reactor temperature as shown in Fig. 4 where the gas flow rate is maintained at 20 c c / m i n and the inner surface of the tube reactor is covered by non-doped Si, p+Si, hydrogen terminated Si and SiO 2. If low-temperature Sill 4 decomposition on the p+Si surface is attributed to the effective dielectric constant as speculated from Eq. (7), the experimental results give the following values for ~rr and y of Eq. (7):

non-dope Si

10

0.1

2

Fig. 6. Temperature dependence of rate constant of the Si3H s molecule decomposition at non-dope Si and p+ Si surface obtained in the same procedure as in Fig. 2.

100

'0

o

1.5

p+ Si

f

(,3

Ea=O.13eV

n-

1000/'r(K)

Activation Energy

100

e

10 IO

100ppm Si2H6 on Various Si Surfaces

f

p+ Si

100

0.01 =1

T(°C ) 500400 300 200

100

Ea=2.15eV

1.5

2 1000/T(K)

2.5

3

Fig. 5. The temperature dependence of the rate constant of Si2H 6 molecule decomposition at non-dope Si and p+ Si surface obtained in the same procedure as in Fig. 2.

12.8×

1 +y~si

y = 0.052

= 5.45 ~ eeff = 1.53, (12)

T. Ohmi/Applied Surfiwe Science 121 / 122 (1997) 44-62

It is found that the molecule adsorbed on the Si surface is subjected to an effective dielectric constant of 1.53. Figs. 5 and 6 show the Arrhenius plot of the rate constant when Si2H 6 and Si3H s interact with the non-dope Si surface and the p+Si surface. Concentration of both Si2H 6 and SiBH s in Ar gas is set at 100 ppm in these experiments. In the reaction with the non-dope Si surface around 400°C, as shown in Fig. 7, the rate constant gets higher in order of S i l l 4, Si e H 6, and Si3H s. In the low-temperature decomposition triggered by the interaction with the p+Si surface, however, the rate constant and the activation energy of Sill 4, Si=H 6, and Si3H s are almost the same. This indicates that a portion of - S i l l 3 in Si=H~ and Si3H ~ is adsorbed on the Si surface. Interaction between a metal surface such as a stainless steel surface and the Sill 4 molecule is also studied. In this experiment, S i l l 4 at a concentration of 100 ppm is introduced at a flow rate of 26.3

3O0

450

Temperature (°C) 400 350

I00

300

.11eV

©

2.0e Z O9

2:10 © © klJ

<~

5

ooo/-r (K) Fig. 7. Temperature dependence of rate constants of Sill 4, Si2H 6 and S i3 H s molecules decomposition interacting with the non-dope Si surface.

51

c c / m i n , into various metal tube reactors featuring a diameter of 1/4" and a length of 1 m. Four different metal tube reactors are used: 100% Ni, Hastelloy (Ni: 50%), SUS316L-EP (Ni: 12%), and Cr203 passivated stainless steel. Ar gas is used as a carrier gas. While Sill 4 is introduced, the temperature of the tube reactors is raised at a rate of 0.33°C/min. Fig. 8 shows the Sill 4 concentration detected at an outlet of the tube reactors. The Sill 4 decomposition characteristics totally depend on the metal surface which it interacts with, when the S i l l 4 concentration is set at the same level. The temperature to trigger Sill a molecule decomposition gets lower as the mixing ratio of Ni is raised. This indicates that Ni features very strong catalytic effects towards Sill 4 molecule decomposition. This is considered because the metal surface containing more Ni is harder to be oxidized even when it is exposed to the air.

3. Electronegativity and energy level: Change in state of electrons on surface due to terminating atoms The concept of electronegativity is often used when atoms are studied in chemistry. Study of metal contamination of the Si surface in solutions has revealed that those metal ions which feature higher electronegativity than Si, such as Cu, Pd, Hg, Ag, Pt, and Au, directly take electrons from Si to chemically bind with Si, and that they are extremely hard to be removed [10,11]. On the other hand, other metals which feature lower electronegativity than Si, such as Fe, Ni, Cr, Na, Ca, and K, do not directly bind with Si, but they are included in native oxides or chemical oxides formed on the Si surface because they are easier being oxidized than Si. These metals featuring lower electronegativity than Si can be easily removed with the diluted HF(DHF) treatment for oxide removal [ 10,11 ]. C u C l 2, FeCl 2, and NiC12 of 3 ppb each are injected to 0.5% HF for the purpose of contaminating the Si surface with Cu, Fe, and Ni. Cu features higher electronegativity than Si while Fe and Ni feature lower electronegativity than Si. This diluted HF solution of 25 cc each is spread on the entire surface of a 6-inch Si wafer which is displaced a little bit to concave shape by vacuum chuck. Then it

T. Ohmi / Applied Surjace Science 121 / 122 (1997) 44 62

52

S A M P L E T U B E : 1/4" X l m 120

0~oO' ~z~4020 60cn 'zl- ~ '~i%,iH`''tel'°y-EP I\~~7~],/' ;"" I", \iiIIc~2°3 0

........................."-"..............

50

1 O0

150

200 250 300 350 T E M P E R A T U R E (C)

400

450

500

Fig. 8. Temperature dependence of Sill 4 concentration in Ar at the outlet of the tube reactor having a diameter of 1/4" and a length of 1 m whose inner surface is pure Ni, electropolished hastelloy, electropolished SUS316L and Cr203, where the gas flow rate is 263 cc/min and the temperature of the tube reactor is raised at a rate of 0.33°C/min.

I015: • BARE Si (P) •Cu 5ppb/HF (0.5%)

10151

.AIR

i0'" -

10~4

• BARE Si (P) • Fe 5ppb/HF (0,5%) 'AIR

10~ ~

"BARE Si (P) T "'AIR Ni 3ppb/HF (0.5%)

10~'

i

Ni

Fe I lo=

Cl

I°"f ~

t

i0 TM

t", 4

1 0 II .

10"

WAFER/)

l

10"

]

0

I I I I I I I 10 20 50 4.0 50 60 70

CENTER

DISTANCE

(mrn)

FROM

CENTER

/) 0 10 CENTER

20 50 40 50 60 70

DISTANCE FROM

(ram)

CENTER

I

0

10 20 30 40 50 60 70 CENTER (mm) DISTANCE

FROM

CENTER

Fig. 9. Metallic impurities adhesion to the bare Si[p-Si(100)] surface and from contaminated diluted HF(0.5%) in air where the diluted HF, of 25 cc contaminated with 3 ppb Cu, Fe and Ni by CuCI 2, FeCI 2 and NiCI 2, is spread on the entire 6" wafer and evaporated by a halogen lamp. The surface concentrations of metals are measured by total X-ray reflection fluorescence. The horizontal axis is the distance in radial direction from the wafers center and the vertical axis is the surface metallic concentration. Fe and Ni each adhere only to the center of the wafer where they especially do not adhere to the peripheral area of the wafer.

T. Ohmi/Applied Surface Science 121 / 122 (1997) 44-62

is evaporated with halogen lamp irradiation to observe segregation of Cu, Fe, and Ni to the bare Si surface. Fig. 9 shows Cu, Fe, and Ni contamination on the vertical axis, and radial location on the Si wafer surface on the horizontal axis. Zero on the horizontal axis stands for the center of the wafer• As the DHF solution gets evaporated, its residual amount decreases, and its droplet gets smaller in diameter while shrinking toward the center of the Si wafer. Eventually the droplet of DHF remaining at the center of the Si wafer is evaporated, and the entire Si wafer surface gets dried. No oxide exists on the Si surface which is in contact with DHF, where hydrogen-terminated bare Si surface is maintained all the time. If metallic ions are more stable when they are absorbed on the Si surface than when they are dissolved in DHF, they must get absorbed onto the Si surface from the very beginning of this procedure. These metals must be found absorbed in an almost uniform way on the entire Si surface. On the other hand, if they are more stable when they are dissolved in DHF, they must remain in DHF solution until the end of the evaporation process. In this case, they

must be found absorbed around at the center of the Si wafer. Fig. 9 shows that Cu featuring higher electronegativity than Si is absorbed almost on the entire Si surface, and that Fe and Ni featuring lower electronegativity than Si are detected only around at the center of the Si wafer. When the same experiment is performed by using a Si surface covered with thermal oxide, selective adsorption of metals is not detected. Unlike the previous experimental result on the bare Si surface, Cu as well as Fe and Ni are found adsorbed just at the center of the wafer. In another test, a HC1 : H202 : H 2 0 (1 : 1 : 6) (HPM) solution to form chemical oxide on the Si surface is spread on the entire Si wafer so as to contaminate the Si surface with Cu, Fe, and Ni. As shown in Fig. 10, these three metals are found distributed on the Si surface in almost the same manner. On this contaminated wafer surface, high purity 0.5%HF solution of 25 cc having no metallic contamination is spread, and it is evaporated just in the same way as described above. Fig. 11 shows radial

10"

10"

• BARE Si (N) /HPM 3ppb (1:1:6) •Cu,Fe,Ni .AIR

i 10"

10" i~

1Oral

.BARE SI (N) ' Cu,Fe,Ni 3ppb /HPM (1:1:6) -AIR

•BARE Si (N) •Cu,Fe,Ni3ppb /HPM (1:1:6) 10 u

N

E ¢.3 oE

i A,RNi

Ou

(.0

10"

53

10 '~

Z o

CO 10,~ W -F

10 '~

10 z

I0'

I0"~

E3

J LU

WAFER f)

J I 0 10 CENTER

I 20

I 30

40

J 50

I 60

] 70

(ram) DISTANCE FROM CENTER

#

~[ I I 0 10 2 0 CENTER

I 30

I 40

I 50

I 60

] 70

(mm) DISTANCE FROM CENTER

#

I[ I I 0 10 2 0 CENTER

[ [ '30 4 0

I 50

I 60

I 70

(ram) DISTANCE FROM CENTER

Fig. 10. Metallic impurities adhesion to the bare Si[n-Si(100)] surface from HPM solution (HCI : H 2 0 2 : H 2 0 = 1 : 1 : 6) contaminated with 3 ppb Cu, Fe and Ni in air. The adhesion profile on the entire wafer surface is almost similar for Cu, Fe and Ni. Fe and Ni adhere even to the peripheral area in this case which are included in the chemical oxide formed in the HPM solution.

54

T. Ohmi / Applied Surface Science 121 / 122 (1997) 44-62

distribution of remaining Cu, Fe, and Ni. Fe and Ni included in chemical oxide are removed with HF solution together with the oxide, and they are detected around the center of the wafer surface. On the other hand, distribution of absorbed Cu remains unchanged. This indicates that metals featuring lower electronegativity than Si, even when they are included in oxide and remain on the Si surface, can be completely removed in the D H F treatment/'or oxide removal. It is also proved that metals featuring higher electronegativity than Si which form direct chemical bonds with the Si surface can not at all be removed in the D H F treatment. Cu included in oxide is dissolved once when the D H F solution etches the oxide, but it gets re-absorbed immediately when the bare Si surface is exposed. These experiments demonstrate the following: • Metallic ions featuring higher electronegativity than Si, such as Cu, Hg, Pd, Ag, Pt, and Au, chemically bind with Si by directly taking electrons from Si. • Metallic atoms featuring lower electronegativity than Si, such as Fe, Ni, Cr, Ca, K, and Na, are

I0~ l

10 ~

2

S

'HF (0.5%) AFTER HPM (1:1:6) BARE Si(N) • Cu;Fe,Ni 5ppb .ArR

\

10'~

AFTER ' HPM (1:1:6) • BARE Si(N) .Cu,ge,Ni 3ppb

10 ~'q

lo"

LLI

:E

.--

10~

± 10~

WAFER f)

.HF (0.5%)

f ("1 0~

0 I~

t <._J

O'B 't~,

• HF (0.5%)

AFTER • HPM 0:1:6) • BARE Si(N) • Cu,Fe,Ni 5ppb , AIR

• AIR

10" ' ~ 10,~

not directly absorbed onto the bare Si surface, but they are included in native oxide or chemical oxide formed on the Si surface as they are easier to be oxidized than Si. Researchers and engineers, whose background is the semiconductor field or the solid state physics field, are familiar with the concept of energy level. As shown in Table 2, the energy level of H and F is - 5 . 1 1 eV and - 9 . 4 2 eV respectively. If these elements are absorbed on the Si surface, the surface band bending shown in Figs. 12 and 13 is generated where the surface band bending of hydrogen terminated and fluorine terminated p type Si is illustrated. The energy level of H is almost the same as the energy at the top of the Si valence band while the energy level of F is deeper than that of the top of the Si valence band by 4.27 eV. When H or F terminates dangling bonds of the Si surface, terminating F atom captures many electrons in the valence band contributing to the S i - S i covalent bond, but terminating H does not capture them. In other words, electron transfer in the valence band is not triggered by hydrogen termination. On the surface where elec-

AFTER HPM

I I I I~ ] I I I 0 10 20 50 40 50 60 70 CENTER (mrn)

,

'l,

Fe

o,.

"' ........ " . . . . . . . . ~ AFTER HPM 10~

b,

Ni

1

AFTER HPM 10 II

l) 10 20 50 40 50 60 70 CENTER (mm)

0 10 E:O 30 40 50 60 70 CENTER (mm)

DISTANCEFROMCENTER DISTANCEFROMCENTER DISTANCEFROMCENTER Fig. l 1. Adhesion profile of metallic impurities of Cu, Fe and Ni before and after the pure diluted HF is spread on the entire wafer surface and evaporated by halogen lamp irradiation, where the wafer surface is initially contaminated by HPM solution in Fig. 10. Fe and Ni are removed by the diluted HF along with the oxide film removal but Cu can not be removed by the diluted HF. The dashed line indicates an initial contamination profile and the solid line indicates diluted HF treated profile.

T. Ohmi /Applied Sur/hce Science 121 / 122 (1997) 44-62

55

® ®® ®® ® -oO- %--d~%-cO-- % -

p-Si V a c u u m Level 1

o•

%

,,0

,o_%

%

a) H Terminated Si Wafer Surface

Conduction Band -4.05eV_ ~

©

(9 @ @ © ©

-

-

H .... ~ H -5.15eV H 5 1 leV Valence B a n d ~ •

©

0(9

@

°--

;'o; .0;;

Fig. 12. Band structure of hydrogen terminated Si surface.

trons exist in large volumes in the conduction band, such as the n + Si surface, the electrons in the conduction band shift toward terminating hydrogen. As shown in Fig. 14(a), the hydrogen-terminated Si surface, although it is a surface, features the same state of electrons as bulk Si crystals for valence

p-Si Vacuum Level t

/

Conduction

b) F Terminated Si Wafer Surface

Fig. 14. Electron state of hydrogen (a) and fluorine (b) terminated Si(100) surface.

electrons. In the case of the fluorine-terminated surface, as electrons are locally concentrated at terminating fluorine atoms, the Si surface terribly runs short of electrons contributing to the covalent bond of the Si crystal as shown in Fig. 14(b). This makes the Si-Si covalent bonds in the vicinity of the surface weaker. The author next describes how this difference in state of electrons on the surface caused by terminating atoms affects various surface reactions.

-4.05eV

4. Oxidation of Si surface at room temperature -5.15eV Valence Band

F 9.42eV

F

Fig. 13. Band structure of fluorine terminated p Si surface.

Even when 50 to 200 Si wafers are treated altogether in a batch process to form a thin oxide of less than 10 nm in thickness, it is required to maintain uniformity of film thickness on the entire wafer surface as well as among all these wafers. To meet this requirement, it is essential to perform wafer loading to oxidation furnace (a step to raise temperature) and wafer unloading (a step to decrease temper-

56

T. Ohmi/Applied Surface Science 121 / 122 (1997) 44-62

ature) in inert gas ambience, such as Ar or N 2 gas, completely excluding O 2 and H 2 0 which contribute to oxidation, and to start the oxidation process by introducing 02 or H 2 0 gas for a predetermined time only when the temperature of every wafer becomes just the same as every other one. If the bare Si surface completely free from oxides is exposed to elevated temperatures of over 600°C in Ar gas or other inert gases, however, hydrogen terminating the surface is removed and surface microroughness is extremely increased [12,13]. Once a monoatomic layer oxide is formed with 0 2 gas on the Si surface at 300°C at which terminating hydrogen is stable, however, surface microroughness does not show any increase even at 900°C in Ar gas, and extremely high-quality thin oxide can be formed [12]. Oxidation at 300°C allows terminating hydrogen to exist stable, and oxidation proceeds as oxygen penetrates into back bonds of Si. Monoatomic-layer oxide (0.4 nm in thickness) is formed on the hydrogen-

O x i d e t h i c k n e s s on F a n d H t e r m i n a t e d Si W a f e r 02

1 O0 %

10

25C25C 200C300C400C •

O

\

5

E

[]

_

F terminated

A

_

/

-v

H terminated

5. Selective W film deposition by means of (Sill 4 + W F 6)

2

u~ u) =~

.2

1

% % %

~0.5

A 0.3

H terminated

A A []

,

I

5

....

•

F terminated •

•

\~ ,

[]

[] []

0.2

0,1

terminated Si surface with 20 min oxidation in 02 gas ambience at 300°C whose thickness is evaluated by XPS [12,13]. When the surface is terminated with fluorine, electrons to contribute to the covalent bond of Si crystals are taken by terminating fluorine atoms, and the binding state of the Si surface gets weakened. This should mean that oxygen can easily penetrate into back bonds of Si at lower temperatures. Fig. 15 shows the time dependence of oxide formation on the hydrogen-terminated Si surface at room temperature, 200°C, 300°C, and 400°C as well as on the fluorine-terminated Si surface at room temperature. It is seen from Fig. 15 that the hydrogen-terminated Si surface is not oxidized even when it is exposed to the 0 2 gas ambience at room temperature for l04 rain. The fluorine-terminated Si surface is easily oxidized in the 02 gas ambience at room temperature. This is because the Si-Si covalent bond of the fluorine-terminated Si surface is weakened as shown in Fig. 14, which allows oxygen atoms to easily penetrate into back bonds of Si. The fluorine-terminated surface is obtained by removing the SiO 2 film on the Si surface with anhydrous HF gas at room temperature [14], where terminated fluorine is not removed even with thermal treatment at 930°C in an inert gas ambience.

o I

10

i

~

o ,

I

50

....

o i

I

100

2O0

Time (rain)

Fig. 15. Oxide thickness as a function of treatment time in dry 02 for various temperatures for hydrogen terminated and fluorine terminated Si(100) surface.

The W film deposition by means of Sill 4 reduction of WF6 can be selectively performed only on the Si or metal surface at low temperatures of 100200°C. What is the mechanism of this selective film deposition? Let us discuss the mechanism of the W film deposition on the hydrogen-terminated Si surface. As described in Section 2, Sill 4 is not decomposed at these temperatures on the hydrogen terminated Si surfaces. As Sill 4 is not decomposed at low temperatures of 100-200°C, W film deposition which is actually observed must be attributed to the decomposition of WF 6 triggered by its interaction with the hydrogen terminated Si surface. WF 6 has been demonstrated to decompose due to the interaction with the hydrogen terminated Si surface even at room temperature although it features a high bond

Z Ohmi/Applied Surface Science 121 / 122 (1997) 44-62

57

WF6 100ppm/Ar ON Si-H SURFACE RT, 10cc/min, 1/2"OD x 45cm 0.1

2316

2188

s_,.:~_

1029

~,

992

712

-~

]

5o

~o.o5 .~

r.=.~,.,,.-.9: , _ . _ _ . _ . , , . -.., o

.. _ _

,.z.:.:.t

/ ..................

"~-"-.-"-:~;

0

.......

:-;/ .... ~

100

......

200 TIME (min)

1

. . . . . . . . ~.-.-I o

300

o ~

~_

•

400

Fig. t6. Time variation of WF6, SiF4 and SiHF 3 concentration at the outlet of the tube reactor having a diameter of 1/2" and a length of 45 cm at room temperature whose inner surface is covered by hydrogen terminated non-dope Si. The flow rate of Ar, including 100 ppm WF0, is 10 c c / m i n . WF6 is confirmed to completely decompose at room temperature by interacting with the hydrogen terminated Si surface, while WF6 is detected at the outlet of the tube reactor after about 50 atomic layers deposition of tungsten on the Si surface.

Sill4 100ppm/Ar ON W-F SURFACE RT, 10cc/min, 1/2"OD x 4 5 c m 0.1 A

2316 SIHF3

2188 Sill4

................

1029 SiF4

=0111l=llJl

992 SiHF3

E

712 WF6 v

200

i.U

O Z < m

m 0

cl.

Z O t-

<

%

nFZ LU

%

0.05

% %

O3 m

% %

<

~tllltltl

I

I

I~lll

100 0Z O 0 "I-

0

a

0

100

=

|

200

t

I

=

I

300 400 TIME (min)

, ,~%1 ......

500

=.- . . . .

600

Fig. 17. Time variation of Sill 4 and SiF4 concentration at the outlet of the tube reactor whose inner surface is covered by fluorine terminated tungsten. The flow rate of Ar including 100 ppm Sill 4 is 100 c c / m i n . Sill 4 is confirmed to completely decompose at room temperature by interacting with fluorine terminated tungsten.

T Ohmi /Applied Surface Science 121 / 122 (1997) 44 62

58

energy. When W F 6 is introduced into the tube reactor whose inner surface is covered by non-dope hydrogen-terminated Si surface at room temperature, W F 6 is not detected at the outlet of this reactor, but SiF4 and SiHF 3 are detected (Fig. 16). This means that W F 6 is completely decomposed at room temperature on the hydrogen-terminated Si surface [15]. W F 6 starts to be detected at the outlet of the tube reactor only when the W film of about 50 atomic layers is deposited on the hydrogen terminated Si surface. This means W F 6 also gets decomposed on the W surface which is terminated with a mixture of hydrogen and fluorine. It has been found that S i l l 4 is decomposed even around room temperature on the fluorine-terminated W surface [16]. As shown in Fig. 17, S i l l 4 is completely decomposed on the fluorine terminated W surface, where the reaction product is SiF4. The W film deposited by means of the H 2 reduction reaction of W F 6 at the Si surface was not adopted in actual manufacturing processes due to the encroachment difficulty. This W film deposition process was found to consume Si at the surface at random, i.e., this is an origin of the appearance of encroachment. When W F 6 and S i l l 4 are used together [17,18] however, Si in the substrate surface is only consumed until the first W monoatomic layer is deposited. Afterward, Si in S i l l 4 is consumed in the reaction. Eq. (13) expresses the basic reaction mechanism: 3 • WI~"6 + 2 S I H 4 ~

3 . W ÷ ~S1F 4 4-

3H 2

(13)

SiHF 3 is sometimes included in the reaction byproduct. In this case, however, SiHF 3 is decomposed on S i O 2 to induce W deposition, which puts an end to selective film deposition on the Si and metal surfaces. It is not clearly understood why the fluorineterminated surface decomposes S i l l 4 at room temperature. Current speculation on this issue is as follows: Regardless of underlying substrate material, a fluorine atom terminating the surface attracts a large number of electrons due to its high electronegativity. Its surface status, therefore, is just like an oxide-free platinum surface having a strong catalytic behavior. Existence of a large number of electrons induces a shielding effect to make the Coulomb force of adsorbed molecules extremely low, which

(a) molecules in gas phase

GGQ

®®® Metal

(b) molecules absorbed on metal surface Fig. 18. Catalytic effect of metal surface and fluorine terminated surface for adsorbed molecules (AB). Electric force line distribution in a molecule (AB) in gas phase (a) and adsorbed on metal or fluorine terminated surface (b).

makes it possible to decompose adsorbed molecules by means of thermal energy at room temperature. Fig. 18 shows the decomposition and dissociation process of adsorbed molecules which is induced by catalytic effects of the fluorine-terminated surface and metal surface to weaken the bond energy of molecules. The mirror image of positive and negative charges in adsorbed molecules is generated on the surface, which makes part of the electric force lines in molecules to be terminated on the surface. This is why the bond energy of adsorbed molecules on the metal surface or on the fluorine terminated surface is weakened.

6. Redox potential and energy level of liquid chemical solution - native oxide formation on the Si surface and metal contaminant adhesion and r e m o v a l with solution The redox (reduction and oxidation) potential is a well-known quantity in the field of solution chemistry. A redox potential of 0 V is defined as the

T. Ohmi/Applied Surface Science 12l / 122 (1997) 44-62

P o t e n t i a l - pH D i a g r a m f o r t h e S y s t e m Cu - W a t e r , at 25°C Cu : l p p m in Water 2.5

V, V/; Y/":,~ //7//"/"/'// 7 7 / / , I

Dissolution

~//// ~,

Dis~o-:.

Y////>,;]

2

1.5 I..U 'I-

cu

Cu(OH)2

1

Z >

0.5

> 0

ILl -0.5

Ctl

"-(~).........

59

coexists however, Cu is not dissolved in solutions in an ionic form but adsorbed on the Si surface unless the redox potential is higher than a certain level. To be more specific, as shown in Fig. 20, the redox potential needs to be higher than + 0.85 V in HF and higher than +0.75 V in the other acid solutions in order to keep Cu dissolved in an ionic form [19,20]. As no oxide is formed on the Si surface in HF, the redox potential should be kept higher by 0.1 V than the other acid solutions. In other words, Cu adsorption is primarily suppressed due to oxidation of the Si surface in the other acid solutions [19,20]. It has been recently revealed that a redox potential of 0 V is equivalent to a energy level of - 4 . 4 4 eV [21]. A redox potential of +0.75 V is equivalent to a energy level of - 5 . 1 9 eV, which is equal to the energy level at the top of Si valence band. This triggers oxidation of the Si surface as solutions take

-1 -1.5

I

I

I

I

I

I

!

*

0

2

4

6

8

10

12

14

pH Fig. 19. Redox potential pH diagram for Cu. Cu is dissolved as a Cu 2+ ion into the solution having a redox potential larger than + 0 . 2 5 V and a pH less than 6.

potential of the reduction-oxidation reaction of a H 2 molecule in solution. Solutions which feature stronger oxidizing forces are expressed with positive redox potentials while those which feature stronger reducing forces are expressed with negative redox potentials. The 'redox potential - pH diagram' is popular in the inorganic chemistry, particularly in the field of metal surface corrosion, where this diagram has been used to fully study two-phase systems of metals and water solutions. The redox potential - pH diagram is effective in investigating metallic contamination on the Si surface. The diagram can not be applied as it is however, because this investigation deals with the three-phase system of metal, water solution, and the Si surface. Fig. 19 shows,the redox potential - pH diagram for Cu. In acid solutions featuring pH of 0 to 6 and redox potential of over + 0.25 V, Cu is dissolved in an ionic form such as Cu 2+. When the Si surface

k

E f f e c t o f P o t e n t i a l a n d pH o f S o l i t i o n s I

I onC ;ZT u,CTOV;;

z

>

............. o51-

"]

.t

J l .... I

-0.5

_;2

0

2

4

6

8

10

12

14

pH Fig. 20. Redox potential - pH diagram for Cu when the Si surface is introduced. The critical potential for Cu dissolution into acid solution as Cu 2+ ion is shifted to + 0 . 7 5 V from + 0 . 2 5 V due to the existence of the Si surface.

T. Ohmi/Applied Surface Science 121/122 (1997) 44 62

60

V a c u u m Level 0

4.0 4.5

-4,44

Conduction_ B _ _ ~

....................

-0.5

/ ~ N o n . ,Dope ~'Na2S203 4.6

0 "1" Iii Z 0.5 ~>

[]

>

5.0

ValenceBand5.15 G =2

5.5 6.O

....... ~

[]

.2-J~'"[ -~"

........ o ~ il........

[]

>

O~. ~,,~ U1O3 UPW

~\ U]H2SO4/O3

1.5

HCI/KCIO

6,5

t

I

I

I

I

I

I

I

I

0

2

4

6

8

10

12

14

2.0

pH UPW UPW UPW He] H2SO4HNO3 UPW (Low02) {High02) (Hig~H2)

•

®

•

3

o

±

Fig. 2l. Energy level ~ of solution converted from redox potential PRED by e = --(PRED +4.44) (eV) is plotted as a function of pH. The Horizontal dash lines are the conduction band energy ( - 4.05 eV) and the valence band energy ( - 5 . 1 5 eV) of Si. The energy level of the acid solution is deeper than the valence band energy.

electrons contributing to the Si covalent bond from the Si surface. The solution featuring the redox potential of PRED features the vacuum energy level ( e ) given by the following equation: = - (4.44 + PRED) (eV)

(14)

which can be considered as the Fermi level of solution. Fig. 21 shows the 'energy level - pH diagram', where the energy level of various cleaning solutions and ultrapure water are plotted. Dotted horizontal lines stand for an energy level at the bottom of the Si conduction band and an energy level at the top of the Si valence band. It is seen from Fig. 21 that the energy level of acid solutions featuring a PREP higher than + 0.75 V are deeper than the energy level at the top of the Si valence band. Electrons in the valence band contributing to the Si covalent bond, therefore, are taken by these acid solutions, i.e., the Si to Si bond at the surface gets weakened, and the Si surface

is easily oxidized. This is why chemical oxide is rapidly formed in acid solutions having a redox potential larger than +0.75 V and a relative dielectric constant of 81. In ultrapure water which comes in contact with the air and contains dissolved oxygen with its saturation solubility of 9 pprn, chemical oxide is formed on the Si surface. In ultrapure water from which dissolved oxygen is removed, on the other hand, chemical oxide is hard to be formed [22]. These facts have something to do with the oxidizing force (force to take electrons) of these solutions against the Si surface. It is revealed from Fig. 21 that the energy level of ultrapure water containing dissolved oxygen of 9 ppm and of 0 ppm is - 5 . 1 5 eV, same to the valence band energy level, and - 4 . 8 0 eV, higher than the valence band energy, respectively. The activation energy of normal high-temperature oxidation by means of dry oxygen is around 1.5 eV. From extrapolation on the basis of this activation energy, roomtemperature oxidation can form oxides of 10 n3 to 10 -14 nm in thickness in a few hours. Growth of chemical/native oxide features a totally different mechanism from that of high-temperature oxidation. The Si surface is oxidized at room temperature in the air [22]. This native oxide growth features the following mechanism: When being exposed to the air, the Si surface is covered with moisture molecules of several tens of molecule layers due to the moisture contained in the air [23]. This means that the Si surface is just in the same state as contacting with water having a dissolved oxygen of 9 ppm. Due to the very high relative dielectric constant of water of 81, Si-Si bonds on the surface get weakened. Besides, electrons contributing to the covalent bond are taken away by the solution, which further weakens Si-Si bonds on the surface. Consequently oxygen can easily penetrate into the back bond of Si. This is why room-temperature oxidation of the Si surface proceeds in the air. In the Si technology, metallic contaminations such as Cu on the Si surface have been removed by SPM cleaning ( H z S O 4 / H 2 0 , 4"1, 120-150°C) accompanying with too much chemical vapor generation and too much liquid chemical consumption. This high concentration chemical solution has been proved to be required to obtain a higher redox potential or lower energy level to take electrons from the surface

T. Ohmi / Applied Surface Science 121 / 122 (1997) 44-62

contaminated metals to dissolve them into the solution as metal ions. Ozonated ultrapure water having a ppm order 03 concentration has been confirmed to exhibit very high redox potentials such as + 1.30 V, which corresponds to the energy level of - 5 . 7 4 eV, as shown in Fig. 21. Thus, ozonated ultrapure water has been speculated to have metal contamination removal capability from the Si surface instead of the SPM solution. This speculation has been demonstrated experimentally as show in Fig. 22, where an initial Cu contamination of up to 1015 atoms/cm 2 is reduced down to the order of 10 l° atoms/cm 2 by ozonated ultrapure water cleaning as well as SPM cleaning. The remaining Cu of 10 l° atoms/cm 2 is included in the chemical oxide tbrmed in ozonated ultrapure and SPM solution, so that this remaining Cu contamination is completely removed by FPM cleaning (HF/H202/H20, 0.05-0.5%: 0.1-1%, room temperature) as shown in Fig. 22. This new wet cleaning technology of the Si surface has been established based on these research results.

61

7. Conclusion This article discusses the energy level of a solid, electronegativity of atoms, and redox potential of solutions in an integrated way in order to reveal the possibility of electron transfer through the surface. Now that technologies have progressed highly enough to realize a surface completely free from contamination both in gaseous and liquid phases, this type of integrated approach makes sufficient sense. This study presents a way to discuss issues of the surface exposed to gaseous and liquid phases in context of two factors: (1) dielectric constant of the solid itself and media contacting the surface, (2) transfer of electrons contributing to the Si covalent bond to the energy level of atoms terminating the surface and the redox potential (energy level, Fermi level) of solutions contacting the surface. The author has also made it possible to understand an interaction between molecules and the solid surface by considering the change of bond energy of molecules and the energy level of electrons in

Contaminated in DHF Solution 1E+I8

Cz n-type (100) Si Substrate

e,l ,~

Initial Contamination

1E+I6

Cleaning

DHF lmin DIW 10min DHF (Cu:lppm) 3mi~ DIW 10min

~_~ 1 E + 1 4 -

Cleaning 10miv DIW 10min ~2Dry

1E+12

1E+I0

1E+8 Contaminated Wafer

Ozonized DIW

SPM

FPM

Fig. 22. Cu cleaning capability of ozonated ultrapure water from the Si surface is illustrated as well as the SPM ( H 2 S O J H 2 0 2 ) cleaning capability. Initial Cu contamination (1015 atoms/cm 2) is decreased down to 10 I° atoms/cm 2 by ozonated ultrapure water and SPM cleaning. The remaining 10 t° atoms/cm 2 Cu contamination is included in chemical oxide, which can be removed by FPM ( H F / H 2 0 2 / H 2 0 ) cleaning.

62

72 Ohmi /Applied Suttee Science 121 / 122 (1997) 44-62

m o l e c u l e s a d s o r b e d on the solid surface. O z o n i z e d ultrapure w a t e r f e a t u r e s a r e d o x potential o f + 1.30 V, i.e. an e n e r g y level o f - 5 . 7 4 eV. T h i s m e a n s o z o n i z e d ultrapure w a t e r f e a t u r e s a s t r o n g e r oxidizing force than H 2 8 0 4 , H N O 3, a n d HCI, all o f w h i c h feature r e d o x p o t e n t i a l s o f a b o u t 1.0 V. O z o n a t e d u l t r a p u r e w a t e r c a n not o n l y r e m o v e o r g a n i c impurities in a s h o r t time, b u t c a n also easily d i s s o l v e a n d remove metals featuring higher electronegativity than Si f r o m the Si surface such as Cu. T h e wet c l e a n i n g o f the Si surface will face a drastic c h a n g e [24]. B y m a k i n g these p h e n o m e n a m u c h clearer, it will b e p o s s i b l e to s i m u l a t e the b e h a v i o r o f the s e m i c o n d u c t o r surface in g a s e o u s p h a s e a n d in solutions. E v e n t u a l l y e x t r e m e l y e f f e c t i v e s e m i c o n d u c t o r prod u c t i o n will b e r e a l i z e d b y u s i n g virtual factory. T h e Si t e c h n o l o g y a n d the s e m i c o n d u c t o r t e c h n o l o g y are n o w c h a n g i n g a n d p r o g r e s s i n g to a n e w scientific stage w h e r e s e m i c o n d u c t o r m a n u f a c t u r i n g t e c h n o l o gies are p r o v i d e d o n scientific u n d e r s t a n d i n g of m e c h a n i s m s o f all processes.

References [1] N.D. Lang, W. Kohn, Phys. Rev. B 3 (1971) 1215. [2] H. Kikuyama, M. Waki. I. Kawanabe, M. Miyashita, T. Yabune, N. Miki, J. Takano, T. Ohmi, J. Electrochem. Soc. 139 (1992) 2239. [3] H. Kikuyama, M. Waki, M. Miyashita, T. Yabune, N. Miki, J. Takano, T. Ohmi, J. Electrochem. Soc. 141 (1994) 366. [4] T. Ohmi, M. Nakamura, A. Ohki, K. Kawada, K. Hirao, T. Watanabe, in: Proc. Abstracts, 23rd Annual Meeting of the Fine Particle Society, vol. 2, Las Vegas, 1992, p. 103. [5] T. Watanabe, M. Nakamura, A. Ohki, K. Kawada, S. Miyoshi, S. Takahashi, M.S.K. Chen, T.Ohmi, Extended Abstracts, 1992 Int. Conf. on Solid State Devices and Materials, Tsukuba, 1992, p. 132.

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