5.1. Physical Vapour Deposition (PVD)

 \*z0\$!z,.+!//z+"zz/! z %!(!0.%z)0!.%(z !,+/%0%+\*\_z!w-!)z!2,+.0%+\*z\* z,100!.¥ ing have been intensively used. The basis different between these two methods is the step coverage: e-Beam shows negligible step coverage while Sputtering produce a film with good /0!,z+2!.#!z/z/\$+3\*z%\*z%#1.!zF^z+.)((5z,1.!z)!0(z(%'!z%\_z0\_z1\_z%\_z(z.!z !,+/%0¥ ed by e-beam evaporation method followed by an oxidation at suitable temperature.

In the e-beam evaporation method, a focused electron beam is used to heat a metal target to evaporate. In the high vacuum chamber, the evaporated metal radiates out from the metal target of which some portion is deposited on the mounted substrate. Generally, the target is placed at bottom and substrate is placed at the top of the vacuum chamber as shown in %#¥ ure 4. A method for producing highly pure, thin oxides is to evaporate metal by electron beam (e-beam) which is highly controllable to small thickness, and to oxidize the deposited metal by ozone or UV assisted oxidation. The advantage of this process is that it produces (!//z )#!z0\$\*z+4% !z/,100!.%\*#z\* z/\$+1( z,.+ 1!z0\$!z,1.!/0z+4% !^z-10z%0z%/z\*+0z\*z!4¥ act method for commercial production.

Figure 4. Schematic diagram of e-Beam evaporation system

layer of AlN/SiO2uIw%z3/z,.!/!\*0! z%\*zc-

deposited oxides are never as good as compared to grown one.

 \*z0\$!z,.+!//z+"zz/! z %!(!0.%z)0!.%(z !,+/%0%+\*\_z!w-

fect when 100 Å SiO2 layer was used.

214 Physics and Technology of Silicon Carbide Devices

5.1. Physical Vapour Deposition (PVD)

5. Depositions method

ing Sputtering

%/!.%z^\_zECCCdz\* z.!2!(/zz(+3z\$.#%\*#z!"¥

(b) (a)

!)z!2,+.0%+\*z\* z,100!.¥

SiO2 can be thermally grown by Thermal oxidation and this growth processes have the great advantage. In similar way, high *K* dielectric must be grown/deposited. It is well known that

ing have been intensively used. The basis different between these two methods is the step coverage: e-Beam shows negligible step coverage while Sputtering produce a film with good /0!,z+2!.#!z/z/\$+3\*z%\*z%#1.!zF^z+.)((5z,1.!z)!0(z(%'!z%\_z0\_z1\_z%\_z(z.!z !,+/%0¥

Figure 3. /;@=E9LA;K@GOAF?KL=H;GN=J9?=9HGGJKL=H;GN=J9?=MKAF?==9EE=L@G<:?GG<KL=H;GN=J9?=MKs

In the e-beam evaporation method, a focused electron beam is used to heat a metal target to evaporate. In the high vacuum chamber, the evaporated metal radiates out from the metal target of which some portion is deposited on the mounted substrate. Generally, the target is placed at bottom and substrate is placed at the top of the vacuum chamber as shown in %#¥ ure 4. A method for producing highly pure, thin oxides is to evaporate metal by electron beam (e-beam) which is highly controllable to small thickness, and to oxidize the deposited metal by ozone or UV assisted oxidation. The advantage of this process is that it produces

ed by e-beam evaporation method followed by an oxidation at suitable temperature.

Sputtering is generally used to deposit refractory materials, compound and alloys, which are difficult to evaporate by e-Beam method. Sputtering exists in the category of the Physical Vapor Deposition (PVD) process, in which metals are removed from the solid cathode. The whole process is carried out by bombarding the cathode with positive ions emitted from ..!z#/z %/\$.#!^z\$!\*z%+\*/z3%0\$z\$%#\$z'%\*!0%z!\*!.#5z.!z%\*% !\*0z+\*z0\$!z0\$+ !\_z0\$!z/1¥ sequent collision knocks loose or sputters atoms from materials. The schematic of Sputtering system is given in Figure 5. Its advantage is that it is broadly available and can produce pure oxides. Its disadvantages are that oxides are insulators so sputtered oxides tend to have plasma-induced damage. Also, PVD methods deposit in line of sight, so they do not give good coverage.

Figure 5. Schematic diagram of sputtering system

#### 5.2. Chemical Vapour Deposition (CVD)

\$!)%(z 2,+1.z !,+/%0%+\*z cdz \* z0+)%z5!.z!,+/%0%+\*z cdz .!z,.!"!..! z%\*¥ dustrial method to deposit gate dielectrics. CVD involves the formation of a thin solid gate dielectric on a desired substrate by a chemical reaction of vapour-phase precursors. In CVD process generally a volatile metal compound as a precursor is introduced into the process chamber/tube and oxidized during deposition onto the desired substrate. CVD is widely 1/! z%\*z0\$!z!(!0.+\*%/z%\* 1/0.5z"+.z)+/0z+"z%\*/1(0+.z !,+/%0%+\*^z 0z#%2!/zz+\*"+.)(z+2!.¥ age even though a three dimension shapes because it is not just line of sight. The other major advantage is that the deposition rate is controllable over a wide range from very slow to high. It can thus be distinguished from physical vapour deposition (PVD) processes, such as evaporation and reactive sputtering, which involve the adsorption of atomic or molecular species on the substrate. The chemical reactions of precursor species occur both in the gas phase and on the substrate. Reactions can be promoted or initiated by heat (thermal CVD), higher frequency radiation such as UV (photo-assisted CVD) or plasma (plasma-enhanced CVD). Figure 6 shows the schematic diagram of horizontal CVD reactor.

Figure 6. Schematic diagram of CVD system

5.3. Atomic Layer Deposition (ALD)

z3/z !2!(+,! z0+z.!+2!.z0\$!z/\$+.0+)%\*#\_z3\$%\$z.%/!z 1!z0+zz,.+!//^zz,.+¥ duce a highly conformal, pinhole-free highly insulating film. It has many advantages over CVD method like able to grow the thinnest films even though all deposition methods, and the most conformal films even into deep trenches. Atomic layer deposition is a method of cyclic deposition and oxidation. In this process, the desired surface is exposed to the suitable precursor, which is further absorbed as a saturating monolayer. The rest of the precursor is then purged from the tube/chamber by passing Ar/N2 gas. A pulse of oxidant such as H2O2, ozone or H2\_z%/z0\$!\*z%\*0.+ 1! z%\*z0\$!z\$)!.u01!\_z3\$%\$z)1/0z0\$!\*z"1((5z+4% %6!z0\$!z ¥ sorbed layer to the oxide and a volatile by-product. The excess oxidant is then purged by a pulse of Ar, and the cycle is repeated. Figure 7 represent a cyclic process of ADL press. ZrO2 and HfO2 was shown as example in figure7. Slow growth rate is a major disadvantage of this process but some time it is very useful to control the thickness of films. It has been seemed that some impurities like Cl, C and H also introduce in the film during deposition process, depending on used precursor. A compatible annealing methodology is needed to remove such type of impurity and densify the deposited oxides films. ALD is an excellent method for producing many high *K* oxides. An addition of an oxide layer, which is usually much less than an atomic layer thick in each and every cycle of ALD process adds an oxide

Materials and Processing for Gate Dielectrics on Silicon Carbide (SiC) Surface

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217

Figure 6. Schematic diagram of CVD system

Plasma

\$!)%(z 2,+1.z !,+/%0%+\*z cdz \* z0+)%z5!.z!,+/%0%+\*z cdz .!z,.!"!..! z%\*¥ dustrial method to deposit gate dielectrics. CVD involves the formation of a thin solid gate dielectric on a desired substrate by a chemical reaction of vapour-phase precursors. In CVD process generally a volatile metal compound as a precursor is introduced into the process chamber/tube and oxidized during deposition onto the desired substrate. CVD is widely 1/! z%\*z0\$!z!(!0.+\*%/z%\* 1/0.5z"+.z)+/0z+"z%\*/1(0+.z !,+/%0%+\*^z 0z#%2!/zz+\*"+.)(z+2!.¥ age even though a three dimension shapes because it is not just line of sight. The other major advantage is that the deposition rate is controllable over a wide range from very slow to high. It can thus be distinguished from physical vapour deposition (PVD) processes, such as evaporation and reactive sputtering, which involve the adsorption of atomic or molecular species on the substrate. The chemical reactions of precursor species occur both in the gas phase and on the substrate. Reactions can be promoted or initiated by heat (thermal CVD), higher frequency radiation such as UV (photo-assisted CVD) or plasma (plasma-enhanced

CVD). Figure 6 shows the schematic diagram of horizontal CVD reactor.

Figure 5. Schematic diagram of sputtering system

216 Physics and Technology of Silicon Carbide Devices

5.2. Chemical Vapour Deposition (CVD)

RF/DC Signal

Vacuum Pump

Counter Electrode

Wafer

Target

Sputter magnet

Reactive Gas

#### 5.3. Atomic Layer Deposition (ALD)

z3/z !2!(+,! z0+z.!+2!.z0\$!z/\$+.0+)%\*#\_z3\$%\$z.%/!z 1!z0+zz,.+!//^zz,.+¥ duce a highly conformal, pinhole-free highly insulating film. It has many advantages over CVD method like able to grow the thinnest films even though all deposition methods, and the most conformal films even into deep trenches. Atomic layer deposition is a method of cyclic deposition and oxidation. In this process, the desired surface is exposed to the suitable precursor, which is further absorbed as a saturating monolayer. The rest of the precursor is then purged from the tube/chamber by passing Ar/N2 gas. A pulse of oxidant such as H2O2, ozone or H2\_z%/z0\$!\*z%\*0.+ 1! z%\*z0\$!z\$)!.u01!\_z3\$%\$z)1/0z0\$!\*z"1((5z+4% %6!z0\$!z ¥ sorbed layer to the oxide and a volatile by-product. The excess oxidant is then purged by a pulse of Ar, and the cycle is repeated. Figure 7 represent a cyclic process of ADL press. ZrO2 and HfO2 was shown as example in figure7. Slow growth rate is a major disadvantage of this process but some time it is very useful to control the thickness of films. It has been seemed that some impurities like Cl, C and H also introduce in the film during deposition process, depending on used precursor. A compatible annealing methodology is needed to remove such type of impurity and densify the deposited oxides films. ALD is an excellent method for producing many high *K* oxides. An addition of an oxide layer, which is usually much less than an atomic layer thick in each and every cycle of ALD process adds an oxide layer, justify its nomenclature. ALD is usually carried out on a native oxide (SiO2) surface followed by ozone cleaning of SiC surface. This limits the crucial lowest EOT that ALD can ,.!/!\*0(5z 00%\*^z w0!.)%\*0! z /1."!\_z 3\$%\$z .%/!/z 5z 0\$!z w(!\*%\*#z 0.!0)!\*0z ,.+!¥ dure, is not favorable surface. It was observed that ALD of HfO2 and ZrO2 from chloride or other organic precursors do not easily nucleate on HF treated SiC surface.

6. Electrical behavior of dielectric material

where V(x) is the potential in the oxide at position X.

Gupta, S.K. 2010b

6.1. Direct tunneling

The quality of dielectric material can be electrically characterized by current-voltage (I-V) and capacitance-voltage (C-V) technique. In order to employ the above techniques, the grown/deposited layers on SiC should be sandwiched between two metal electrodes. A different type of current conduction mechanisms were observed and presented in this section, based on dielectric thickness and applied electric field across electrode. In this section SiO2 z3/z+\*/% !.! z /z z#0!z %!(!0.%z+\*z %z /1."!^z1,0\_z ^^z \* z\$%/z .!¥ search group are continuously working to investigate the current conduction mechanism and charge management of gate dielectric material and SiC system (Gupta, S.K. 2010a

Materials and Processing for Gate Dielectrics on Silicon Carbide (SiC) Surface

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Schrödinger equation describes that there is a finite probability that a particle can tunnel through a non-infinite potential barrier. As the width of potential barrier decreases, the ,.+%(%05 z +" z ,.0%(!/ z c!(!0.+\*/ z \* z \$+(!/d z ,!\*!0.0%\*# z 0\$.+1#\$ z 0\$! z ..%!. z 5 z -1\*¥ tum-mechanical tunneling, rises exponentially. In sufficiently thin oxides (below 5 nm), %.!0z-1\*01)z)!\$\*%(z01\*\*!(%\*#z0\$.+1#\$z0\$!z,+0!\*0%(z..%!.z\*z+1.^z\$%/z-1\*¥ tum-mechanical phenomenon can easily be understood by recognizing that the electron or hole wave function cannot immediately stop at the barrier (SiO2/4H-SiC interface), but rather it decreases exponentially into the barrier with a slope determined by the barrier height. If the potential barrier is very thin, there is non-zero amplitude of the wave "1\*0%+\*z .!)%\*%\*#z 0z 0\$!z!\* z +"z 0\$!z..%!.z)!\*/z z\*+\*w6!.+z,.+%(%05z "+.z 0\$!z!(!¥ tron or hole to penetrate the barrier. It is well known that the barrier heights of hole tunneling in the SiO2 layer from the metal gate and from the Si substrate are higher than the corresponding values for electrons, moreover, the hole mobility in SiO2 is lower than the electron mobility, therefore the main contribution to conduction in SiO2 is due to electrons. Since in n-type 4H-SiC mobility of electrons is much higher than that of hole, therefore, the described conduction mechanism in case of Si can be fully applied to 4H-SiC. The metal/SiO2 and SiO2/4H-SiC interfaces are at the position X = 0 and X = toxt respectively, in our notation. Voxt= V (0)-V (toxt) is the voltage drop in the oxide layer,

At low gate voltages (figure 8 (a)), electrons can move from the gate metal through SiO2 0+z 0\$!zGw%z/1/0.0!z+\*(5z5z 01\*\*!(%\*#z %.!0(5z 0\$!z!\*0%.!z+4% !z 0\$%'\*!//z%^!^z5z 01\*¥ \*!(%\*# z 0\$! z 0.,!6+% ( z ,+0!\*0%( z ..%!. z !03!!\* z #0! z \* z Gw% z /1/0.0!^ z \$! z -1\*¥ tum-mechanical phenomenon of a trapezoidal barrier tunneling is termed as direct tunneling effect. It contributes significantly to the conduction through the SiO2 only in ultra thin oxide layers (toxtTzHz\*)d^z0z\$%#\$!.z#0!z2+(0#!z c"%#1.!zKz cdd\_z 0\$!z\* z!\* ¥ ing causes the potential barrier shape to become triangular. Electron tunnel from the gate to the SiO2 conduction band, through the triangular potential barrier and finally, moves in the SiO2 conduction band to the 4H-SiC substrate. The conduction mechanism

; Gupta, S.K. 2011b ; Gupta, S.K. 2012).

;

219

Figure 7. Schematic of the cyclic process of Atomic layer deposition process
