Real dimensional simulation of anisotropic etching of silicon in CF4+O2 plasma

Vacuum 66 (2002) 39–47

Real dimensional simulation of anisotropic etching of silicon in CF4+O2 plasma R. Knizikevic$ iusa,*, A. Galdikasa,b, A. Grigonisa a

Department of Physics, Kaunas University of Technology, 73 K. Donelai$cio st., LT-3006 Kaunas, Lithuania b Lithuanian Energy Institute, 3 Breslaujos st., LT-3035 Kaunas, Lithuania

Abstract The reactive ion etching (RIE) of silicon in CF4+O2 plasma is considered. The profiles of etched grooves are calculated as a function of mask dimensions, fluxes of chemically active and non-active plasma components and parameters of ion bombardment. To achieve this goal the chemical composition of CF4+O2 plasma is calculated and the one-dimensional plasmochemical etching (PCE) of Si in this plasma is considered. The values of phenomenological constants are found by extrapolation from experimental results. Using values of phenomenological constants, found by analysis of chemical composition of plasma and one-dimensional etching, the etched groove profiles at real dimensions are calculated. Special attention is given to the etching anisotropy and lateral etching. The influence of oxygen addition to fluorocarbon plasma on etched groove profile is considered. The conditions under which anisotropic etching prevails are found. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: CF4+O2 Plasma; Silicon; Reactive ion etching

1. Introduction Silicon integrated circuits are most widespread today. Silicon is easily processed compare to other semiconductor materials (GaAs, InP), and its oxide is used as an insulator. Control of reactions and their kinetics is very important using plasma etching methods. During radical, ion, electron and photon bombardment of solids following processes take place [1]: adsorption of radicals, reactions to form intermediate or stable products, desorption, influence of ion bombardment on the *Corresponding author. Fax: +370-7-350737. E-mail address: [email protected] (R. Knizikevi$cius).

processes listed above, particle reflection during ion bombardment, ion implantation and production of defects, sputtering, diffusion (on the surface, through the reaction layer, ion enhanced diffusion), redeposition of desorbed products, surface roughening, electron induced desorption, electron emission, photostimulated processes, metastable induced processes. Many of these processes influence the etching rate. An increase of performance rate of integrated circuit can be achieved: (1) by use of metals with low resistivity for interconnections (for example, replacing aluminum by copper [2]), (2) by decrease of geometrical dimensions of feature (now grooves with width 0.18 mm are etched [3]). In the last case, the etching anisotropy must be as high as possible.

0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 4 1 8 - 3

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R. Knizikevi$cius et al. / Vacuum 66 (2002) 39–47

Elimination of reactions at the sidewall or exact balancing of etching and deposition rates without ion bombardment gives possibility to achieve the straight sidewall. The O2 addition to CF4 plasma is used to control the lateral etching of silicon. As O2 content in the feed increases the oxidation processes become dominant. The SiOxFy layer is formed on the surface [4]. Fluorine atoms react with Si to form SiFx (xp4). Some of them react with oxygen atoms on the Si surface to become SiOxFy. The etching rate of SiOxFy strongly depends on energy of bombarding ions. At high O2 content in the feed etching rates at the sidewall vanish. While at the groove bottom, subjected to ion bombardment, the etching rates are high. A better understanding of etched groove shape dependences on O2 content in the feed and mask dimensions will allow more efficient integrated circuit manufacture. In previous work [5,6], the etching of Si trough a mask in fluorine-based plasma was considered. However, the values of frequency probabilities used in these models were freely choosen and the profiles of etched grooves were calculated in relative units. In this work, the main reactions occurring in a CF4+O2 plasma are considered and the concentration of plasma components are calculated by extrapolation from experimental data. Using the derived composition of the flux of particles from the plasma, one-dimensional etching of Si is investigated. The values of frequency probabilities, obtained from analysis of one-dimensional etching, are used for the calculation of the real dimensions of etched grooves.

2. Modelling of plasma composition In Table 1, the homogeneous reactions, taking place in a CF4+O2 plasma, in a general case are presented. In order to reduce the number of frequency probabilities only the main reactions are included in the model: CF4 þ e-CF3 þ F þ e;

ð1aÞ

O2 þ e-2O þ e;

ð1bÞ

Table 1 List of values of enthalpies and free Gibbs energies of reactions, taking place in the CF4+O2 plasma Reaction

DH (kcal/mol)

DG (kcal/mol)

CF4+e2CF3+F+e CF3+e2CF2+F+e O2+e22O+e 2CF3+M2C2F6+M 2F+M2F2+M CF3+O2-COF2+OF CF2+O2-COF+OF CF3+O-COF2+F CF2+O+M-COF2+M CF3+OF-COF2+2F CF2+OF-COF2+F COF+F+M-COF2+M COF2+e-CO+F2+e COF2+O-CO2+F2 CO+O+M-CO2+M O+F+M-OF+M OF+O-O2+F 2OF-O2+2F

123.7 104.6 119.2 71.6 37.8 2.5 18.5 76.7 181.3 31.7 136.3 125.6 125.3 2.0 127.3 45.0 74.2 29.2

112.6 94.1 110.8 F 29.6 4.3 F 76.4 170.5 37.7 131.8 F 115.2 1.7 116.9 38.7 72.1 33.4

CF3 þ F þ M-CF4 þ M;

ð1cÞ

CF3 þ 2O-CO2 þ 3F;

ð1dÞ

2F þ M-F2 þ M;

ð1eÞ

2O þ M-O2 þ M:

ð1fÞ

Reaction rates are characterized by frequency probabilities of dissociation Gi and reaction Rij : G1 ¼ gCF4 þe-CF3 þFþe ½e=N;

ð2aÞ

G2 ¼ gO2 þe-2Oþe ½e=N;

ð2bÞ

R34 ¼ kr;CF3 þFþM-CF4 þM ½M=N;

ð2cÞ

R35 ¼ kr;CF3 þ2O-CO2 þ3F ;

ð2dÞ

R44 ¼ kr;2FþM-F2 þM ½M=N;

ð2eÞ

R55 ¼ kr;2OþM-O2 þM ½M=N;

ð2fÞ

where gi is the dissociation rate constant, N is the total neutral particle concentration in the plasma, kr;i is the reaction rate constant, and M is a third particle or wall of the reactor. Equation (1d) is a resultant reaction, which includes sequence of reactions, during oxidation of CF3 radicals in the

R. Knizikevi$cius et al. / Vacuum 66 (2002) 39–47

plasma. The values of enthalpy and free Gibbs energy of this reaction are DH=40.9 kcal/mol and DG=48.5 kcal/mol. Let us assume that all the neutral components of the plasma are pumped out from the reactor with the same frequency probability of exhaust E at a fixed O2 content in the feed. The concentrations of electrons and ions in the plasma are not taken into account, as the ionization degree of plasma does not exceed 1–2%. It follows that seven chemical species exist in the plasma: CF4, O2, CF3, F, O, CO2 F2 with relative concentrations n1=[CF4]/N, n2=[O2]/N, n3=[CF3]/N, n4=[F]/N, n5=[O]/N, n6=[CO2]/N, and n7=[F2]/N, respectively. The following system of rate equations describes the kinetics of chemical composition of a CF4+O2 plasma: dn1 ¼ I1  G1 n1 þ R34 n3 n4  En1 ; dt dn2 ¼ I2  G2 n2 þ R55 n25  En2 ; dt dn3 ¼ G1 n1  R34 n3 n4  R35 n3 n25  En3 ; dt dn4 ¼ G1 n1  R34 n3 n4 þ 3R35 n3 n25  2R44 n24  En4 ; dt dn5 ¼ 2G2 n2  2R35 n3 n25  2R55 n25  En5 ; dt dn6 ¼ R35 n3 n25  En6 ; dt dn7 ¼ R44 n24  En7 ; ð3Þ dt where Ii ¼ Fi =V0 N is the injection rate of ith component of gas mixture, Fi is the flow rate of ith component, and V0 is the volume of the reactor.

3. Modelling of one-dimensional etching The one-dimensional PCE of a Si substrate in a CF4+O2 plasma is considered. The composition of plasma is calculated from the above model of plasma composition. When a substrate is immersed in the plasma, O atoms produced by dissociation of O2 molecules react with Si atoms: Si+2O-SiO2. Phenomenological constant k2 characterizes the reaction rate. F atoms produced by dissociation of CF4 molecules and oxidation of

41

CF3 radicals (Eqs. (1a) and (1d)) react with Si and SiO2: Si+4F-SiF4 and SiO2+4F-SiF4+O2. Phenomenological constants k1 and k3 characterize these reaction rates, respectively. The SiF4 molecules formed leave the surface. A frequency probability of desorption o is equal to o ¼ n0 expðEd =kTÞ;

ð4Þ

where n0 is the frequency of oscillation of atoms in the solid, and Ed is the activation energy of desorption. It follows that three components exist on the surface: Si, SiO2, and SiF4 with relative concentrations c1=[Si]/C, c2=[SiO2]/C, and c3=[SiF4]/C (C is the concentration of surface atoms C=1.36  1019 m2). The relative concentrations 3 P must fulfill the following condition ci ¼ 1: We i¼1

do not include O2 molecules on the surface, as activation energy of O2 desorption is much lower than for SiF4. The following system of balance equations describes the kinetics of surface concentrations of components:   dc1 ¼  k1 n4F þ k2 n2O c1 þ oc3 ; dt dc2 ¼ k2 c1 n2O  k3 c2 n4F : ð5Þ dt The solutions of the system of linear and homogeneous equations (5) have the following form: ci ðtÞ ¼

3 X

  Aij exp lj t ;

ð6Þ

j¼1

where Aij is the coefficient, defined from the initial conditions, lj is the solution of the characteristic equation. The stationary surface concentrations of components are equal to R3 o ; ð7aÞ c1;st ¼ R3 ðR1 þ R2 Þ þ oðR2 þ R3 Þ c2;st ¼

R2 o ; R3 ðR1 þ R2 Þ þ oðR2 þ R3 Þ

ð7bÞ

c3;st ¼

R3 ðR1 þ R2 Þ ; R3 ðR1 þ R2 Þ þ oðR2 þ R3 Þ

ð7cÞ

where R1 ¼ k1 n4F ; R2 ¼ k2 n2O ; and R3 ¼ k3 n4F : The etching rate is proportional to the removal rate of formed SiF4 molecules. According to

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R. Knizikevi$cius et al. / Vacuum 66 (2002) 39–47

Eq. (7c) the etching rate at the steady-state regime is equal to Vst ¼ h0 oc3;st ¼

h0 R3 oðR1 þ R2 Þ ; R3 ðR1 þ R2 Þ þ oðR2 þ R3 Þ

ð8Þ

( is the thickness of one monowhere h0=2.72 A layer.

4. Modelling of two-dimensional etching The etching of grooves on a silicon substrate in a CF4+O2 plasma is considered. In the calculation of profiles of etched grooves, the results obtained from previous models of chemical composition of plasma and one-dimensional etching of Si are used. As etching takes place through a mask, the number of atoms, reaching the surface, depends on the position of an arbitrary point M on the surface (Fig. 1). Two-dimensional etching has some peculiarities, related to the geometry of the mask. At first, the assumption that the mean free path of particles arriving at the surface is much larger than feature size was taken into account. This assumption is always fulfilled during etching in the lowpressure plasma. The second assumption is that the angular distribution of particles in the plasma is isotropic. During two-dimensional etching the

concentrations of atoms at the surface of an etched groove depend on the position of the arbitrary point Mðx; yÞ (Fig. 1). Let us assume that the concentration of atoms is equal to ni ðx; yÞ ¼ n0;i Yðx; yÞ=p; where n0;i is the relative concentration of the ith plasma component, and Yðx; yÞ is the angle. The groove bottom is subjected to ion bombardment, and the processes of activation of surface particles and physical sputtering take place there. The activation of Si atoms and SiO2 molecules on the surface are defined by the following n reactions: Si þ CFþ x þ e-Si þ CFx and SiO2 þ n þ þ CFx -SiO2 þ CFx ; which are characterized by the frequency probabilities of activation G1 ¼ gSi I0 =C;

ð9aÞ

G2 ¼ gSiO2 I0 =C;

ð9bÞ

where gi is the activation constant of the ith surface component and I0 is the ion flux. The activated particles have excess of energy but not enough to leave the surface. The activated surface particles relax: Sin -Si and SiOn2 -SiO2 : These processes are characterized by frequency probabilities of relaxation Rr;3 ¼ 1=tSin ;

ð10aÞ

Rr;4 ¼ 1=tSiOn2 ;

ð10bÞ

where ti is the mean relaxation time of ith component. Let us assume that Rr;3 ¼ Rr;4 ¼ R: The sputtering of the Sin atoms, SiOn2 ; and SiF4 molecules is characterized by frequency probabilities of sputtering oi ¼ Yi I0 f ðaÞ=C;

ð11Þ 0:5

where Yi is the sputtering yield, f ðaÞ ¼ cos a is the sputtering yield dependence on the incident ion angle a measured from the surface normal [7]. It follows that the frequency probability of removal of SiF4 molecules consists of frequency probabilities of sputtering and desorption o5 ¼ o5;s þ o5;d ¼ YSiF4 I0 f ðaÞ=C þ n0 expðEd;SiF4 =kTÞ

Fig. 1. The schematic presentation of RIE through the mask.

ð12Þ

The activated silicon atoms and SiO2 molecules more intensively react with fluorine and oxygen atoms, and the following reactions take place:

R. Knizikevi$cius et al. / Vacuum 66 (2002) 39–47

1.0

Relative concentration

Si*+4F-SiF4, Si*+2O-SiO2 and SiOn2 þ 4F-SiF4 þ O2 : Phenomenological constants k4 ; k5 and k6 characterize reaction rates, respectively. It follows that five components exist on the surface: Si, SiO2, Si*, SiOn2 ; and SiF4 with relative c2=[SiO2]/C, c3 ¼ concentrations   c1=[Si]/C,  Sin =C; c4 ¼ SiOn2 =C; and c5=[SiF4]/C. The relative concentrations must fulfill the following 5 P condition: ci ¼ 1: The following system of

43

F 0.8

O

0.6

0.4

0.2

i¼1

balance equations describes the kinetics of surface concentrations of components:   dc1 ¼  G1 þ k1 n4F þ k2 n2O c1 þ ðRr þ o3 Þc3 dt þ o 4 c4 þ o 5 c5 ;   dc2 ¼ k1 c1 n4F  G2 þ k3 n4F c2 þ k4 c3 n4F þ Rr c4 ; dt   dc3 ¼ G1 c1  k4 n4F þ k5 n2O þ Rr þ o3 c3 ; dt   dc4 ¼ G2 c2  k6 n4F þ Rr þ o4 c4 : ð13Þ dt The etching rate V is proportional to the sum of removal rates of Si* atoms, SiOn2 and SiF4 molecules: Vst ¼ h0 ðo3 c3 þ o4 c4 þ o5 c5 Þ:

ð14Þ

Knowing the surface concentrations of components from the solutions of Eq. (13), the etching rate as a function of coordinate and time is obtained. The point Mðx; y; tÞ after time interval Dt will take position M 0 ðxþDx; yþDy; tþDtÞ; where Dx ¼ Vnx Dt and Dy ¼ Vny Dt; nx and ny are the components of the surface normal n% (Fig. 1). In this way, the groove profile as a function of time is obtained.

5. Results and discussions 5.1. Chemical composition of plasma The experimental results presented in Fig. 2 [8] are used in the model in order to calculate the chemical composition of a plasma. It was observed that the maximum concentration of F atoms is achieved for 20%. The conditions during experiments [8] were the following: the plasma was

0.0

0

10

20

30

40

50

O2 content in the feed, % Fig. 2. Experimental [8] (points) and theoretical (curves) dependences of the relative concentration of F and O atoms on O2 content in the feed.

created in a cylindrical 130 ml volume Al2O3 reactor, the flow rate was 30 cm3/min at standard conditions, the pressure was 133 Pa. From these data, assuming that temperature of plasma T=500 K (the typical temperature in such experiments is 400–600 K), the total neutral particle concentration N and total gas injection rate I are estimated and was found to be N=1.93  1022 m3 and I=5.47 s1, respectively. The total gas injection rate was estimated using expression I ¼ F=ðV0 NÞ; where the flow rate is measured in molecule/s. The plasma composition is calculated using Eq. (3). Experimental [8] and theoretical dependences of the relative concentration of F and O atoms in the plasma on O2 content in the feed are shown in Fig. 2. The following values of frequency probabilities of dissociation and reaction were found by extrapolation from experimental results: G1=0.8 s1, G2=20 s1, R34=200 s1, R35=2.0  104 s1, R44=0 s1, R55=0 s1. It is important to note that the values of frequency probabilities R44 and R55 are equal to zero, i.e. the reactions defined by Eqs. (1e) and (1f) did not took place, during the experiment. The chemical composition of the plasma as a function of O2 content in the feed calculated using Eq. (3) is presented in Fig. 3. The concentration of F atoms with increase of O2 content in the feed increases first due to the reaction of O atoms with CF3 radicals (Eq. (1d)). As the concentration of F

R. Knizikevi$cius et al. / Vacuum 66 (2002) 39–47

44

100

100

Concentration, %

F x4

Conversion coefficient, %

CF4 80

O

60

40

CF3 x4

20

0

0

O2

CO2 x4

20

40

60

80

100

O2

1

80

2 3

60

40

CF4

20

0

1 2 3

0

20

40

60

80

100

O2 content in the feed, %

O2 content in the feed, %

Fig. 3. The dependences of the concentrations of CF4+O2 plasma components on O2 content in the feed.

Fig. 4. The dependences of the conversion coefficients of CF4 and O2 molecules on O2 content in the feed at different flow rates F (in cm3/min): 1–15, 2–30, and 3–60.

atoms approaches the maximum value, almost all CF3 radicals have reacted. With further increase in O2 content in the feed the concentration of F atoms starts to decrease in proportion to the amount of injected CF4 molecules. The maxima for the dependences of CO2 molecules and F atoms are achieved at the same value of O2. This is caused by the reaction of oxygen atoms with CF3 radicals (Eq. (1d)). The dissociation rate constant of O2 molecules is greater than for CF4 [9,10]. As a result of intensive dissociation the concentration of O atoms in CF4+O2 plasma is much higher than the concentration of O2 molecules. The conversion coefficient of the ith component of the gas mixture indicates a part of molecules converted to reaction products and is equal to

calculate Si etching rate. In the calculation of etching rates the results obtained from the previous modelling of plasma composition (Fig. 3) are used. Experimental and theoretical calculated using Eq. (8) Si etching rate dependences on O2 content in the feed are shown in Fig. 5. The following values of phenomenological constants and frequency probability of desorption are determined: k1=2.0  106 s1, k2=4.0  104 s1, k3=4.0  104 s1, o=250 s1. When Si etching rate is expressed as a function of concentration of F atoms, the dependence shown in Fig. 6 is obtained. It is observed that a fixed concentration of F atoms in the plasma corresponds to two different values of Si etching rates, which depend on O2 content. It is due to the formation of sufficiently high amount of SiO2 molecules on Si surface. As F atoms react with SiO2 molecules more slowly than with Si atoms, the etching rate is reduced at high O2 content in the feed. When phenomenological constants and frequency probability of desorption are found, it is possible to analyze the silicon etching in the ambient of F atoms only. In this case, the stationary etching rate is calculated using expression Vst ¼ h0 R1 o=ðR1 þ oÞ: Using L’ Hospital’s rule, the maximum etching rate at a fixed temperature is be expressed as

Zi ¼ 1  Eni =Ii :

ð15Þ

The dependences of the conversion coefficients of CF4 and O2 molecules on O2 content in the feed at different flow rates are presented in Fig. 4. The amount of converted molecules increases with decrease of flow rate. The increase of the conversion coefficient of CF4 molecules at low O2 content in the feed (o20%) is related to the reaction of O atoms with CF3 radicals. 5.2. One-dimensional etching The experimental results presented in Fig. 5 (points) [8] are used in the model in order to

h0 k1 n4F o ¼ h0 o: nF -N k1 n4 þ o F

Vmax ¼ lim

ð16Þ

R. Knizikevi$cius et al. / Vacuum 66 (2002) 39–47

3.5

45

1.0

3.0

0.8

2.5 Si=(2.16

V/ Vmax

V, m/min

0.6

2.0

± 0.40)·10 -2

0.4

1.5 0.2

1.0 0.0

0.5

0.0

0.0

0

10

20

30

40

50

O2 content in the feed, % Fig. 5. Experimental [8] (points) and theoretical (curves) dependences of the Si etching rate on O2 content in the feed.

0.2

0.4

0.6

0.8

1.0

Concentration of F atoms, x10 22 m-3

Fig. 7. The dependence of Si etching rate on concentration of F atoms at a fixed substrate temperature. Vmax =40.8 mm/min.

many silicon atoms are removed from the surface by one incident F atom, is calculated. The obtained value of reaction constant is equal to e=(2.1670.05)  102. The value of reaction constants e coincide with experimental one [11]: eexp =4.0  103 defined at 1001C. The concentration of F atoms was varied in the interval of (1.6– 7.7)  1021 m3 during the experiment. Moreover, in work [12] the reaction constant defined at temperature 1001C is equal to eexp =1.6  102. 5.3. Two-dimensional etching Fig. 6. Experimental [8] (points) and theoretical (curve) dependences of Si etching rate on relative concentration of F atoms. Arrows indicate the direction of increasing O2 content in the feed.

It is observed that maximum etching rate Vmax depends only on frequency probability of desorption o: Calculated ratio V =Vmax is equal to V 1 ¼ : Vmax 1 þ o=k1 n4F

ð17Þ

The dependence of Si etching rate on concentration of F atoms at a fixed substrate temperature is shown in Fig. 7. In interval of (0.05–0.95)Vmax the etching rate almost linearly depends on concentration of F atoms. In this interval the dependence of etching rate on concentration of F atoms is fitted line and the reaction constant e; which show how

The chemical composition of plasma and the values of phenomenological constants, found by analysis of one-dimensional etching, are used for the calculation of real etched grooves profiles. The evolution of groove profile during RIE is shown in Fig. 8. It is observed that at initial stages the etching process is mainly influenced by PCE. The influence of ion bombardment is more pronounced at later stages. The following values of phenomenological constants and frequency probabilities were used: k1=2.0  106 s1, k2=k3=4.0  104 s1, k4=2.0  107 s1, k5=k6=4.0  105 s1, G1=40 s1, G2=60 s1, R=1.0  102 s1, o3= 10 s1, o4=2.0 s1, o5,d=250 s1, o5,s=2.3  103 s1. Phenomenological constants k4, k5 and k6 characterize the reactions of reactive atoms with activated particles on the surface. The values of these constants are assumed to be ten

R. Knizikevi$cius et al. / Vacuum 66 (2002) 39–47

46

9 8 7

Aspect ratio

6 5 4 3

RIE

2

PCE

1 0 0.2

0.4

0.6

0.8

1.0

Mask width, m

Fig. 8. The evolution of groove profile during RIE. Mask width is 0.5 mm, mask height is 0.1 mm, and etching time is 40 min. The groove profile is shown every 8 min.

6

45

4

PCE (x5) 0.7

3 2

0.6

RIE 1

0.5

0 0

20

40

60

80

100

O2 content in the feed, %

Etching anisotropy

5

PCE / RIE

0.8

50

Groove depth, m

Groove width, m

0.9

Fig. 10. The dependences of aspect ratio on mask width. The groove depth is 0.5 mm (PCE) or 2.5 mm (RIE), mask height is 0.1 mm, and O2 content in the feed 13%.

40 35 30 25 20 15 10 5 0.2

0.4

0.6

0.8

1.0

Mask width, m

Fig. 9. The dependences of the depth and width of etched grooves on O2 content in the feed. Mask width is 0.5 mm, mask height is 0.1 mm, and etching time is 40 min.

Fig. 11. The dependence of etching anisotropy on mask width during RIE. The groove depth is 2.5 mm, mask height is 0.1 mm, and O2 content in the feed 13%.

times higher than ones with non-activated particles. The values of frequency probabilities G1, R, o3, and o5,s were choosen taking into account the results of the analysis of ion beam assisted etching [13]. The dependences of the depth and width of etched grooves on O2 content in the feed are shown in Fig. 9. The maximum depth of etched groove during RIE is achieved for pure CF4. As O2 content in the feed increases, the depth of etched groove decreases due to the oxidation process. The width of etched grooves for both PCE and RIE reaches the maximum value at the maximum concentration of F atoms in the plasma.

The aspect ratio ðymax =2xmax Þ describes the shape of the etched groove. The dependences of the aspect ratio on mask width at a fixed depth of etched grooves during PCE and RIE are shown in Fig. 10. The aspect ratio during RIE at low values of mask width is quite high. As mask width increases, the aspect ratio decreases due to the intensive PCE. The etching anisotropy is equal to the ratio of etching rates in the vertical and horizontal directions. The dependence of the etching anisotropy on mask width at the fixed depth of etched groove is shown in Fig. 11. It is observed that the depth grooves with a high value of etching anisotropy are etched at low values of mask width.

R. Knizikevi$cius et al. / Vacuum 66 (2002) 39–47

2. The aspect ratio and etching anisotropy increase with decrease of mask width. 3. Calculated value of reaction Si+4F-SiF4 constant e, which show how many silicon atoms are removed by one incident fluorine atom, coincide with one measured experimentally and is following: eSi =(2.1670.05)  102.

50

40

Etching time, min

47

RIE 30

20

10

PCE 0 0.2

0.4

0.6

0.8

1.0

Mask width, m

Fig. 12. The dependences of etching time on mask width. The groove depth is 0.5 mm (PCE) or 2.5 mm (RIE), mask height is 0.1 mm, and O2 content in the feed 13%.

In integrated circuit manufacture it is required to determine the etching time needed to etch a fixed groove depth. The dependences of the etching time on mask width are shown in Fig. 12. During PCE narrow grooves are etched very long. Meanwhile during RIE due to ion bombardment the etching time needed to etch the fixed groove depth is constant at low values of mask width.

6. Conclusions 1. For O2 content in feed greater than 20%, the etching anisotropy increases due to the decrease of the concentration of fluorine atoms in the plasma.

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