Model-based Etch Profile Simulation of PSM Films...In the next simulation, normal vertical etching of both the photoresist and chrome occurs. The starting resist profile has The starting

Model-based Etch Profile Simulation of PSM Films

Michael Grimbergen, Madhavi Chandrachood, Jeffrey Tran, Becky Leung, Keven Yu, Amitabh Sabharwal and Ajay Kumar

Applied Materials, 974, E. Arques Avenue, Sunnyvale, CA 94085

ABSTRACT For advanced binary and PSM mask etch, final profile control is critically important for achieving desired mask specifications. As an aid to attain profile control, an etch profile simulation method has been developed. The method starts with an initial photoresist profile and incorporates etch rate and directionality information to predict the final etch profile. In this paper, simulated results are compared to measured etch profiles for PSM substrates. The results highlight the importance and implications of incoming resist profile and etch selectivity on final profile. Key Words: photomask, mask etch, etch simulation, binary, phase-shift, photoresist, etch profile

INTRODUCTION As the drive to reach smaller technology nodes continues, thinner photoresists and new etch materials are being introduced to achieve greater resolution. Attainment of that resolution requires precise etch profile control. In order to optimize profiles of newly introduced PSM (phase shift mask) etch structures, it is helpful to gain further understanding of the basic processes governing PSM photomask etching. Greater knowledge of the role of different plasma characteristics affecting chrome and MoSi (molybdenum silicide) etch profiles can lead to improvements in reactor design. In addition, assessment of the impact of incoming photoresist profiles also can aid in the selection of optimal process conditions. In the literature various models have been developed for silicon etch applications. Early models (such as Coburn and Winters) described the fundamental etch mechanisms. 1,2 Singh developed a model for fluorine etching of Si using a linear combination of reactive ion etching and chemical etching. 3 Levinson applied this model to chlorine etching of Si.4 Toward the goal of understanding PSM etch, we developed an etch simulation model for photomasks based on these prior efforts. In this paper we present predictions of the numerical etch model. Limiting cases are considered to illustrate its validity. Results show the effects of varying flux (etch rate) conditions and the initial photoresist (PR) profile. Model etch parameters can be described in a dimensionless way to match experimental etch profiles as a function feature height. We first outline the basis of the model, and then compare predicted behavior to measured cross-sections. Example evolutionary profiles for current PSM (Cr and MoSi) applications are presented.

THEORY

The geometry of the PSM etching feature is shown schematically in Figure 1. A masking layer, usually photoresist (PR), covers the chrome and MoSi layers. The coordinate system is oriented so that the trench lies parallel to the x-axis. A surface element is shown with an azimuthal angle θ and a rotational angle φ for the purpose of calculating solid angles. The neutral flux J and ion angular distribution functions i(θ) are shown schematically above the mask.

Photomask Technology 2013, edited by Thomas B. Faure, Paul W. Ackmann, Proc. of SPIE Vol. 8880, 88800X · © 2013 SPIE · CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2029006

Proc. of SPIE Vol. 8880 88800X-1

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z

y

x

θ

φPRCrMoSi

i(θ)J

Figure 1. Geometrical layout of feature being etched. The y and z component local ion fluxes seen by the etch surface element for this geometry are (1) (2) where i(θ) is the angular ion density distribution. The y and z components of the elemental local neutral radical flux at the surface are (3) (4) where J is the neutral flux density on a surface without shadowing. The etch front can be considered as a series of contiguous small elements. The movement of the etch front depends on the ion and radical flux. The two terms can be linearly summed, where N is the ratio of radical to ion flux, and Γ is the total flux. They can be separated into two coordinate directions as (5) (6) The proportionality relation is shown because the quantitative etch rate will also depend on the yield per ion and kinetic factors such a sticking coefficient and surface transport. Since the quantitative measurement of system parameters such as ion and neutral fluxes, and ion energy distribution are difficult to make in an etch chamber, they are estimated for these studies from the measured etch rates. The surface elements were advanced by calculating the flux components in each dimension on each element by a cellular arrangement, then iteratively repeating the procedure, which is outlined in block form in Figure 2.

∫Ω=)(

2 )(cossinx

y iddI θφθφθ

∫Ω=)(

)(cossinxz iddI θθθφθ

∫Ω=)(

2 cossin1xy Jddj φθφθ

π

∫Ω=)(

cossin1xz Jddj θθφθ

π

NJNI

dtdz

NJNI

dtdy

zzz

yyy

++

=Γ∝

++

=Γ∝

1

1



Ion flux

Ion distribution Function (IAD)

Neutral radicalFlux

Feature geometry

Ion Solid angle CalculationΩion(X)

NeutralSolid angle Calculation Ωneutral(X)

ElementalIon fluxCalculationΓion(y,z)

ElementalNeutral fluxCalculationΓneutral(y,z)

Y-Component ElementalFlux – etch rateCalculationΓy(y,z)

Z-component ElementalFlux – etch rateCalculationΓz(y,z)

ProfileEvolutionCalculationP(y,z,t)

Iteration

Final Profile

Model Inputs

Figure 2. Computational procedure for profile simulation.

MODEL PREDICTIONS To test the model, some limiting cases were verified with a stack of PR/Cr/MoSi. The fluxes on the photoresist were artificially kept at zero to not allow any etching of the photoresist for this test. Figure 3 shows three snapshots of the graphical output of the model. The top snapshot shows the film stack before etching, with photoresist partially covering the blanket PSM Cr and MoSi layers. The photoresist profile was chosen to have a representative shape, with different slopes near the top and bottom of the film. On the left side of Figure 3 (a), the chrome was allowed to etch in the limiting case of neutral radical etching only, producing a spherical undercut profile, as expected. On the right side of Figure 3 (b), the chrome was etched only by a flux of ions, thus producing a profile due to ion shadowing.

PR

Cr

MoSi

Plasma

Pre-Etch

Radical EtchingOnly

Ion EtchingOnly

a) Model showing chemical etch behavior

b) Model showing ion etch behavior

Figure 3. Initial profile before etch (top), and limiting cases of neutral-only etching (a), and ion-only etching (b). In the next simulation, normal vertical etching of both the photoresist and chrome occurs. The starting resist profile has been chosen to have a 20-degree top slope, followed by a main slope of 88 degrees. In the comparison of Figure 4, the left side (4a) has only vertical etching, while the right side (4b) shows the effect of 1:15 lateral-to-vertical etching. The slightly darker pink regions outline the original profile before etch. The lateral component of etching in (4b) produces slight taper in the upper half of the (incomplete) etch profile.



I

1

PR

Cr

MoSi

Etched PR

Lateral shiftand tapering

Figure 4a and 4b. Vertical etching only (left), and vertical combined with 1:15 lateral etching. The next calculation shows the predicted effect of a foot at the base of the starting resist profile. The left illustration (5a) has an 88-degree photoresist profile with no foot, while the right illustration (5b) shows a small incoming photoresist foot. The resulting etch shows some taper in the chrome due to lateral etching in both cases, but a small chrome foot is seen on the right (5b). With additional etching time, the chrome footing would be reduced (not shown).

Incoming PR

Final PR Incoming foot

Cr Foot

Figure 5a and 5b. Effect of near-vertical PR profile (left) and PR foot (right) on Cr etch. In a similar way, less-than-vertical resist profiles can also taper the chrome profile. Figure 6a and 6b show the predicted profile with minimal over etch for the case of two different initial slopes. Notably increased tapering can be seen in the top half of the chrome layer for the 80-degree case. The degree of tapering can also be affected by surface transport and re-deposition conditions, which are not included here.

80o88o

Figure 6a and 6b. Effect of initial photoresist profile slope for slope of 88 degrees (left) and 80 degrees (right). Oval highlight indicates tapered region. Once the chrome is etched, the model can be used to predict MoSi etch profiles given initial profiles for the chrome layer, and photoresist layer, if present. As an example of MoSi etching, the lateral etching effect was considered and can be seen in Figure 7. Decreasing lateral etching makes the MoSi slope more vertical, as can be seen in the Figure, in which lateral-to-vertical ratios are varied from 1:10 to 1:30.



i

Cr

MoSi

Lateral:Vertical 1:10 1:15 1:30

Figure 7 a,7b,and7 c.. Effect of decreasing lateral etch rate on MoSi profiles.

COMPARISON WITH ETCH RESULTS

To check the model’s ability to predict chrome etch profile evolution, cross-sections of a mask were taken at various times in the chrome etch process. The images in Figure 8 show the cross-section at four different etch times and their corresponding predicted profiles. The blue band in the snapshots is the material etched. At time t1, about 60% of the chrome film is etched, showing a very gradual profile. At time t2, only a small amount of film remains in the open areas and the top of the profile has become more vertical. By time t3, the open area is fully etched and the profile has become less curved. At a later time t4 the profile has sharpened up additionally with the top portion of the profile nearly vertical.

t3t2t1 t4

PRCr etched

Etch Time

Figure 8. Chrome etch evolution and predicted profiles. With additional process time to complete the etch, the chrome profile becomes more vertical. Figure 9 shows a cross-section with the photoresist stripped, together with a modeled near-vertical profile.

Figure 9. Chrome final cross-section and model profile.



An example of PSM MoSi etch is shown in Figure 10. In the MoSi cross-section on the left, the chrome layer has been removed. The interface between the MoSi and the substrate is not visible due to the coating required to make the image. In the simulation on the right, the underlying substrate is not shown. Both cross-section and model show a matching sloped profile.

MoSi

Substrate

Cr

MoSi

Figure 10. PSM MoSi cross-section (left) and model profile (right).

CONCLUSIONS

A new model for PSM etch profile evolution has been presented. The model includes the effects of simultaneous photoresist etching on chrome etching, and chrome etching during MoSi etching. The model was tested under various conditions. The predicted time evolution for chrome etching was compared with experimental cross-sections at several time intervals. The effects of lateral etching and varying the ratio of vertical-to-lateral etch ratio on chrome and MoSi profiles were presented. The impact of initial photoresist profile was also seen to play a role in the resulting chrome profile. The model can be extended to new materials such as multilayer chrome absorber layers and to multilayer MoSi attenuation layers.

REFERENCES [1] Coburn, J. W., and Winters, H. F., “Plasma etching: a discussion of mechanisms,” J. Vac. Sci. Technol., 16, 391 (1979) [2] Jewett, R.E., Hagouel, P.I., Neureuther, A.R., and van Duzer, T., “Line Profile Resist Development Simulation Techniques,” Polym. Eng. Sci. 167, 381 (1977) [3] Singh, V.K., Shaqfeh, E. S. G., and McVittie J.P., ”Simulation of etch profile evolution in silicon reactive ion etching with re-emission and surface diffusion,” J. P., J. Vac. Sci. Technol. B 10, 1091 (1992) [4] Levinson, J. A., Shaqfeh, E. S. G., Balooch, M., and Hamza, A.V., “Ion-assisted etching and profile development of silicon in molecular chlorine,” J. Vac. Sci. Technol. A 15, 1902 (1997)



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Model-based Etch Profile Simulation of PSM Films...In the next simulation, normal vertical etching of both the photoresist and chrome occurs. The starting resist profile has The starting