PHOTOLITHOGRAPHY USING THE AERIAL ILLUMINATOR IN A VARIABLE NA WAFER STEPPER

Richard Rogoff, Guy Davies, Jan Mulkens, Jos de Klerk, Peter van Oorschot,ASM Lithography,

De Run 1110, 5503 LA Veldhoven. The Netherlands.

Gabrielle Kalmbach, Johannes Wangler, Wolfgang Rupp,Carl Zeiss, Oberkochen, Germany.

This paper was first presented at theSPIE Symposium on Microlithography,

Santa Clara, March 10-15, 1996.

1

PHOTOLITHOGRAPHY USING THE AERIAL ILLUMINATOR IN A VARIABLE NA WAFER STEPPER

Richard Rogoff, Guy Davies, Jan Mulkens, Jos de Klerk, Peter van Oorschot,ASM Lithography,

De Run 1110, 5503 LA Veldhoven. The Netherlands.

Gabrielle Kalmbach, Johannes Wangler, Wolfgang Rupp,Carl Zeiss, Oberkochen, Germany.

ABSTRACT

This paper shows the suitability of i-line photolithography for production at 0.30 µm. The process performance isdemonstrated through the use of off-axis illumination, high NA projection lens, and a state of the art photoresist system.

The minimum required depth of focus for a suitable 0.30 µm process is derived as 0.95 µm over at least a 10% processwindow. This will result in a 0.60 µm common corridor over a square 22 mm imaging field.

In addition to the dense and isolated lines, a preliminary investigation of contact hole performance using chrome andphase shift masks was completed.

1. INTRODUCTION

Process requirements for the latest generationproducts (i.e. 64 MB DRAM and high speed, (133) MHzmicroprocessors) demand enhanced lithographicresolution, while maintaining depth of focus and processlatitude. Unfortunately, smaller dimensions result in atighter process window. This is due to reductions in depthof focus and process latitude. The switch from i-line toDUV wavelength can overcome this problem but thetechnical and cost implications restrict the introduction ofthis technology for all but the most critical productionlevels. For this reason the extension of i-line technologyhas positive benefits and many IC manufacturers arelooking at ways to extend the lifetime of their currentlithography investment.

Several methods of improving the imagingperformance of optical lithography tools are currentlyaccepted in the industry. These methods include off-axisillumination, chemically amplified photoresist, Phase ShiftMasks (PSM) and Optical Proximity Correction (OPC).Several of these techniques are used to examine theperformance of i-line lithography at 0.30 µm and below.

In this paper the value of such techniques for use inproduction is investigated by examining a range offeature types, including dense lines, isolated lines andcontact holes. Using a PAS 5500/200, this experimentstudies a wide range of illumination settings through theuse of the new AERIAL1 (Automated EnhancedResolution Illuminator for Advanced Lithography)

illuminator. This new illuminator provides a high degreeof flexibility in setting illumination modes in a way whichis both automated and easy to use. A description of thesystem is given, including an overview of the wide rangeof illumination profiles that can be used to improve theimaging performance of an i-line optical projectionsystem. This illumination principle can also be adaptedfor future use in DUV lithography.

2. IMAGING AND ILLUMINATION

Design requirements of lithographic lenses used fortodays production and tomorrows development demandthe ability to print smaller and smaller feature sizes.

The adapted Rayleigh’s criterion for resolution isshown in equation 1. This describes the resolution limit ofthe lens (Rlimit) and shows that the projection lens as wellas the illumination optics play an important role in theimaging performance of a photolithographic system.

(1)

The main design parameter of the projection lens isNumerical Aperture (NA), while the illumination systemdetermines the other parameters: λ (wavelength of thelight source), σ (spatial partial coherence factor) and θ

Rlimit α λ

NA 1 σ Θ( )sinNA

-----------------+ +

----------------------------------------------------

Where σ Θ( )sinNA-----------------+

1<

2

(angle of incidence of rays at the reticle as seen from thewafer level). Illumination sources with θ > 0 are oftendescribed as “off-axis”.

In a photolithographic system, the light passes fromthe illumination source through several opticalcomponents of the illuminator which increases theuniformity and establishes the angular distribution of thelight incident on the reticle. The light then passes throughthe reticle, where it is diffracted into orders. The ordersare collected by the entrance pupil of the lens andrecombined at the focal plane to create an image on thewafer. At the resolution limit of the lens (Figure 12) theentrance pupil only captures the zero and first orders ofthe diffracted light.

Changing the illuminator setup can have a markedeffect on image contrast; this is especially true whencomparing conventional (θ = 0) to off-axis illumination(θ > 0). The reason can be explained by comparing theway in which the first and second orders interfere (Figure1). Conventional illumination results in a three beaminterference effect, while only two beams interfere in theoff-axis case. This results in the different contrast curvesshown in Figure 2. The off-axis case shows a flat responseover the frequency range, while the conventional caseindicates degrading contrast with increasing frequency.Comparing these graphs shows that off-axis illuminationgives higher contrast, which will result in an improveddepth of focus at smaller feature sizes and yields a

degraded depth of focus at larger feature sizes. As aconsequence, each feature size requires the optimizationof both coherence and angle of incidence to produceoptimal contrast, depth of focus and exposure latitude.

Figure 1 Two versus three beam imaging.

Figure 2 Contrast curves for different illuminationconditions and spatial frequencies(image size).

Figure 3 AERIAL illuminator schematic.

3

3. AERIAL ILLUMINATION SYSTEM

In this study the AERIAL illuminator is integrated intoan i-line stepper, to evaluate the impact of differentillumination schemes on the imaging performance. Theoptical path is illustrated in Figure 3.

The AERIAL illuminator is able to create a largerange of conventional and annular (off-axis) illuminationmodes. The implementation is such that illuminationcontrol is done from the operator console by translatingNA and σ values into position of some refractive elementsin the pupil shaping optics. Moreover, with the use ofspecial aperture blades, semi-automatic quadrupoleillumination can be obtained.

The characterization of the illumination mode is doneusing a physics definition of partial coherence andannularity and is based on encircled energy. The innerradius is obtained from:

(2)

and the outer radius from:

(3)

Where I(r) represents the intensity profile in the lenspupil, r represents the illumination spot size inside thepupil, and R represents the NA diaphragm. In Figure 4,some examples of illumination profiles are presented. Thecases shown represent the optimum conditions for0.30 µm dense lines as determined in this study throughsimulation and experiment.

σinner I r( )r r = 0.1 I r( )r rd

0

R

∫d

0

r inner

∫=

σouter I r( )r r = 0.9 I r( )r rd

0

R

∫d

0

r outer

∫=

Figure 4a Example of illumination profile forconventional illumination(NA = 0.6, σ = 0.7).

Figure 4b Example of the illumination profile forannular illumination(NA = 0.58, σinner = 0.5, σouter = 0.8).

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4. PROCESS REQUIREMENTS

To obtain a robust, production worthy,photolithographic process, the process engineer mustcontrol many parameters. These parameters can be splitinto three major categories: lithography tools, wafers,and process. In previously published articles3,4,5 thelithography tool and wafer influences have been welldescribed. An example of relevant parameters is given inTable 1.

Where UDoF (Usable Depth of Focus) represents thecommon corridor across the image field and CDoF(Critical Depth of Focus) represents the minimum requiredDoF.

In order to ensure the most robust manufacturingprocess, all of the parameters in Table 1 are continuallyundergoing improvements to minimize their contributionsto the error budget. For example, in the area of wafererror contributions, there have been improvements inwafer non-uniformity due to the advances in the polishingof the starting material. Similarly, increased use ofChemical Mechanical Polishing (CMP) of critical layershas almost eliminated the effects of wafer topography.Using the parameters given in Table 1 and equation 4, theminimum required Depth of Focus (CDoF) for an

acceptable production process can be defined as:

(4)

Where LEC represents the lithography tool errorcontribution, WEC is the wafer error contribution, andFPD is the focal plane deviation across the exposure field.As can be seen, the minimum required Depth of Focus foran acceptable 0.3 µm production process is 0.95 µm.From equation 4, one would select an operating conditionthat yields a DoF as large as possible above the CDoF.However, this equation leaves out a very significant factorin a manufacturing process. This is common day to dayvariations. Common process variations consist of factorssuch as: resist uniformity, develop uniformity, andenvironmental control. To ensure that a process is capableof operating successfully in a production environment,these variations must be accounted for in the selection ofthe exposure condition. Therefore, it is necessary toassign a minimum process latitude for the CDoF range.

Process latitude is defined as the range of exposureenergies over which the minimum required Depth of Focusis achieved. In Figure 5, an example Exposure - Defocus(E-D) window is shown, where the two boxes representDepth of Focus Maximum (DoFMax) and Depth of Focuswith a 10% Process Latitude (DoF@10%). It is clear fromFigure 5 that accounting for process variations will resultin a lower Depth of Focus. It will also ensure a moreuniform CD performance over a large process window.

During the experimentation, the process was onlyoptimized for 0.30 µm dense lines. In order to guaranteethat the process will be applicable in a productionenvironment, the CDoF has to be maintained over at leasta 10% process latitude. The 10% criteria can vary andstrongly depends on the specific process and how well itcan be controlled.

Table 1 Tool and wafer influences.

Min. Feature size (µm)

Tool and wafer parameters 0.35 µm 0.30 µm

Lithography Tool Error Contribution (LEC)

• Focus Settling Error (rss)

• Focus Reproducibility (rss)

• Imaging Instability (rss)

• Leveling Reproducibility (rss)

• Exposure Chuck Flatness (rss)

± 0.10

± 0.10

± 0.10

± 0.10

± 0.10

± 0.05

± 0.05

± 0.10

± 0.10

± 0.10

Wafer Error Contributions (WEC)

• Process Layer Sensitivity (rss)

• Wafer non-uniformity (rss)

• Wafer Topography (lin)

± 0.10

0.30

0.15

± 0.10

0.25

0.10

UDoF (µm) 0.73 0.60

Focal plane deviations (µm) (lin) 0.35 0.35

CDoF (µm) 1.08 0.95

CDoF LEC2 WEC2+ FPD+=

Figure 5 Example of Exposure Defocus (E-D)window for 0.35 µm dense lines.

5

5. EXPERIMENTATION

5.1 Process

The experimental variables investigated fell into twoareas, the stepper and the photoresist process.

The stepper parameters, which will be discussed laterin this paper, consisted of different combinations of NAand coherence (σ for conventional illumination and σouterand σnner for annular illumination)..

The photoresist process chosen was ASML’s standard0.35 µm lens qualification photoresist (SumitomoPFI38A), which typically yields the results shown in Table2. For the 0.3 µm evaluation it was decided to reduce thephotoresist film thickness to 0.8 µm. The primary reasonfor lowering this photoresist thickness was to keep theaspect ratio between 2.5:1 and 3:1, while stillmaintaining sufficient adhesion, etch resistance andreliable metrology. This thickness represents a point ofmaximum incoupling or minimum energy to clear. Thestandard PEB and development process used consists of60 seconds PEB followed by a 60 seconds spray/puddledevelopment. A summary of the process parameters canbe found in Table 3.

.

5.2 Simulations

Simulations were utilized to estimate the optimum NAand coherence settings for DoFMax and DoF@10%. Thesimulations were carried out using SOLID-C and Prolithbased on an optimum set of conditions: no lensaberrations, top hat illumination profiles, PFI38A resistand a ring width (σouter - σinner) of 0.3. Previous Prolithsimulations indicated that this ring width is the optimumfor 0.30 µm dense lines. Only dense lines were simulatedand all experimental optimization was carried out for thissituation. Results from the simulations are given in Figure6, where both DoFMax and DoF@10% conditions areshown. In the figures and throughout the remainder of thepaper, σcentre is defined as (σouter + σinner)/2.

The graphs clearly demonstrate that a trade-off existsbetween maximum DoF and maximum process latitude. Ifthe goal of the process optimization is to maximize DoFone would choose a low NA and a high σ. While if thegoal was to maximize process latitude one would choosea high NA and an intermediate σ. In most cases, theprocess will need to be maximized for both parameters;therefore, one would choose an NA in the middle of therange and a high σ. This should allow sufficient DoF,while yielding better CD control.

5.3 Design Of Experiments

A statistical Design Of Experiments (DOE) based on acentral composite design was derived from thesimulations. The layout of the DOE is shown in Figure 7.Twelve wafers were exposed according to the DOE andthe resulting depth of focus was determined on an Opal7830 automated CD SEM. The measurements were then

Table 2 Typical results for 0.35 µm imaging with thePAS 5500/200 and the AERIALilluminator.

DoFMax DoF@10%

Conventional Illumination(NA = 0.56, σ = 0.7) 2.25 µm 1.6 µm

Annular Illumination(NA = 0.49, σouter= 0.72,σ inner= 0.42)

2.7 µm 1.95 µm

Table 3 Standard photo-resist process parameters.

Parameter Process conditions

Photo-resist Sumitomo PFI38A

Photo-resistThickness

0.98 µm for the 0.35 µm process0.80 µm for the 0.30 µm process

Photo-resistUniformity

± 6 nm T.I.R.

SoftbakeTemperature

90 oC hard contact

Softbake Time 60 s

Developer OPD 262 (2.38% TMAH)

Develop Time 60 s. spray/puddle

PEB Temperature 115 oC hard contact

PEB Time 60 s

6

analyzed using Linnovations E-D Forest software. Ingeneral, experiment and simulation demonstrated similartrends towards high NA and σ for increased processlatitude and low NA and high σ for increased DoF.

In order to more accurately determine the optimumsetting six additional wafers (three with a fixed NA andthree with a fixed σ) were exposed under the conditionslisted in Table 4. The settings were chosen as a result ofnew simulations which were based on the initial DOEexposures

.

The wafers were measured using the Opal SEM andthe results are shown in Figure 8. The optimum conditionfor dense lines was determined to be: NA = 0.58,σouter = 0.8 and σinner = 0.5. This gave the bestcombination of DoFMax and DoF@10%.

A similar DOE was performed for conventionalillumination and 0.30 µm dense lines. The resultingoptimal condition (NA = 0.6 and σ = 0.7) gave a smallerDoF than its annular counter part.

Table 4 Exposure conditions used to determineoptimum operating point for 0.30 µmlines and spaces.

DoFMax

NA σouter σinner

0.50 0.90 0.60

0.54 0.90 0.60

0.58 0.90 0.60

DoF@10%

NA σouter σinner

0.58 0.90 0.60

0.58 0.80 0.50

0.58 0.70 0.40

Figure 6a Simulations for DoFMax using annularillumination with a ring width of 0.3.

Figure 6b Simulations for DoF@10% using annularillumination with a ring width of 0.3.

Figure 7 DOE layout for central composite design.

7

5.4 Process performance

Using the photoresist process previously described inconjunction with the optimum settings for annular andconventional illumination, DoF performance of 0.30 µmdense lines was evaluated using silicon substrates. TheExposure Defocus (E-D) window of each wafer wasanalyzed using the automated top down metrologypackage of the Opal 7830 low voltage SEM. In order tocalculate the process worthiness of each condition, 270points were measured, 9 steps in energy and 30 steps infocus. The results were analyzed using the E-D Forestprogram from Linnovation. This program calculates thedimensions of the process window and allows the depthof focus to be determined for different process latitudes.The software and CD metrology were verified using aPhilips XL-50 analytical SEM tool, which allows tilted end-on evaluation.

The measurements indicate that the process is capableof achieving 1.4 µm DoF at 10% process latitude for theconventional illumination condition and 1.8 µmDoF@10% for the annular illumination. The Bossungcurves and E-D windows for both conditions are shown inFigure 9.

Figure 8b DoF versus NA comparing actual andtheoretical results for 0.30 µm dense linesusing annular illumination.

Figure 8a DoF versus sigma center comparing actualand theoretical results for 0.30 µm denselines using annular illumination.

8

Since both of these conditions exceeded the requiredCDoF for a suitable 0.3 µm process, the next step was todetermine whether the performance is maintained onprocess films most often used for critical manufacturinglayers. The process films investigated in this study werepolysilicon at 250 nm, oxide at 800 nm and nitride at200 nm. A summary of the results can be found in Table 5.In all cases, the CDoF requirement of 0.95 µm over aprocess window of 10% is met with a significant margin,where the margin is defined as:

The analytical SEM photos for the annular conditions,showing consistent profiles throughout defocus, can befound in Figure 10.

Measured DoF - Required DoF( )Required DoF---------------------------------------------------------------------------------- 100%×

Figure 9c E-D window for conventional illumination for0.3 µm dense lines using conventionalillumination, (NA = 0.6, σ = 0.7).

Figure 9d E-D window for 0.3 µm dense lines usingannular illumination,(NA = 0.58, σinner = 0.5 σ outer = 0.8).

-1.50 -1.20 -0.90 -0.60 -0.30 -0 0.30 0.60 0.90 1.20 1.50120

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Linear Exposure (m

J/cm2) Li

near

Exp

osur

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J/cm

2 )

Defocus (µm)

DoF max

DoF @ 10%

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Defocus (um)

Linear Exposure (m

J/cm2)

Defocus (µm)

Line

ar E

xpos

ure

(mJ/

cm2 )

DoF max

DoF @ 10%

Figure 9a Bossung curves for 0.3 µm dense linesusing conventional illumination,(NA = 0.6, σ = 0.7).

140 mJ/cm2145 mJ/cm2150 mJ/cm2155 mJ/cm2160 mJ/cm2165 mJ/cm2170 mJ/cm2175 mJ/cm2180 mJ/cm2

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Figure 9b Bossung curves for 0.3 µm dense lines usingannular illumination,(NA = 0.58, σinner = 0.5, σ outer = 0.8).

140 mJ/cm2145 mJ/cm2150 mJ/cm2155 mJ/cm2160 mJ/cm2165 mJ/cm2170 mJ/cm2175 mJ/cm2180 mJ/cm2

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Table 5 Summary of the 0.3 µm dense line DoF results from measurement of process layers.

Dense Lines

Film stack Illumination mode DoFMax DoF@10% Margin

Silicon Conventional 1.7 1.4 45%

Annular 2.5 1.9 93%

Polysilicon Conventional 1.9 1.4 52%

Annular 2.4 2.1 120%

Oxide Conventional 1.8 1.4 42%

Annular 2.4 2.0 114%

Nitride Conventional 1.9 1.3 41%

Annular 2.5 1.9 96%

Focus (µm) Silicon Polysilicon Oxide Nitride

-1.0

-0.5

0.0

0.5

1.0

Figure 10 Analytical SEM photos for or 0.30 µm dense lines using the annular illumination conditions, showingconsistent profiles throughout defocus.

10

5.5 Linearity

Another relevant parameter in a manufacturingprocess is CD linearity. This is defined as the region overwhich the reticle CD is directly translated to the wafer.

CD linearity for all conditions exposed are plotted inFigure 11. The corresponding analytical SEM photos forthe annular conditions can be found in Figure 12.

The results clearly demonstrate that linearity can bemaintained between 0.30 µm and 0.45 µm. To obtainlinearity below 0.30 µm further optimization of the photoresist process would be needed (i.e. a new photoresist orthinning of the photoresist). In all cases, the resultsindicate an undersizing of the CDs. This has primarilybeen shown to be due to reflections from the surface of thewafer6.. Therefore, the more absorbing layers, (e.g.polysilicon) experience less undersizing.

Figure 11 CD linearity for all film stack conditions exposed using both conventionaland annular illumination.

0.25

0.3

0.35

0.4

0.45

0.5

Reticle CD (µm)

Act

ual

CD

(µm

)

0.25 0.275 0.3 0.325 0.35 0.375 0.4 0.425 0.45 0.475 0.5

Si conventional Si annular Poly conventional Poly annular Oxide conventional Oxide annular Nitride conventional Nitride annular Nominal

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CD(µm) Silicon Polysilicon Oxide Nitride

0.45

0.425

0.4

0.375

0.35

0.325

0.3

0.275

0.25

Figure 12 Analytical SEM photos of linearity for the annular conditions.

12

5.6 Isolated Line performance

Though the process evaluated in this study wasoptimized for dense line structures, 0.30 µm isolated lineperformance was also collected for information. Asummary of the DoF for isolated lines can be found inTable 6. The results indicate reduced performance ascompared to the dense lines. However, furtheroptimization should give better results. This may requiretrading off dense line performance for improved isolatedline results.

5.7 ODoF performance

Also shown in Table 6 is the overlapping depth offocus (ODoF7) between the dense and isolated lines. Toincrease the ODoF between the dense and isolated lineone needs to minimize the isolated-dense bias. Theexperimentation in this area for 0.30 µm structures is stillongoing.

Previously, this type of optimization was performed for0.35 µm structures. The results of the 0.3 ring widthannular illumination condition are shown in Figure 13.Analyzing the results, it was determined that the isolated-dense bias depends heavily on σouter and is almostindependent of NA. The same phenomea is true forconventional illumination, except that it is dependent onσ. Additionally, it was found that a zero bias conditioncan be obtained without a reduction in DoF.

Table 6 Summary of the 0.30 µm isolated lines performance.

Dense Lines Isolated Lines ODoF

Film stack DoFMax 10% DoFMax 10% DoFMax 10%

Silicon Conventional 1.7 1.4 1.1 0.9 1.1 0.9

Annular 2.5 1.8 1.4 1.3 1.3 1.1

Polysilicon Conventional 1.9 1.4 1.1 0.8 1.1 0.4

Annular 2.4 2.1 0.8 0.7 0.8 0.7

Oxide Conventional 1.8 1.4 1.4 1.2 1.4 1.2

Annular 2.4 2.0 1.6 1.1 1.6 1.1

Nitride Conventional 1.9 1.3 1.1 1.0 1.1 1.0

Annular 2.5 1.9 1.2 1.0 1.2 0.9

Figure 13b DoF@10% for 0.35 features using annularillumination with a ring width of 0.3.

Figure 13a Dense-isolated bias using annularillumination with a ring width of 0.3. Blackrepresents zero bias.

Numerical Aperture

Sig

ma

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5.8 UDoF performance

Characterization of the process window and theODoF is not the only information needed to ensure arobust production process. The DoF performance acrossthe entire imaging field, which is better known as UsableDepth of Focus (UDoF), is also important.

In Table 7, the UDoF measurements of a 0.3 µm denselines are summarized for three radii of 22 mm squareimage field. Figure 14 shows the SEM verification photosfor the silicon substrate used during this study. As isdemonstrated in Table 7 and Figure 14, the UDoF for allfilm stacks exceeds the minimum requirement given inTable 1.

Focus(µm)

Conventional Annular

Center 11 mm 15.6 mm Center 11 mm 15.6 mm

-0.5

-0.3

-0.1

0.1

0.3

0.5

Figure 14 SEM verification photos for 0.30 µm dense lines UDoF on silicon substrate.

14

The 0.30 µm isolated line UDoF performance wasmeasured at the optimum conditions for dense lines andis summarized in Table 8. With only one exception, theminimum UDoF requirement discussed in Table 1 wasexceeded. This one exceptional case was unexpected,since the dense performance on oxide was equal to orbetter than the silicon control condition.

A true UDoF calculation should be comprised bothdense and isolated lines, therefore this information wasgathered and summarized in Table 9. Combining thedense and isolated line results from Tables 7 and 8 givesthe overlapping UDoF shown in Table 9.

Generally the annular illumination condition resultedin better or equal UDoF performance than theconventional illumination condition. The only substratenot meeting the minimum required UDoF was oxide. Aspreviously mentioned, this is driven by the isolated lineperformance. As the conditions used were optimized fordense lines, which typically yielded DoF performance

between 20 and 100% larger than the isolated lines, itcan be seen that further optimization of the NA, σouter,σinner, and process to incorporate both dense andisolated lines lead to an increase in overlapping UDoFperformance.

The final feature type investigated was contact holes.No optimization was done on the illuminator and lenssettings. The NA was set to 0.6 and the σ was set to 0.7(the same conventional illumination condition used for thedense and isolated line evaluation) also the standardphotoresist process was used. Both binary mask andattenuated PSM with an 8% transmission were utilized,and the feature size remained at 0.30 µm. A summary ofthe DoF and UDoF found for the two conditions exposedis given in Table 10. Figures 15 and 16 show thecorresponding analytical SEM photos and CD versusdefocus plots. The results indicate good performance forboth conditions (binary and PSM) with the PSM improvingthe results by less than equal to 50%.

Table 7 Summary of 0.30 µm dense line UDoF performance.

Silicon Polysilicon Oxide Nitride

Radial Position Conventional Annular Conventional Annular Conventional Annular Conventional Annular

Center 1.4 2.2 1.4 2.3 1.5 2.4 1.0 1.2

11 mm 1.5 1.9 1.6 2.2 1.7 2.3 1.0 1.6

15.6 mm 1.3 2.0 1.6 2.3 1.5 2.2 1.5 1.2

UDoF 1.1 1.9 1.4 2.0 1.3 2.2 0.9 1.1

Table 8 Summary of 0.30 µm isolated line UDoF performance.

Silicon Polysilicon Oxide Nitride

Radial Position Conventional Annular Conventional Annular Conventional Annular Conventional Annular

Center 1.0 1.1 0.9 1.2 0.5 0.4 1.0 0.9

11 mm 0.9 1.2 1.0 1.0 0.8 0.6 0.8 1.1

15.6 mm 1.1 1.2 0.9 1.0 0.5 0.4 1.1 0.8

UDoF 0.7 0.8 0.6 0.8 0.5 0.4 0.8 0.8

Table 9 Summarized UDoF results for dense and isolated lines.

Silicon Polysilicon Oxide Nitride

Radial Position Conventional Annular Conventional Annular Conventional Annular Conventional Annular

Center 1.0 1.1 0.9 1.2 0.5 0.4 0.8 0.9

11 mm 0.9 1.2 1.0 1.0 0.8 0.6 0.8 1.1

15.6 mm 0.9 0.8 0.9 1.0 0.5 0.4 1.1 0.8

UDoF 0.7 0.8 0.6 0.8 0.5 0.4 0.8 0.8

15

.

Table 10 Summary of DoF and UDoF for 0.30 µm contact hole performance.

Center 11 mm 15.6 mm UDoF

Chrome 1.0 0.8 0.9 0.7

PSM 1.0 1.2 1.0 1.0

Focus(µm)

Binary Mask 8% Attenuated Phase Shift Mask

Center 11 mm 15.6 mm Center 11 mm 15.6 mm

+0.4

+0.2

0

-0.2

-0.4

-0.6

Figure 15 Analytical SEM photos for 0.30 µm contact holes.

16

6. CONCLUSIONS

Results have been presented which demonstrate theperformance of i-line optical lithography in conjunctionwith a production orientated off-axis illuminator(AERIAL). This combination together with a variableNA lens allows for a continuously varying range of lensand illumination settings. Through the optimization ofthese settings, it has been established that the imagingresults have sufficient depth of focus over a large processwindow to allow for a robust 0.30 µm production processEven though the optimization was performed for denselines, the combination of dense and isolated lines gaveacceptable ODoF and UDoF performance as comparedto the minimum requirements (see Table 1). Improvedresults can be obtained by optimizing dense and isolatedlines simultaneously, which has been demonstrated in thestudy for 0.35 µm structures.

Finally, initial results from 0.30 µm contact holeperformance have been presented. The results show largeDoF performance across the entire imaging field usingboth chrome and attenuated phase shift (PS) masks.

All results presented in this study clearly demonstratea production worthy 0.3 µm process. With furtherprocess development the authors believe extension of i-line lithography beyond 0.3 µm will be possible.

7. ACKNOWLEDGMENTS

The authors would like to thank all those involved inthe PAS 5500/200 project. Special thanks are due to Tedder Kinderen, Jos Beerens, Jenny Swinkels, and Yin FongChoi for the their assistance in completing the requiredSEM analysis. To Peter Lambert for his assistance in thelayout and illustrations of the paper.

Special Acknowledgments to Gabrielle Kalmbach,Wolfgang Rupp, Eckhart Piper, and Johannes Wangler ofCarl Zeiss, Oberkocker for their technical guidance.

Finally, the authors would like to thank PaulLuehrmann, Paul van Attekum, and others for theirencouragement and advice in completing this paper.

8. REFERENCES

1) J. Mulkens et al, “High Throughput Wafer Stepperswith Automatically Adjustable Conventional andAnnular Illumination Modes”, Proceedings oftechnical seminar: Semicon Japan, Ciba, December1995.

2) S. Wittekoek, “Optical Lithography: present status andcontamination below 0.25 µm” MicroelectronicEngineering 23, October 1994, pp 43-55.

3) B. Katz et al, “I-line Lithography for Sub Half MicronDesign Rules” Proceedings of SPIE vol. 1927 p298-310. (1993)

4) L. vd Hove et al, “CD - Control Issues for sub 0.25 µmProcesses”, presented at the ASML European UsersMeeting 1995.

Figure 16a CD versus defocus for 0.3 µm Binarycontacts.

Figure 16b CD versus defocus for 0.3 µm AttenuatedPhase Shift contacts.

-1.1

-0.9

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0.4

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)

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Binary 15.6 mm

I-07648.EPS

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0.4

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)

I-07649.EPS

Binary center

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5) S. Graca et al, “A Novel Inorganic Arc for sub 0.4micron Optical Lithography”. OCG MicrolithographySeminar, (1995).

6) M. Gehm et al “Intra-chip Linewidth Variation for 0.5micron Poly Gates”. OCG Microlithography seminarp315-328 (1993).

7) S. Hansen, “Computer simulations for sub 0.35 µmNovolak Resists”, presented at the second AdvancedLithography Workshop hosted by OCG (1995).

8) P.F. Luehrmann et al, 0.35 µm Lithography Using OffAxis Illumination, Proceedings of SPIE Optical/LaserMicrolithography VI Vol 1927, pp 103-124, (1993).

9) T. Ogawa et al, “Subquarter Micron OpticalLithography with Practical Super ResolutionTechnique”. Proceedings of SPIE Optical/LaserMicrolithography VII, Vol 2440, pp772-783, March(1995).

10)P. Tzviatkov et al, “0.30 µm and Sub 0.30 µm i-lineLithography for Random Logic Poly Gates”, OCGMicrolithography Seminar, page 1 - 23, (1995).

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