Basic Concepts

1. Basic Concepts • Antireflective coating is used to prevent reflections from the chrome coming back into the resist • Occasionally AR coatings are deposited on wafers also • Develop the resist and etch to remove the metal • We get good dimensional control because the Cr is very thin (~80nm) • It is critical that the areas beneath where the Cr is removed be highly transparent at the wavelength of the light used in the wafer exposure system. • Masks (reticules) for steppers (step and repeat systems) are 4x to 5x larger than what is printed • Relaxes minimal feature requirements on mask • Masks for steppers print usually only one or two die at a time; any defect in the mask gets reproduced for every die!

2. Reflectivity • At the interface of two bulk layers http://www.mellesgriot.com/products/optics/images/fig5_12.gif

3. Antireflectivity Coatings • For l/4 thick films • Ideal index of refraction for antireflective coating is √(nairnglass)

4. Basic Concepts • We generally separate lithography into three parts • The light source • The exposure system • The resist • The exposure tool creates the best image possible on the resist (resolution, exposure field, depth of focus, uniformity and lack of aberrations) • The photoresist transfers the aerial image from the mask to the best thin film replica of the aerial image (geometric accuracy, exposure speed, resist resistance to subsequent processing)

5. Light Source • Historically, light sources have been arc lamps containing Hg vapor • A typical emission spectra from a Hg-Xe lamp • Low in DUV (200-300nm) but strong in the UV region (300-450nm)

6. Light Source • To minimize problems in the lens optics, the lamp output must be filtered to select on of the spectral components. • Two common monochromatic selections are the g-line at 436 nm and the i-line at 365 nm. • The i-line stepper now dominates the 0.35 m market

7. Light Sources • For 0.18 and 0.13, we use two excimer lasers (KrF at 248 nm and ArF at 193 nm) • These lasers contain atoms that do not normally bond, but if they are excited the compounds will form; when the excited molecule returns to the ground state, it emits • These lasers must be continuously strobed (several hundred Hz) or pulsed to pump the excitation • Can get several mJ of energy out • Technical problems have been resolved for KrF and these are used for 0.25 and 0.18 m • ArF is likely for 0.18 and 0.10 m; technical problems remain

8. Exposure System • There are three classes of exposure systems • Contact • Proximity • Projection

9. Exposure System • Contact printing is the oldest and simplest • The mask is put down with the Cr in contact with the wafer • This method • Can give good resolution • Machines are inexpensive • Cannot be used for high-volume due to damage caused by the contact • Still used in research and prototyping situations

10. Wafer Exposure Systems • Proximity printing solves the defect problem associated with contact printing • The mask and the wafer are kept about5 – 25 m apart • This separation degrades the resolution • Cannot print with features below a few microns • The resolution improves as wavelength decrease. This is a good system for X-ray lithography because of the very short exposure wavelength (1-2 nm).

11. Wafer Exposure Systems • For large-diameter wafers, it is impossible to achieve uniform exposure and to maintain alignment between mask levels across the complete wafer. • Projection printing is the dominant method today • They provide high resolution without the defect problem • The mask (reticule) is separated from the wafer and an optical system is used to image the mask on the wafer. • The resolution is limited by diffraction effects • The optical system reduces the mask image by 4X to 5X • Only a small portion of the wafer is printed during each exposure • Steppers are capable of < 0.25 m • Their throughput is about 25 – 50 wafers/hour

12. Optics Basics • We need a very brief review of optics • If the dimensions of objects are large compared to the wavelength of light, we can treat light as particles traveling in straight lines and we can model by ray tracing • When light passes through the mask, the dimensions of objects are of the order of the dimensions of the mask • We must treat light as a wave

13. Optics Basics • Diffraction occurs because light does not travel in straight lines • Pass a light through a pin-hole; we see that the image is larger than the hole • This cannot be explained by ray tracing

14. Diffraction of Light

15. Diffraction of Light • The Huygens-Fresnel principle states that every unobstructed point of a wavefront at a given time acts as a point source of a secondary spherical wavelet at the same frequency • The amplitude of the optical field is the sum of the magnitudes and phases • For unobstructed waves, we propagate a plane wave • For light in the pin-hole, the ends propagate a spherical wave.

16. Diffraction of Light

17. Young’s Single Slit Experiment sinq = l/d http://micro.magnet.fsu.edu/optics/lightandcolor/diffraction.html

18. Amplitude of largest secondary lobe at point Q, eQ, is given by: eQ = a(A/r)f(c)d where A is the amplitude of the incident wave, r is the distance between d and Q, and f(c) is a function of c, an inclination factor introduced by Fresnel. http://micro.magnet.fsu.edu/optics/lightandcolor/diffraction.html

19. Young’s Double Slit Experiment http://micro.magnet.fsu.edu/optics/lightandcolor/interference.html

20. Basic Optics • This diffraction “bends” the light • Information about the shape of the pin hole is contained in all of the light; we must collect all of the light to fully reconstruct the pattern • The following diagram shows how the system works • Note that the focusing lens only collects part of the diffraction pattern • The light diffracted at higher angles contains information about the finer details of the structure and are lost

21. Basic Optics

22. Basic Optics • The image produced by this system is

23. Basic Optics • The diameter of the central maximum is given by • Note that you get a point source only if d  

24. Basic Optics • There are two types of diffraction • Fresnel, or near field diffraction • Fraunhofer, or far field diffraction • In Fresnel diffraction, the image plane is near the aperture and light travels directly from the aperture to the image plane (see Figure 5-4) • In Fraunhofer diffraction, the image plane is far from the aperture, and there is a lens between the aperture and the image plane (see Figure 5-6) • Fresnel diffraction applies to contact and proximity printing while Fraunhofer diffraction applies to projections systems • There are powerful simulations systems for both cases

25. Fraunhofer Diffraction • We define the performance of the system in terms of • Resolution • Depth of focus • Field of view • Modulation Transfer Function (MTF) • Alignment accuracy • throughput

26. Fraunhofer Diffraction • Imagine two sources close together that we are trying to image (two features on a mask) • How close can these be together and we can still resolve the two points? • The two points will each produce an Airy disk (5-7) • Lord Rayleigh suggest that we define the resolution by placing the maximum from the second point source at the minimum of the first point source

27. Fraunhofer Diffraction

28. Fraunhofer Diffraction • With this definition, the resolution becomes • For air, n=1 •  is defined by the size of the lens, or by an aperture and is a measure of the ability of the lens to gather light

29. Fraunhofer Diffraction • This is usually defined as the numerical aperture, or NA • This really is defined only for point sources, as we used the point source Airy function to develop the equation • We can generalize by replacing the 0.61 by a constant k1 which lies between 0.6 and 0.8 for practical systems

30. Fraunhofer Diffraction • From this result, we see that we get better resolution (smaller R) with shorter wavelengths of light and lenses of higher numerical aperture • We now consider the depth of focus over which focus is maintained. • We define  as the on-axis path length difference from that of a ray at the limit of the aperture. These two lengths must not exceed /4 to meet the Rayleigh criterion

31. Depth of Focus

32. Depth of Focus • From this criterion, we have • For small 

33. Fraunhofer Diffraction • From this we note that the depth of focus decreases sharply with both decreasing wavelength and increasing NA. • The Modulation Transfer Function (MTF) is another important concept • This applies only to strictly coherent light, and is thus not really applicable to modern steppers, but the idea is useful

34. Fraunhofer Diffraction • Because of the finite aperture, diffraction effects and other non-idealities of the optical system, the image at the image plane does not have sharp boundaries, as desired • If the two features in the image are widely separated, we can have sharp patterns as shown • If the features are close together, we will get images that are smeared out.

35. Modulation Transfer Function

36. Fraunhofer Diffraction • The measure of the quality of the aerial image is given by • The MTF is really a measure of the contrast in the aerial image • The optical system needs to produce MTFs of 0.5 or more for a resist to properly resolve the features • The MTF depends on the feature size in the image; for large features MTF=1 • As the feature size decreases, diffractions effects casue MTF to degrade

37. Change in MTF versus Wavelength

38. Contrast and Proximity Systems • These systems operate in the near field or Fresnel regime • Assume the mask and the resist are separated by some small distance “g” • Assume a plane wave is incident on the mask • Because of diffraction, light is bent away for the aperture edges • The effect is shown in the next slide • Note the small maximum at the edge; this results from constructive interference • Also note the ringing • As a result, we often use multiple wavelengths

39. Fresnel Diffraction

40. Fresnel Diffraction • As g increases, the quality of the image decreases because diffraction effects become more important • The aerial image can generally be computed accurately whenwhere W is the feature size • Within this regime, the minimum resolvable feature size is

41. Depth of Focus http://www.research.ibm.com/journal/rd/411/holm1.gif

42. Summary of the Three Systems