Chapter 5: Lithography

1. Chapter 5: Lithography

2. Introduction • The mechanism to print 2-D patterns to a thin film layer on the wafer surface. • Masks are glass plates (soda lime or quartz glass) that contain the patterns. • The patterns are first transferred from the mask to photoresist (PR), a light-sensitive polymer. • After opening windows in the PR, the pattern is transferred to the thin film using etching techniques. • Complexity of a fabrication process is often measured by the number of photolithographic masks used in the process.

3. Introduction • The concept is simple • Spin on a thin layer of light-sensitive photoresist • Selectively expose it to UV light • Causing chemical bonds to either form or break • Develop to selectively remove the lighter weight PR • The resist may be used as a mask for either etching or for ion implantation • Because of constraints of resolution, exposure field, accuracy, throughput, and defect density, the implementation is not so simple • Very expensive • Very complex

4. Introduction

5. Introduction Mask Maker

6. Introduction

7. Introduction

8. Introduction Mask Aligner

9. UV 5,6 6 Introduction 1 2 3,4

10. 7,8 9 10 Introduction Some common PRs: 1800 series (for thin)  will be developed in MF 319 9200 series (for thick)  will be developed in AZ 400 Be aware that there are two different types of PR: Positive PR: exposed areas will be developed Negative PR: exposed areas will not be developed

11. Alignment Markers • Once a photolith process is done, the pattern developed is used to perform some additional process selectively on the wafer • Etching trenches in Si or SiO2 • Making metalization runs • Implantation of dopants • Then the wafer will come back for another photolith step • Alignment markers are registration patterns that mate from one mask to another so that the multiple pattern sets match one another.

12. Introduction • Positive resists provide better controllability for small features. • Positive resists are easier to work with and use less corrosive developers and chemicals. • Positive resists are the dominant type of photoresists today.

13. Clear Field and Dark Field Masks • Most photolith engineers prefer clear field masks when possible • Easier to detect pattern on the wafer itself as there is more clear glass in the mask

14. Introduction • Demands placed on this process for • Resolution: smaller device structures • Exposure field: ever-increasing chip sizes • Placement accuracy: aligning with existing layers • Throughput: manufacturing cost • Defects: yield and cost

15. NTRS Lithography Requirements

16. Introduction • The National Technology Roadmap for Semiconductorsdefines the future needs • Note especially • The driving force is the reduction of feature size • For every factor of two in reduction of area, there is a reduction of 0.7 in the linear dimensions • The reduction is required every three years • The most commonly quoted feature size is not as small as isolated MOS gate lines • Critical dimension (CD) control must improve (about 10% of minimum feature size) • Alignment accuracy must be about 1/3 of minimum feature size • The printing area increases with time since we must print one full die at a time

17. Introduction • About 1/3 of the cost of a wafer cost (about $1000 for an 8-inch wafer) is associated with lithography; we have only a few hundred dollars per wafer to spend • Optical lithography is used down to 0.13 m (130nm) generations • For smaller dimensions, X-ray, direct e-beam, or extreme UV (EUV) processes are used.

18. Basic Concepts • We generally separate lithography into three parts • The energy source (photons or electrons) • The exposure system • The resist • The exposure tool, which includes the light source and the exposure system, creates the best image possible on the resist (resolution, exposure field, depth of focus, uniformity and lack of aberrations) • Optimization of the photoresist with the settings on the exposure tool transfers the aerial image from the mask to the best thin film replica of the aerial image

19. 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)

20. Light Source • A much smaller set of wavelengths used to expose the resist • to minimize optical distortion associated with the lens optics. • to match the properties of the resist • Pick the wavelength that is heavily absorbed and causes changes in resist chemical properties • Two common monochromatic selections are the g-line at 436 nm and the i-line at 365 nm.

21. UV Light Sources • To expose < 250nm wide lines, we need to use shorter wavelength light • 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 UV light • These lasers must be continuously strobed (several hundred Hz) or pulsed to pump the excitation; can get several mJ of energy out

22. Excimer Lasers Low reliability due to etching of the electrodes and the optical windows by the energitic F ions

23. E-beam Source Field Emission Gun (3), which provides the source of the electron beam, is a W or LaF6 filament. Condenser Lens (7) are pairs of electromagnets that are used to collimate the beam of electrons. Beam Booster, composed of Anode (5), Vacuum Tube (6), Apertures (8), Alignment Coils (9a, b, c), Stigmator (13), and Isolating Valve (15) is used to determine the energy of the electrons and to remove the electrons moving off-axis. Objective Lens (10,11) is another set of electromagnets that focuses the electron beam onto the specimen (12), also containing the Deflecting System (14), which is another set of electromagnetics that sweep the electrons across the field of view and off of the sample . http://cmi.epfl.ch/metrology/img/LEO1550/LEOColumn.gif

24. X-Ray Source • High energy electrons collide with a metal. The transfer of energy results in the release of x-rays (short wavelength photons).

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

26. Exposure System • Contact printing is the oldest and simplest • The mask is put with the absorbing layer face down 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

27. 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 b/c of the very short exposure wavelength (1-2 nm).

28. Projection/Step and Repeat • For large-diameter wafers, it is impossible to achieve uniform exposure and to maintain alignment between mask levels across the complete wafer. • Masks are now called reticules • Projection printing is the dominant method today • They provide high resolution without the defect problem • The mask 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

29. 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

30. Snell’s Law and Reflectivity n1 sin(q1) = n2 sin(q2) 1 = T+R+A, where T is transmission R is reflection A is absorption If q1 = p/2, q2 = sin-1(n1/n2) R = [(n1-n2)/(n1+n2)]2 http://scienceworld.wolfram.com/physics/SnellsLaw.html

31. Refractive index of SiO2 Transmission through two air-glass surfaces is less than 93.1%. R = 3.5 in air l = 365nm http://www.mellesgriot.com/products/optics/images/fig5_12.gif http://www.ioffe.ru/SVA/NSM/nk/Oxides/Gif/sio2.gif

32. Snell’s Law/Antireflective Coatings when the layer thickness,t, is t = (m+1)l/4; m = 0,1,2… R = 0 when n = (n1n2)1/2 n1 n2 n t http://en.wikipedia.org/wiki/File:Optical-coating-1.png

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

34. 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

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

36. Diffraction of Light

37. 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.

38. Diffraction of Light

39. Basic Optics

40. Basic Optics • 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 • If only part of the diffraction pattern is collected and focused on the substrate, the image created is not identical to the one on the mask. • The light diffracted at higher angles contains information about the finer details of the structure and are lost

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

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

43. 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. • In Fraunhofer diffraction, the image plane is far from the aperture, and there is a lens between the aperture and the image plane. • Fresnel diffraction applies to contact and proximity printing while Fraunhofer diffraction applies to projections systems

44. 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

45. 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. • Lord Rayleigh suggested that the minimum resolution be defined by placing the maximum from the second point source at the minimum of the first point source.

46. Fraunhofer Diffraction

47. 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

48. Fraunhofer Diffraction • This is usually defined as the numerical aperture, or NA • Defined only for point sources as the point source Airy function was used to develop the equation • A more generalized equation replaces 0.61 by a constant k1 which lies between 0.6 and 0.8 for practical systems.

49. 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

50. Depth of Focus