Major Fabrication Steps in MOS Process Flow

Transcription

1 Major Fabrication Steps in MOS Process Flow UV light Mask oxygen Silicon dioxide photoresist exposed photoresist oxide Silicon substrate Oxidation (Field oxide) Photoresist Coating Mask-Wafer Alignment and Exposure Exposed Photoresist Photoresist Develop Ionized CF 4 gas photoresist oxide Ionized oxygen gas oxide oxygen gate oxide Dopant gas Silane gas polysilicon Ionized CCl 4 gas oxide Oxide Etch CF 4 or C 3 F 8 or CHF 3 O 3 CF4 +O 2 or CL 2 Photoresist Strip Oxidation (Gate oxide) Polysilicon Deposition Polysilicon Mask and Etch Scanning ion beam silicon nitride Contact holes Metal contacts ox S G D Ion Implantation top nitride G G S D S D S G D Active Regions Nitride Deposition Used with permission from Advanced Micro Devices Contact Etch drain G S D Metal Deposition and Etch

2 Eight Steps of Photolithography UV Light HMDS Resist Mask 1) Vapor prime 2) Spin coat 3) Soft bake 4) Alignment and Exposure 5) Post-exposure bake 6) Develop 7) Hard bake 8) Develop inspect

3 Lithography. Basic lithographic concept:

4 Photolithography Processes Negative Resist Wafer image is opposite of mask image Exposed resist hardens and is insoluble Developer removes unexposed resist Positive Resist Mask image is same as wafer image Exposed resist softens and is soluble Developer removes exposed resist

5 Negative Lithography Chrome island on glass mask Shadow on photoresist Ultraviolet light Exposed area of photoresist Areas exposed to light become crosslinked and resist the developer chemical. Island Photoresist Window Photoresist Oxide Silicon substrate Oxide Silicon substrate Resulting pattern after the resist is developed.

6 Positive Lithography Ultraviolet light Chrome island on glass mask Shadow on photoresist Areas exposed to light are dissolved. Island Window Exposed area of photoresist photoresist Photoresist Photoresist photoresist Oxide oxide Silicon silicon substrate Oxide oxide Silicon silicon substrate Resulting pattern after the resist is developed.

7 Contrast The Contrast of a photoresist is a welldefined property Contrast might be the most important property of any photoresist

8 Eight Steps of Photolithography UV Light HMDS Resist Mask 1) Vapor prime 2) Spin coat 3) Soft bake 4) Alignment and Exposure 5) Post-exposure bake 6) Develop 7) Hard bake 8) Develop inspect

9 Post-Exposure Bake Required for Deep UV Resists Typical Temperatures 100 to 110 C on a hot plate Immediately after Exposure Has Become a Virtual Standard for DUV and Standard Resists

10 Eight Steps of Photolithography UV Light HMDS Resist Mask 1) Vapor prime 2) Spin coat 3) Soft bake 4) Alignment and Exposure 5) Post-exposure bake 6) Develop 7) Hard bake 8) Develop inspect

11 Photoresist Development Process Summary: Soluble areas of photoresist are dissolved by developer chemical Visible patterns appear on wafer - windows - islands Quality measures: - line resolution - uniformity - particles and defects To vacuum pump Develop dispenser Vacuum chuck Spindle connected to spin motor

12 Resist Development Parameters Developer Temperature Developer Time Developer Volume Developer Concentration Rinse Exhaust Flow

13 Development Function of Rinse: Stop development at specific time Remove resist fragment

14 Continuous Spray Development Used in Wafer Track Systems To vacuum pump Vacuum chuck Spindle connected to spin motor

15 Puddle Resist Development Puddle formation Developer dispenser (a) Puddle dispense (b) Spin-off excess developer (c) DI H 2 O rinse (d) Spin dry

16 Negative Resist Development

17 Positive Resist Development

18 Line width as function of development

19 Development Process Immersion possible but same drawbacks as primer immersion. Spray, Rinse, Dry Puddle, Spray, Rinse, Dry Preferred!

20 Scum and Descumming Thin layer of resist or developer might be present on surface after rinse. Can be removed by plasma oxidation, if substrate is not damaged too severely by this

21 Eight Steps of Photolithography UV Light HMDS Resist Mask 1) Vapor prime 2) Spin coat 3) Soft bake 4) Alignment and Exposure 5) Post-exposure bake 6) Develop 7) Hard bake 8) Develop inspect

22 Hard Bake A Post-Development Thermal Bake Evaporate Remaining Solvent Improve Resist-to-Wafer Adhesion Higher Temperature (120 to 140 C) than Soft Bake

23 Hard Bake Hardening of the resist film after development. Makes it more durable for subsequent processes. Typical temperatures C and 30 min in convection oven. Other oven designs shorter duration Don t bake too hot! This will make the resist flow!

24 Photolithography Track System

25 Automated Wafer Track for Photolithography Load station Vapor prime Resist coat Develop and Rinse Edge-bead removal Transfer station Wafer stepper (Alignment/Exposure system) Wafer Transfer System Soft bake Cool plate Cool plate Hard bake

26 Photolithography Concepts Patterning process Photomask Reticle Critical dimension generations Light spectrum and wavelengths Resolution Overlay accuracy Process latitude

27 Three Basic Exposure Methods 1:1 Exposure 1:1 Exposure ~5:1 Exposure

28 Contact printing capable of high resolution but has unacceptable defect densities. May be used in Development but not manufacturing. Proximity printing cannot easily print features below a few m in line width. Used in nano-technolgy. Projection printing provides high resolution and low defect densities and dominates today. Typical projection systems use reduction optics (2X - 5X), step and repeat or step and scan. They print» 50 wafers/hour and cost $5-10M.

29 Contact lithography implementation. Normally mask is aligned with substrate and then brought into contact with substrate before exposure. Alignment Exposure

30 Contact Lithography Since illumination almost perfectly follows mask, almost perfect features can be produced regardless of contrast of resist. Contact printing produces superb images. Image quality under contact printing conditions is not a reflection of quality of resist. Disadvantages of contact printing: Contact can cause defects in resist. Contact can damage mask. Throughput limited. Contact not used in commercial production of silicon devices. However it can be used in laboratory situations as well as in special low volume production. Smallest feature sizes not easily attainable due to problems making the mask.

31 Commercial contact aligners. Suss laboratory contact aligner. Suss MA6a contact aligner. Quintel contact aligner.

32 Commercial contact aligners.continued. K-W MA1006 contact aligner Suss MA8 contact aligner

33 Contact vs. Proximity Lithography

34 Proximity lithography. In proximity lithography, the mask is held above the substrate by a fixed distance or gap. (better protection for mask) Limiting factor: Fresnel diffraction. General rule: resolution ~ (g ) 0.5 where g is gap. Derivation: Chang and Sze, ULSI Technology (McGraw Hill, 1996), p 274. Take g = 20 micron and = 0.4 micron: resolution ~ 3 micron. Since it is difficult to maintain g < 20 micron, proximity lithography is rarely used in commercial production. Some versions are widely used in research and small scale production (Suss, 250 nm wavelength).

35 Lithographic exposure and equipment. Diffraction pattern from coherently illuminated mask.

36 Projection lithography: General considerations. source Collimating optics mask Projection Lens.

37 Properties of optical lenses used in lithography. Simple lens can be characterized by diameter D and focal length, f. Source distance d 1 from lens will be focused at distance d 2 according to the relation: 1 f 1 d 1 Thus if d 1 = then d 2 = f. 1 d 2 Image will be magnified by factor m=d 2 /d 1. Could alternately define simple lens in terms of numerical aperture NA and D where NA = D/(2f). Lithographic lenses are extremely complex lenses with multiple elements to minimize aberrations resulting from several phenomena. However they can to first order be described in terms of simple lens equivalent.

38 Projection lithography: Resolution. Limitations are due to Fraunhofer diffraction. Consider the following geometry: Souce. Collection optics. Mask Projection lens: NA Image onto surface of wafer.

39 Projection lithography: General considerations. Frauenhoffer Diffraction: Resolution = k 1 / NA Minimum intensity at space is X maximum intensity. Theory: k 1 = 0.61 for optical resolution. k 1 can vary with esoteric optical systems or if response of resist is included.