SEMICONDUCTOR MANUFACTURING

TOPIC 3.6: PATTERNING TECHNOLOGY (ETCH)

I. Objectives

• At the end of this topic, students should be able to:

  • Understand the keywords associated with etching.

  • Compare and contrast wet and dry etch.

  • Appreciate the need to shift from wet to dry etch due to shrinking linewidths.

II. Introduction to Etching

• Overview of Etching: Etching is a process for removing material from a substrate or thin films on the surface of a substrate. A mask layer protects specific regions of the wafer surface during this process. The goal is to remove material precisely in areas not covered by the mask.

• Ideal Etch Process: An ideal etching process is not entirely possible; thus, etching processes may not perfectly transfer patterns established by the mask.

III. Parameters in Etching

• Bias: The difference in lateral dimensions between the etched image and the mask image.

• Etch Rate (ER): The rate at which material is removed during etching.

• Uniformity: Refers to the evenness of the etching process across the surface.

• Selectivity: The ratio of etch rates between the target material and other materials present on the wafer. High selectivity allows for the complete removal of the selected layer without damaging the underlying layer or mask.

IV. Etching Process

1. Mass transport of reactants to the surface to be etched:

   • Occurs through a boundary layer.

2. Reaction between reactants and the film surface:

   • Takes place at the surface to be etched.

3. Mass transport of reaction products:

   • Exiting from the surface through the boundary layer.

V. Etch Rate and Selectivity

• Etch Rate (ER) Calculation:

  • $ext{Uniformity} = rac{ ext{Max ER} + ext{Min ER}}{ ext{Time taken}} imes 100 ext{<br>}$

• Selectivity (S) Calculation:

  • $S = rac{R<em>m}{R</em>s}$

  • Where:

  • $R_m$ = Etch rate of the target material.

  • $R_s$ = Etch rate of the underlying material.

• Degree of Anisotropy: Defined as the lateral etch ratio:

  • Isotropic Etching: $R_L = 1$ (etch rates are equal in all directions).

  • Anisotropic Etching: 0 < R_L < 1 (etch rate is different across directions).

  • Perfectly Anisotropic Etching: $R_L = 0$.

VI. Categories of Etching

1. Wet Etching:

   • Uses liquid-based etchants.

   • Isotropic etching predominantly.

2. Dry Etching:

   • Involves plasma-based etching.

   • Includes different methods such as Reactive Ion Etching (RIE), which provides more control and anisotropic profiles.

VII. Wet Etching

• Process:

  1. Immersing wafers in etchant solutions.

  2. Rinse steps to remove the acid and etchant residues.

• Advantages:

  • Simple equipment, batch processing, and high throughput.

  • High selectivity for various materials.

• Disadvantages:

  • Can lead to undercutting due to isotropic nature, potentially causing damage to features.

  • Requires large volumes of etchant and higher disposal costs.

VIII. Typical Wet Etchants

• Etchants Overview:

  1. KOH: Etching Rate ~ 6-600 nm/min (high selectivity) for Si.

  2. HNO3 + H2O + HF: Etching Rate ~ 100 nm/min for SiO2; Selectivity > 50:1.

  3. H3PO4: Etching Rate ~ 10 nm/min for GaAs.

  4. HCl + HNO3: ~ 40 nm/min for Au; Selectivity > 50:1.

IX. Wet Etching Methods

• Immersion Wet Etching:

  • Wafers are submerged in etchant; simple and economical.

  • Agitation of the solution can improve uniformity.

• Spray Wet Etching:

  • Etchants are sprayed onto surfaces, allowing quick etchant removal and enhanced definition.

X. Dry Etching

• Emergence: Developed due to limitations in wet etching when dealing with smaller geometries.

• Advantages:

  • Anisotropic etching possibility, easy disposal of byproducts, low gas consumption.

• Disadvantages:

  • Complex equipment, low throughput, selectivity issues.

XI. Types of Dry Etching Techniques

1. Physical Sputtering: Ion milling and plasma sputtering.

2. Plasma Enhanced Etching: Combines plasma-assisted chemical reactions.

3. Reactive Ion Etching (RIE): Involves both chemical reactions and ionic bombardment for more effective etching.

XII. Conclusion: Wet vs. Dry Etching

• In modern Integrated Circuit (IC) manufacturing, dry etching has gained prominence due to shrinking features that require greater precision.

• A comparison shows:

  • Wet Etching: More suitable for non-critical features; faster but isotropic and risks undercutting.

  • Dry Etching: Better control for smaller feature sizes and allows for non-isotropic etching patterns essential for advanced technologies.

XIII. References

• Accessed URLs for further information and examples included in the original content.

XIV. Tutorial Questions

1. Identify the two fundamental types of etching and their differences.

2. Explain the main purpose of the etching process in semiconductor manufacturing.

3. Discuss how wet etching differs from dry etching regarding precision and feature size.

4. What are the advantages and disadvantages of wet etching in semiconductor processing?

5. Why is dry etching preferred in integrated circuit design as feature sizes decrease?

6. Define etching selectivity and its importance in the etching process.

7. Describe the three steps involved in the etching process.

8. Distinguish between anisotropic and isotropic etching.

9. What benefits does reactive ion etching present compared to other methods?

10. Discuss the challenges in achieving accurate pattern transfers in etching processes.