MICROCHIP MANUFACTURING by S. Wolf

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1 MICROCHIP MANUFACTURING by S. Wolf Chapter 7: BASICS OF THIN FILMS 2004 by LATTICE PRESS

2 Chapter 7: Basics of Thin Films CHAPTER CONTENTS Terminology of Thin Films Methods of Thin-Film Formation Stages of Growth of Amorphous & Polycrystalline Thin Films Structure & Grain Growth Surface Roughness Mechanical Properties of Thin Films Adhesion Stress Measurement of Electrical Properties of Thin Films MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 7-2

3 Chapter 7: Basics of Thin Films TERMINOLOGY OF ULSI THIN-FILMS Thin films of many kinds are used to fabricate silicon ICs They are formed using a variety of techniques: Chemicalreaction of gases with the Sisubstrate (wafer) to form the film (e.g., O 2 (g) + Si (s) --> SiO 2 ) CVD PVD Electroplating Spin-On MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 7-3

4 Chapter 7: Basics of Thin Films REQUIRED CHARACTERISTICS OF ULSI THIN-FILMS The Thin-Films used in ULSI fabrication must satisfy many chemical, structural, and electrical requirements: Controlled Chemical Composition Controlled & Uniform Film-Thickness Low-Density of Defects & Pinholes Excellent Adhesion Low Stress Conformal Step-Coverage Smooth Surface (low Surface-Roughness) Dielectric Layers used in IC interconnects should have small dielectric constants (low-k) Single-Crystal Thin-Films should exhibit a highdegree of crystalline perfection (epitaxial films) MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 7-4

5 Chapter 7: Basics of Thin Films STAGES OF GROWTH OF POLYCRYSTALLINE & AMORPHOUS THIN FILMS Illustration of the various stages in the growth of polycrystalline & amorphous thin-films Grain growth in poly-si thin-film at elevated temperature with time MICROCHIP MANUFACTURING 2004 by LATTICE PRESS 7-5

6 Chapter 7: Basics of Thin Films ADHESION & STRESS IN THIN FILMS Excellent adhesion is critical to maintaining structural integrity of ICs. Film-delamination would cause device failure Glue-layers are often needed Tape-test monitors adhesion Thin-Films adhering to a substrate (e.g., a Si-wafer) can be in a stressed state: Tensile, or Compressive The following slide ( 7-7 ) shows one way how such stress can arise Stress in Thin-Films can cause cracking, change in resistivity, delamination due to poor-adhesion, hillocks, etc. MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 7-6

7 Chapter 7: Basics of Thin Films HOW STRESS ARISES IN THIN-FILMS Example of how compressive stress arises in a thin-film adhering to a thicker substrate, using coiled springs (and an Al thin-film on a thicker Si-wafer) as an analogy Thermal-expansion differences between Al & a Si-substrate lead to stresses in the Al-film. Compressivestress develops in the Al-film. The bottom figure shows the bending of the structures that occurs due these forces. Schematic of hillock-formation due to compressive-stress in Al-film on a Si-wafer (with an SiO 2 -layer on it s surface). Aldiffusion along grain-boundaries is shown by LATTICE PRESS 7-7

8 Chapter 7: The Basics of Thin Films MEASURING THE ELECTRICAL PROPERTIES OF THIN-FILMS The sheet-resistance R S of a conducting Thin- Film is one of its key electrical characteristics If the film-length equals its width, R = ρ/t = R S (expressed in Ω/sq) where R = resistance (Ω), ρ = resistivity (Ω-cm), and t = film-thickness (cm) The sheet-resistance R S is the same for both of these squares, even though they differ in size MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 7-8

9 Chapter 7: Basics of Thin Films MEASURING THE ELECTRICAL PROPERTIES OF THIN-FILMS While R S can be measured using a test structure as shown in the right-hand figure of 7-8 a second method that does not require such a conducting strip, is the 4-point probe method The 4-point probe is more widely used to find R S In commercial systems, a fixed current I is passed between the two outerprobes, & the voltage V is measured between the two inner-probes. Then: R S = 4.53 (V / I) MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 7-9

10 Chapter 7: Basics of Thin Films SUMMARY OF KEY CONCEPTS Many kinds of Thin-Films are used to fabricate silicon Integrated-Circuit structures The form of such Thin-Films can be: single-crystalline, polycrystalline, or amorphous They can be deposited by a variety of methods They must satisfy many materials characteristics, as were discussed in the chapter The growth of all polycrystalline & amorphous Thin-Films follows the same general sequence Stress in a Thin-Film impacts many of its characteristics, including: adhesion, cracking, surface morphology, resistance, and electromigration & corrosion resistance The electrical properties of Thin-Films are expressed in terms of such quantities as its resistivity (ρ) and sheet resistance (Ω/sq) MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 7-10