Developments in Surface Contamination and Cleaning: Methods for Surface Cleaning

Volume 9

Rajiv Kohli, Kashmiri L. Mittal

• Publisher: William Andrew
• Year: 2016
• Print ISBN: 9780323431576
• eBook ISBN: 9780323431729
• Topic: Physical Sciences
• Subtopic: Industrial & Technical Chemistry

Developments in Surface Contamination and Cleaning: Methods for Surface Cleaning, Volume 9, part of the Developments in Surface Contamination and Cleaning series provide a state-of-the-art guide to the current knowledge on the behavior of film-type and particulate surface contaminants and their associated cleaning methods. This newest volume in the series discusses methods of surface cleaning of contaminants and the resources that are needed to deal with them. Taken as a whole, the series forms a unique reference for professionals and academics working in the area of surface contamination and cleaning. A strong theme running through the series is that of surface contamination and cleaning at the micro and nano scales. - Provides a comprehensive coverage of innovations in surface cleaning - Written by established experts in the surface cleaning field, presenting an authoritative resource - Contains a comprehensive review of the state-of-the-art, including case studies to enhance the learning process

Chapter 1

Nonaqueous Cleaning Challenges for Preventing Damage to Fragile Nanostructures

Takeshi Hattori, Hattori Consulting International, Chigasaki, Japan

Abstract

With semiconductor device geometry shrinking and becoming more complex, highly diluted chemicals are used during aqueous silicon-wafer cleaning. The use of physical cleaning aids such as megasonic agitation with dilute chemistry or high-pressure atomizing jet sprays to avoid reduction of particle-removal efficiency is prone to cause structural damage to high-aspect-ratio large-scale integrated (LSI) circuit structures with low mechanical strength as well as fragile freestanding microelectromechanical system (MEMS) structures. The surface tension of water used in wafer rinsing after wet chemical cleaning can also cause pattern collapse of nanostructures during the subsequent wafer drying. In addition, ultrapure water can cause several other problems and result in the degradation of device characteristics. These problems make the development of novel damage-free nonaqueous cleaning methods a high priority. In this article, several water-caused problems that damage advanced semiconductor devices are addressed with possible solutions. As the ultimate solution to overcome the shortcomings of water-based cleaning, various alternative damage-free nonaqueous cleaning techniques are overviewed and discussed, including low-pressure, elevated-temperature hydrofluoric (HF) vapor cleaning, cryogenic aerosol nitrogen cleaning, supercritical carbon dioxide cleaning, and pinpoint dry cleaning that employs lasers, nanoprobes, or nanotweezers. There will be more research challenges and business opportunities in these environmentally benign damage-free cleaning technologies in the near future.

Keywords

Nonaqueous cleaning; nanostructure; silicon wafer; pattern collapse; water-caused problems; surface tension; HF vapor; supercritical carbon dioxide (SCCO₂); pinpoint cleaning; cryogenic aerosol; particle removal

Chapter Outline

1 Introduction 1

2 Water-Caused Problems in Device Fabrication 3

2.1 High Resistivity of Water 3

2.2 High Reactivity of Water with Silicon 5

2.3 High Permittivity of Water 6

2.4 Dissolution and Corrosion of Metals in Water 6

2.5 High Surface Tension of Water 6

3 HF Vapor Processing 9

4 Cryogenic Aerosol Nitrogen Cleaning 11

5 Supercritical Fluid Cleaning 13

5.1 Wafer Drying with SCCO₂ 13

5.2 Wafer Cleaning with SCCO₂ 14

6 Pinpoint Cleaning 18

6.1 Laser Cleaning 18

6.2 Atomic Force Microscopy Nanoprobe Sweeping 19

6.3 Nanotweezers Pickup 20

7 Summary 21

References 22

1 Introduction

As semiconductor devices become ever more highly integrated and their geometry continues to shrink, even slight silicon and oxide etching loss during silicon wafer cleaning can have a negative impact on the characteristics of metal oxide semiconductor (MOS) transistors [1]. Therefore, highly diluted chemicals are used during aqueous silicon-wafer cleaning [2,3] to minimize the material loss. Dilute chemistry is also preferable from the viewpoints of both microroughness control of the silicon surface and environmental control of the chemical consumption [4]. However, it is difficult to remove particles to the extent desirable with highly diluted chemicals.

To enhance the particle-removal efficiency, physical aids such as megasonic agitation are generally employed [5], but they tend to cause structural damage to fragile large-scale integrated (LSI) circuit patterns [6]. Reducing the megasonic power can reduce the megasonic damage to the fragile structures, but it also reduces the particle-removal efficiency. To reduce the device damage by the megasonic energy, modifications have been proposed [6]. In general, conventional aqueous cleaning methods using any kind of sonification become less effective as the particle diameters become smaller.

An alternative physical technique—atomizing jet-spray cleaning employing water–gas mixtures—has been proposed [7]. This technique has an advantage over megasonics because it can remove smaller particles due to the greater impact of droplets at near-sonic speed. Thus, the water–gas mixture atomizing jet spray at a high flow rate or at high carrier gas speed without any addition of chemicals can remove particles on unpatterned wafer surfaces without material loss from chemical etching.

But we must be mindful of the possibility of structural damage on patterned wafers, particularly both fragile freestanding microelectromechanical system (MEMS) and high-aspect-ratio LSI circuit structures with low mechanical strength that can be caused by this type of physically assisted treatment as well as by megasonic agitation. In fact, the water–gas mixture atomizing jet spray at a high speed can easily damage fragile nanostructures on patterned wafers [8]. As the speed decreases, the degree of damage decreases; at the same time, the particle-removal efficiency also decreases. In other words, there is a trade-off between efficient particle removal and minimization of damage to fragile device structures when the atomizing jet spray is used.

To remove particles using the water–gas mixture jet spray at comparatively low speed, an additional chemical pretreatment has been proposed to weaken particle adhesion to the substrate by chemically undercutting or etching native silicon dioxide under the particles [9]. But the use of elevated-temperature ammonium hydroxide–hydrogen peroxide–water mixture (APM) [10] before or during this spray jet causes significant material loss by undesirable chemical etching even though the particle-removal efficiency is increased.

To overcome this issue, an ultradiluted (ppm order) hydrofluoric (HF)–nitrogen gas jet flow spray procedure at room temperature has been proposed [8,11]. Maintaining the high particle-removal efficiency while avoiding structural damage and minimizing material loss has become the key to wafer-surface cleaning in the manufacturing of advanced LSI devices as well as MEMS devices, but there are trade-offs involving particle removal, material loss, and structural damage. The process window for particle removal using physical aids without causing damage is becoming narrower as the device geometry continues to shrink.

Another issue of aqueous cleaning in terms of structural damage to fragile LSI circuit patterns during the subsequent wafer drying is the capillary force due to the surface tension of rinsing water in aqueous cleaning, which will be discussed in the next section in detail. The several issues already mentioned make the development of new damage-free nonaqueous cleaning methods a high priority [4,12].

In this article, several problems associated with water in cleaning during device fabrication will be discussed and possible solutions offered. As the ultimate solutions to overcome shortcomings of water-based cleaning, various alternative damage-free nonaqueous cleaning techniques are overviewed and discussed, including low-pressure, elevated-temperature HF vapor cleaning, cryogenic aerosol nitrogen cleaning, supercritical carbon dioxide cleaning, and pinpoint dry cleaning that employs lasers, nanoprobes, or nanotweezers.

2 Water-Caused Problems in Device Fabrication

Semiconductor fabrication plants (fabs) are generally located at the sites where abundant clean water is available. Ultrapure water (UPW) plays an essential and important role in the semiconductor industry for cleaning and rinsing the surfaces of silicon wafers and photomasks, as well as for diluting aqueous chemicals used for polishing, etching, and cleaning the surfaces [13]. Recently, in advanced photolithography for finer circuit pattern exposure (so-called immersion lithography), UPW has been used as a medium placed between the final lens and the silicon-wafer surface in the exposure system to enhance patterning resolution, thereby achieving smaller feature sizes of the integrated circuit...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• List of Contributors
• About the Editors
• Preface
• Chapter 1. Nonaqueous Cleaning Challenges for Preventing Damage to Fragile Nanostructures
• Chapter 2. Gas-Phase Cleaning for Removal of Surface Contaminants
• Chapter 3. Laser-Induced Spray Jet Cleaning
• Chapter 4. Brush Scrubbing for Post-CMP Cleaning
• Chapter 5. Contamination Removal From UV and EUV Photomasks
• Chapter 6. Aqueous Displacement of Water-Immiscible Cleaning Solvents: Cleaning Enhancement Using Ultrasonics
• Index