Polymers for Microelectronics and Nanoelectronics - American

Chapter 12

Porous Low-k Dielectrics: Material Properties 1

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C. Tyberg , E. Huang , J. Hedrick , E. Simonyi , S. Gates , S. Cohen , K. Malone , H. Wickland , M . Sankarapandian , M . Toney , H.-C. Kim , R. Miller , W. Volksen , P. Rice , and L . Lurio 1

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Downloaded by UNIV OF LEEDS on May 18, 2016 | http://pubs.acs.org Publication Date: February 7, 2004 | doi: 10.1021/bk-2004-0874.ch012

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IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598 I B M Microelectronics, Hopewell Junction, NY 12533 I B M Almaden Research Center, San Jose, CA 95120 Northern Illinois University, Dekalb, IL 60115 2

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Abstract Improvements in back end of the line (BEOL) interconnect performance require the reduction of resistance and capacitance. The semiconductor industry, led by IBM, has migrated from aluminum to copper wiring in a scaled manner to lower resistance and capacitance in appropriate wiring levels and enhance performance. More recently, industry focus has centered on decreasing capacitance by reducing the dielectric constant of the insulator. Numerous low k dielectric candidates exist ranging from PECVD materials to organic thermosets. Unfortunately, no potential candidate possesses properties comparable to silicon dioxide, which has been the primary insulator in semiconductor chips for over 30 years. In this paper, properties and challenges of integrating low k dielectrics will be described. In addition, extendibility of low k dielectrics by incorporating porosity is discussed.

© 2004 American Chemical Society

Lin et al.; Polymers for Microelectronics and Nanoelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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Introduction For over 30 years silicon dioxide (S1O2) has been the dielectric insulator of choice for the semiconductor industry. Silicon dioxide possesses excellent dielectric breakdown strength, a high modulus, good thermal conductivity and excellent adhesion to metallic liners, PECVD barrier cap layers, etc. However, with ground rule reductions and the need for improved interconnect performance, Si0 is being replaced with materials possessing lower permittivity to achieve reduced capacitance. Fluorosilicate glass (FSG), for instance, has replaced Si0 in high performance logic and SRAM technologies. IBM integrated FSG with copper and implemented the technology at the 0.18 μπι technology node (1). For the 130 nm technology generation IBM selected the SiLK™ Semiconductor Dielectric integrated with copper for advanced BEOL interconnects (2). The combination of SiLK™ and copper reduces the normalized resistance/capacitance (RC) delay by 37% compared to silicon dioxide and aluminum structures. At the 90 nm technology generation, the target effective dielectric constant (keff) according to the 2001 International Technology Roadmap for Semiconductors (3) is 2.6-3.1. The is a composite value comprised of the dielectric, hardmask (if present), barrier cap layer, and etch stop layer (if present) as shown in Figure I. To achieve a keff of -3.0 the dielectric constant of the intermetal dielectric (IMD) must be -2.7. Therefore, the introduction of ultra low k dielectrics is not expected until the 65 nm technology generation, where the target effective dielectric constant is 2.3-2.6 (3). To achieve this, ultra low dielectric constant, materials with k < 2.2 will be required. In order to attain sufficiently low dielectric constants, porosity must be incorporated. Extendibility of unit processes and tooling in die fabrication of interconnecte is dependent on the extendibility of the dielectric material and integration scheme. Therefore, the dielectric choice for the 90 nm generation ideally should provide a pathway toward ultra low k porous dielectrics for the 65 nm generation and beyond. 2


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Low k Dielectrics The integration of low-k dielectrics into BEOL interconnects is not trivial. In fact, the immensity of the task parallels the transition from aluminum to copper wiring. The challenges can be attributed to the absence of low-k dielectrics available having electrical, thermal, mechanical, or thermal conductivity properties comparable to silicon dioxide. Low-k materials, in general, are less dense and typically possess a lower modulus and hardness as well as decreased thermal conductivity. Table I shows a comparison of die properties of an organic low-k dielectric (SiLK™), SiCOH (a class of materials containing Si, C, Ο , and H), and silicon



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Figure I. Back end of the line (BEOL) wiring structure.

Table I. Comparison of Low-k Dielectric Material properties to Silicon Dioxide Property Dielectric Constant Leakage Current at 1 MV(A/cm ) at 150°C Breakdown Field (MV/cm ) Modulus (GPa) Hardness (GPa) Toughness MPam Thermal Conductivity (W/mK)

Organic Dielectric (SiLK™) 2.62 3.3 χ 10'

10

Si C O H w

x

y

z

2.7-3.0 ~1 χ 10

1U

Silicon Dioxide 3.9-4.5 11

~2 χ 10"

2

~4

6-10

>8

2.7 0.25 0.62

9-15 1.3-2.4 -0.28

72 8.7 0.8

0.19

0.40

1.07

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164 dioxide. Comparison of the organic low-k dielectric and SiCOH demonstrates that SiCOH possesses a significantly higher modulus and hardness, but the organic low-k dielectric (SiLK™) possesses superior toughness. However, in terms of material properties, Si0 is far superior to both low k dielectrics. Silicon dioxide possesses: a modulus that is 5 to 25 times higher; thermal conductivity that is 2.5 to 5 times higher; and a hardness value that is 4 to 30 times higher than SiCOH and SiLK™. 2


Extendibility of Low-k technologies with Porous Dielectrics With the prospect of changing dielectrics, liners, hardmasks and barrier cap layers in future technologies to enable performance enhancements and maintain reliable interconnects with ground rule reductions, it is essential to evaluate the extendibility of current technologies. Extendibility is important to reduce development costs and capital expenditures, thus it is critical to maximize the lifetime of development efforts and avoid re-tooling of the semiconductor infrastructure with each technology generation. Thus, 90 nm generation unit processes and structures ideally should enable evolution to the 65 nm technology node and beyond. With keff targets of -2.3-2.7 for the 65 nm generation, the incorporation of porosity into dielectrics to enable reduced permittivity is inevitable. The utilization of porous dielectrics in BEOL interconnects will pose additional challenges beyond conventional low-k technology. A thorough characterization of the porous structure along with the accurate measurement of electrical and mechanical properties will be critical in the selection of ultra low-k porous dielectrics. Integration of ultra low dielectric constant porous dielectrics is extremely challenging due to the reduction of mechanical properties in comparison to dense low-k dielectrics. Mechanical properties such as modulus, hardness, fracture toughness, adhesion, and coefficient of thermal expansion are critical parameters in screening materials to achieve successful integration. Porous materials with a dielectric constant of 2.2 may have a ~30% reduction in modulus and hardness due to the incorporation of porosity. Similarly, the fracture toughness and adhesion may also be lowered by the incorporation of porosity. This reduction in the mechanical properties can result in delaminations during chemical mechanical polishing or failure during reliability testing or packaging. Another property of importance is the coefficient of thermal expansion. Any mismatch in the coefficient of thermal expansion of the dielectric insulator and the metal interconnects can create stress on liners and metal lines during processing and reliability testing. This is especially critical for organic dielectrics that have significantly higher thermal expansion than the metal wiring.



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Table II shows a comparison of the properties of an organic spin-on ultra low dielectric constant material (porous SiLK™), to silicon based ultra low-k dielectrics by both spin-on (primarily silsesquioxane materials) and CVD processes (4). Porous organic dielectrics have slightly lower modulus and hardness, lower breakdown voltages, and higher coefficients of thermal expansion than the SiCOH type dielectrics of the same k value. In addition, the pore size of porous organic dielectrics is often larger than that of the porous SiCOH based dielectrics of an equivalent dielectric constant. However, porous organic dielectrics, such as porous SiLK™, are significantly tougher than porous SiCOH dielectrics and are not susceptible to cracking; whereas, most silsesquioxane (spin-on porous SiCOH) based porous dielectrics have a crack threshold of less than 2 microns, and are susceptible to stress corrosion cracking. In addition, the porous organic dielectric evaluated exhibits superior adhesion behavior during blanket metalized chemical mechanical polishing, as delaminations are not observed at down forces up to 9 psi. Porous silsesquioxane dielectrics tend to show delaminations at down forces as low as 12 psi. Therefore, as with the non-porous low-k dielectric candidates, the competing ultra low-k porous dielectrics exhibit similar trade-offs in properties as organic dielectrics have lower modulus, higher CTE, and larger pore size, but significantly improved toughness and adhesion in comparison to silicon based porous dielectrics. These trade-offs make the choice of the best material for integration difficult. However, due to the need for an extendable integration approach, the choice of porous dielectric will likely depend on the dielectric choice and integration success of the 90 nm technology generation.

Characterization of Porous Dielectrics Mechanical and thermal characterization of ultra low k dielectrics is very similar to the characterization of dense low k dielectrics; however the introduction of porosity requires the development of new characterization techniques in order to understand the pore structure. The dielectric constant, dielectric breakdown and coefficient of thermal expansion can be measured using the same techniques used for dense low k dielectrics. Modulus and hardness can also be measured by the same techniques, however, if using nanoindentation, measurements from porous dielectrics may have larger substrate contributions at equivalent film thicknesses. Therefore, the modulus values



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Table Π. Mechanical Properties of Porous Dielectric Property

2.15

Porous SiwC O H (Spin-on) 1.4-2.3

2.1-2.5 0.10-0.15 ~ 65-70 >25

1.5-3 0.10-0.35 -14

Polymers for Microelectronics and Nanoelectronics - American

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