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`Stress-induced
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`Many of the processes used in the fabrication
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`of silicon integrated circuits lead to the
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`Given enough stress, the substrate will yield
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`by generating dislocations. We examine the
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`formation of stress-Induced dislocations in
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`integrated circuit structures. Examples are
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`integrated circuit structures that were created
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`generation during the formation of a shallow-
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`dislocations is illustrated using structures
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`a process used to form a trench capacitor In a
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`DRAM cell. The effect of device layout
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`solely Identifying those process conditions
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`under which dislocations are generated. By
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`combining TEM observations with stress
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`analysis, we show how the sources of stress
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`1. Introduction
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`Given enough stress, a silicon substrate will yield by
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`generating dislocations. Although it would certainly be
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`preferable for this not to occur during the fabrication of
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`silicon integrated circuits (ICs), its occurrence appears to
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`be an unavoidable by-product of the processes used to
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`fabricate such circuits; dislocations often appear at some
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`Once present in the silicon substrate, dislocations can lead
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`to charge leakage and electrical shorting between
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`elements—effects that can seriously degrade or prevent
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`device operation [1, 2].
`There are many sources of stress that arise during IC
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`fabrication processes. Some important examples are the
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`imbedding of materials with thermal expansion coeflicients
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`different from that of silicon, deposition of films with
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`intrinsic stress, and oxidation of nonplanar surfaces. A
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`°Copyrlght 1992 by International Business Machines Corporation. Copying in printed fonn for private use is permitted without payment of royalty provided that (1) each
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`reproduction is done without alteration and (2) the Journal reference and IBM copyright notice are included on the first page. The title and abstract, but no other portions. of
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`this paper may be copied or distributed royalty free without further permission by computer-based and other information-service systems. Permission to republish any other
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`portion of this paper must be obtained from the Editor.
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`P. M. FAHEY ET AL.
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`IBM J. RES. DEVELOP. VOL. 36 NO. 2 MARCH 1992
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`SAMSUNG ET AL. EXHIBIT 1043
`Page 1 of 25
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`recent paper by Hu [3] gives an excellent overview of
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`these and other stress—related problems. In the present
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`paper, we present examples of stress-induced dislocations
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`that occur during the fabrication of advanced bipolar and
`MOS-based integrated circuits. This is a timely subject
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`because of the growing importance of stress-induced
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`defects in evolving IC processes.
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`For each succeeding generation of integrated circuits,
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`two major trends are evident: The packing density of
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`circuits on a chip increases, and device dimensions are
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`reduced. Increasing packing density permits a greater level
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`of integration per chip; reducing critical device dimensions
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`leads to improved device performance for both M08 and
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`bipolar structures. Unfortunately, the two trends lead to
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`processes in which stress levels increase.
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`Higher packing densities are achieved by developing
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`devices that occupy smaller areas of the silicon substrate
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`and by packing the devices closer together. However, such
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`scaling is subject to the constraint that devices must be
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`electrically isolated from one another by isolation regions.
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`In general, the reduction of the area required for isolation
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`has been found to result in larger substrate stresses. In
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`addition, as the area occupied by an active device
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`continues to shrink, more of the device in each isolated
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`cell is in proximity to the edges of its isolation regions, and
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`some of the largest stresses develop at these locations.
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`Scaling of device dimensions may lead indirectly to
`increased substrate stress. To maintain critical device
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`dimensions (e.g., the channel length of a MOSFET or the
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`base width of a bipolar transistor), control of dopant
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`diffusion is essential. A large degree of such control is
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`accomplished by minimizing the temperatures of thermal
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`processing. But for oxidation processes, lowering the
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`temperature of oxidation leads to higher stress levels in the
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`substrate [4~6]. The fundamental reason for this is that an
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`oxide growing on a nonplanar silicon surface must
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`constantly deform, and oxides become more resistant to
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`strain as temperatures are lowered. As an oxide thus
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`becomes more rigid, a greater amount of the stress that
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`develops during oxidation is accommodated by the
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`development of strain in the substrate. Given the pervasive
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`use of oxidation steps in IC fabrication, oxidation-induced
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`stress is likely to become an increasingly important
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`concern in the continuing drive toward lower-temperature
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`processing.
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`Since stress is responsible for the unwanted appearance
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`of dislocations, a question that naturally arises is how
`much stress a silicon wafer can tolerate before dislocations
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`are generated. There is no simple answer to this question.
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`A variety of studies indicate that stress levels of the order
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`of 10’ dynesfcmz or higher should be considered
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`significant. In practice, however, the strengths of silicon
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`wafers vary depending on a number of factors--in
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`particular, the oxygen content and thermal history of the
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`wafer [’?~9]. Also, the generation of stress—induced
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`dislocations is a two~step process. Dislocations must first
`be nucleated and must then grow or move into regions
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`where they affect devices. The stress necessary to nucleate
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`dislocations depends greatly on the particular process of
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`nucleation. Nuclei may be found in the as-grown wafer
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`[7—9] or may be introduced into the wafer during
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`processing. Two common process steps that assist the
`nucleation of dislocations are oxidation and ion
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`implantation. Oxidation generates silicon self-interstitials,
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`which can coalesce preferentially in strained silicon ll}.
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`Vanhellemont et al. [10, 11] have discussed in detail the
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`homogeneous nucleation of dislocations by condensation of
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`self-interstitials. Ion implantation disrupts the crystalline
`structure of the silicon lattice and creates an excess of
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`point defects. Although the details of the nucleation
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`process following ion implantation are not thoroughly
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`understood, it is well established that implantation damage
`can lead to the nucleation of dislocations. The effect of
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`implantation on dislocation generation in stressed material
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`can be quite dramatic; we demonstrate this with examples
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`from bipolar and DRAM technologies (Sections 4 and 5).
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`Once nucleated, dislocations can move great distances
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`under an applied shear stress, by the process of glide. (The
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`level of stress necessary to move dislocations depends, as
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`in the case of nucleation, on oxygen content and thermal
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`history [l2].) Thus, dislocations initially created in a
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`locally stressed area can propagate to other parts of a
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`device cell (Sections 3 through 5).
`The examples presented in this paper are taken from
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`structures used in developing IC processes. During the
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`developmental stage, process conditions are often varied to
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`extreme cases, yielding important information regarding
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`relevant process windows and information regarding
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`process extendability: for example, extendability by placing
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`devices closer together (vs. changing the layout design or
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`isolation scheme) or by lowering processing temperatures
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`and thereby minimizing dopant diffusion. We have chosen
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`examples gathered from process experiments performed as
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`part of a few different chip development efforts at several
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`IBM laboratories. The examples illustrate the underlying
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`causes of stress, how they cause dislocation generation,
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`and how solutions to defect generation can be found.
`In Section 2 we examine the underlying causes for the
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`development of stress in oxide layers on silicon substrates
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`and relate its development to oxidation parameters
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`(temperature, pressure, ambicnce). In Section 3 we present
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`an example illustrating the increased susceptibility of a
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`process to defect generation as oxidation temperatures are
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`lowered. The example explores dislocation generation
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`resulting from a shallow-trench isolation process
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`compatible with the requirements of a 16Mb DRAM
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`technology. In addition to the temperature of oxidation,
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`the importance of cell layout geometry is shown to be a
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`IBM 1. RES. DEVELOP. VOL. 36 NO. 2 MARCH 1992
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`P. M. FA}-IEY ET AL.
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`SAMSUNG ET AL. EXHIBIT 1043
`Page 2 of 25
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`key factor in determining the onset of defect generation.
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`The very important process of defect nucleation by ion
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`implantation is discussed in Section 4, where we examine
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`the effects of implantation on a deep-trench isolation
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`process used in a bipolar technology. Damage created by
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`ion implantation can develop into dislocations that glide
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`large distances in stressed material. Substrates that
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`otherwise show no signs of defects can become heavily
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`dislocated after ion implantation. The geometry of cell
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`layout again plays a key role in affecting this behavior.
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`Finally, in Section 5 we present a detailed stress analysis
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`for dislocation generation and propagation in a 4Mb
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`DRAM process.
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`2. Oxldatlon-induced stress
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`Oxidation can introduce stress into silicon substrates in
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`two different ways. The first is associated with the volume
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`expansion of SiO2 from silicon. A given volume of silicon
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`will produce about 2.2 times as large a volume of SiO,. If
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`the oxide is not free to expand, it pushes on whatever
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`material is constraining its growth. This occurs frequently
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`during IC fabrication, for example, during oxidation along
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`the sidewalls of polysilicon-filled trench structures or at the
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`edges of regions masked with Si3N4. A second cause of
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`stress is the strain that oxide layers experience when
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`oxidation is performed on nonplanar structures. As a
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`nonplanar silicon surface is oxidized, the oxide layer is
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`constantly forced to stretch or contract as it grows out of
`the silicon surface.
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`How much stress is translated from an oxide layer into
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`the substrate is determined by how easily the oxide
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`deforms as it grows. In the following discussion, we
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`consider the deformation of an oxide layer growing on a
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`nonplanar substrate. The parameters that determine the
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`ability of an oxide layer to deform in response to stress are
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`examined, with particular attention focused on the effect of
`oxidation conditions on oxide stress.
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`0 Development of stress in Si02
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`Upon oxidation, the growth of oxide at the Si/SiO,
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`interface pushes the oxide above it away from the Si
`surface. On a convex-shaped surface, this causes the oxide
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`to experience a tangential tensile stress; on a concave-
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`shaped surface, compressive stress results. We are
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`primarily interested in how stress develops in the silicon
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`substrate during an oxidation step. This requires foremost
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`an understanding of how an SiO, film deforms in response
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`to the stresses that arise during its growth. Our knowledge
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`of such stress-induced deformation processes is not yet
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`satisfactory, and the subject remains an active area of
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`research. However, in the past few years there has been a
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`growing consensus that the best starting point for
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`understanding nonplanar oxide growth is to View Si02 as a
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`nonlinear viscoelastic material [13, 14]. In recent papers,
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`Rafferty, Borucki, and Dutton [13] and Hu [15] have
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`provided an excellent illustration of the viscoelastic
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`properties of SiO2. They considered the simple case of
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`oxidizing a cylinder of silicon. The geometry of this
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`problem lends itself to relatively simple analyses of stress
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`induced in a growing oxide, yet the oxidizing cylinder
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`example brings out most of the essential features of
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`oxidation-induced stress on nonplanar surfaces. Hu has
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`modeled oxide stress on a cylinder under a variety of
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`process conditions. Rafferty et al. have modeled oxide
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`stress under a more limited set of conditions than Hu, but
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`with a more realistic viscoelastic model for SiO2. In the
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`following treatment we use the oxide model of Rafferty
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`et al. and reexamine the model predictions of Hu for a
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`variety of process conditions. We also use the oxidizing
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`cylinder as a vehicle to demonstrate how different model
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`assumptions used in process modeling programs manifest
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`themselves in predicting oxidation-induced stress.
`Consider a cross section of a silicon cylinder having a
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`thin surface oxide layer, the outer surface of the oxide
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`located at a radial distance ru. When the cylinder is
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`oxidized, the newly formed oxide at the silicon surface will
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`push out the original oxide layer. After a given oxidation
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`time, the outermost layer of oxide moves from ro to r,
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`stretching out in the process. The stretching process
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`requires a tangential strain of s = (r — ro)/ru. If the oxide
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`grows at a rate X”, it is a simple matter to show that
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`i‘ = (1 — B)Xox, where [3 = 0.44 is the amount of silicon
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`consumed for a given thickness of oxide grown. Therefore,
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`the strain rate of the outermost oxide layer on the cylinder
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`is given by
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`, _ <1 — 13))‘,
`8 -
`re
`ox '
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`1
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`(
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`The question is how this strain is accomplished. If the
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`oxide is highly resistant to deformation, large stresses
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`develop in the oxide during the oxidation and,
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`consequently, also in the underlying silicon. On the other
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`hand, if the oxide can be stretched easily, little stress is
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`developed during the oxidation. In general, stress is
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`accommodated by a combination of elastic deformation
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`and viscous flow. Following the treatment by Hu [15]
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`and Rafferty et al. [13], we illustrate the viscoelastic
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`deformation process by analogy to a spring and dashpot
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`(i.e., damped piston) in series-—-a combination known as a
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`Maxwell element (Figure 1).
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`Initially, the system is assumed to be in a state of zero
`stress. Stress is developed by applying a force that moves
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`the end of the spring. The total strain in the system is e =
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`(A — A0)/A0, where A0 is the original position ofA. If the
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`end of the spring moves at a speed 11, the strain rate of the
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`system is é = v/A0. The total strain is the sum of elastic
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`P. M. FAHEY ET AL.
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`IBM J. RES. DEVELOP. VOL. 36 NO. 2 MARCH 1992
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`SAMSUNG ET AL. EXHIBIT 1043
`Page 3 of 25
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`elastic
`and viscous deformation processes, i.e., 3 = e
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`VlSCO|JS '
`The elastic strain is given by
`s
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`0'
`(A — B)
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`0
`elastic _ T_ E ’
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`S
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`+
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`where G is the spring constant, which we identify with the
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`elastic modulus of rigidity of the oxide. The stress—strain
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`relation for viscous flow is expressed as
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`’
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`0'
`B
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`= —-— = —
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`0
`77
`Eviseous A
`where 27 is the viscosity of the dashpot (i.e., the viscosity
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`of the oxide). Equations (2) and (3) lead to the differential
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`equation
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`0'
`6'
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`-6 + ; = é.
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`(4)
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`Equation (4) is a simple but useful expression for
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`understanding many of the important factors that
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`determine evolution of stress in oxides during growth. By
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`using values for G and 1; corresponding to SiO2 and using
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`the expression for é in Equation (1), relative levels of
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`stress generated with different oxidation conditions can be
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`investigated for the oxidizing cylinder example [13, 15].
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`For example, one can calculate the relative stress levels in
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`different thicknesses of an oxide film grown at a given
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`temperature, or the relative stress levels in an oxide film of
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`a given thickness grown at different temperatures.
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`In the simplest case, both the growth rate and Viscosity
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`are assumed constant. In this case, Equation (4) has the
`solution
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`0' = né(1 — e'°"").
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`(5)
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`Viscoelastic model of oxide strain. Analogy is made to a spring
`and dashpot (i.e., damped piston) system.
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`This equation states that the oxide starts at zero stress and
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`asymptotically approaches a value of né. Physically, this
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`means that the oxide deforms elastically in the initial
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`stages of growth; then, as the oxide continues to grow, its
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`stress is relieved by viscous flow. In steady state, the
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`stress in the oxide is directly proportional to the oxidation
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`rate times the viscosity. Obviously, the less viscous the
`oxide, the smaller the saturation value of its stress. Oxide
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`viscosity decreases with higher temperatures and
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`increasing hydroxyl content (see the papers by Hu [15] and
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`Stiffler [14] for a summary of data). Thus, performing
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`oxidations at the highest acceptable temperature and in a
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`wet rather than a dry oxygen ambience would seem to be
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`preferable. However, oxidation rates increase with
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`increasing temperature and are higher in a wet oxygen
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`ambience than in a dry oxygen ambience. Faster oxide
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`growth means that the strain rate is higher and viscous
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`flow must increase to avoid stress buildup. Predictions
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`regarding such trade-offs in processing conditions can be
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`obtained by solving Equation (4). In that regard, Equation
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`(5) predicts that growing oxides at high temperatures in a
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`wet oxygen ambience does indeed minimize stress levels,
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`despite growth at relatively high rates. Hu [15] has shown
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`that this same qualitative behavior occurs for the more
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`general situation of linear-parabolic oxide growth. In
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`addition, the analysis of Hu predicts that high-pressure
`steam oxidation should lead to further reductions in oxide
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`stress from the levels of atmospheric oxidation—again,
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`despite the increased oxidation rates at high pressure.
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`The above analysis applies under the assumption that
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`oxide viscosity remains constant during growth. The
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`Viscosity of SiOz was considered a function of temperature
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`and hydroxyl content only. However, it has been shown
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`experimentally that the viscosities of glasses decrease
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`under high stress [16—18]. Recent treatments [13, 14, 19,
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`20] of oxidation—induced stress have attempted to take
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`account of this fact. Sutarjda and Oldham [19, 20], Rafferty
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`et al. [13], and Stiffler [14] have favored the use of
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`Eyring’s model [21] to describe viscosity. In Eyring’s
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`model, the viscosity of the oxide decreases dramatically at
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`IBM J. RES. DEVELOP. VOL. 36 NO. 2 MARCH 1992
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`P. M. FAHEY ET AL.
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`SAMSUNG ET AL. EXHIBIT 1043
`Page 4 of 25
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`where no is the zero-stress viscosity and Vm is a
`characteristic volume associated with the space cut out by
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`a diffusing molecule during flow. Eyring [21] has analyzed
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`the dependence of viscosity on stress from the viewpoint
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`of absolute rate theory. However, the quantitative
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`predictions of this atomistic approach do not agree well
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`with experiment [16]. To obtain agreement with
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`experiment, Vm has been viewed as a fitting factor.
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`Donnadieu et al. [18] have empirically determined a value
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`for Vm of approximately 180 A3.
`As an example of how oxide stress varies as a function
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`of process Variables, we show in Figure 2 calculations for
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`stress in the outermost layer of oxide grown on a silicon
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`cylinder 1 pm in radius. To produce these plots, we have
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`solved Equation (4) using the expression for viscosity :1
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`given in Equation (6). We have used a value of G =
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`3 X 10“ dynes/cmz [22, 23] and viscosity values for no
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`from Stiflier [14], and have assumed oxidation rates for
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`{100} surfaces using the Deal—Grove formulation [24]
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`Xi. X...
`k— + 7;» _ 1,
`p
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`(7)
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`where kp and k, are the parabolic and linear growth-rate
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`constants. For wet oxidations, we have used values for kp
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`and k, from Razouk et al. [25]; for dry oxidations, we have
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`used values from Hess and Deal [26, 27].
`In Figure 2(a) we show calculated stress values for
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`varying thicknesses of oxide grown at different
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`temperatures. This plot indicates that stress should
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`decrease as the oxidation temperature is increased. This is
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`primarily due to the decrease of viscosity with increasing
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`temperature. Similar plots by Hu [15], using a stress-
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`independent viscosity model, show a stronger temperature
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`dependence on stress than that shown in Figure 2(a). A
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`secondary effect involves the quicker transition at higher
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`temperatures from linear growth rate (Xox o< t) to parabolic
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`growth rate (Xox 0: K“). In the Viscous flow regime,
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`stress is expected to decrease as the strain rate decreases
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`[Equation (3)]. Therefore, as the oxidation rate decreases
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`during parabolic growth, stress should also decrease. The
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`effect of parabolic oxide growth is most pronounced for
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`the 1100°C case in Figure 2(a), where more of the oxide
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`growth takes place in the parabolic regime compared to the
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`lower temperatures of oxidation. Similar plots by Rafferty
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`et al. [13], using the same type of viscoelastic oxide model,
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`do not show this behavior. For simplicity, Rafferty et al.
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`ignored the parabolic growth regime in their treatment and
`assumed oxidation rates to be constant with time.
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`Including full linear-parabolic growth-rate behavior for the
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`oxide, Figure 2(a) shows that for a given temperature there
`should be a maximum value in the amount of stress in the
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`oxide. Once the oxide growth has moved from the elastic
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`to viscous regimes, no further increase in stress is
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`Strcssinouteroxide(aynes/cm’)
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`iso
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`Oxide thickness (urn)
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`(A)
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`Oxidation at 900°C
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`3 3 §.
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`5 3
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`Calculated oxide stress as a function of oxide thickness at different
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`conditions, for growth on a silicon cylinder, initially 1 pm in ra-
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`(a) Stress in the outermost oxide layer for different oxidation
`dius:
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`temperatures.
`(b) Stress developed at 900°C for different ambient
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`conditions.
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`high stresses. The high-stress behavior is described by the
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`relation
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`(¢rVm/2kT)
`" = "° sinh(¢rVm/2kT)’
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`(6)
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`P. M. FAHEY ET AL.
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`IBM J. RES. DEVELOP. VOL. 36 N0. 2 MARCH 1992
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`SAMSUNG ET AL. EXHIBIT 1043
`Page 5 of 25
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`€
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`expected. As long as the maximum stress level is not
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`enough to generate dislocations in the bulk, it should be
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`possible to grow any thickness of oxide without defect
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`generation. The prediction that the growth of the thicker
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`oxide does not necessarily lead to more substrate stress is
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`not at all intuitive, and demonstrates the value of the
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`viscoelastic analysis. Conversely, the viscoelastic analysis
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`indicates that growing relatively thin oxides is no
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`guarantee that high levels of stress will not be generated.
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`Supportive experimental results for the above predictions
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`are presented in Section 3.
`In Figure 202) we show the calculated effects of oxidant
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`ambience on stress. To take account of changes in
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`pressure, we have scaled Ice and kp linearly with pressure
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`[24, 25]. Reductions in stress going from dry to wet
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`oxidations and from atmospheric to high-pressure wet
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`oxidations are caused by corresponding decreases in
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`viseosities. The predicted stress reduction for high-
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`pressure wet oxidation compared to atmospheric
`conditions is dramatic. Note also that in the 5—atm
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`oxidation case there is no large elastic contribution to
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`stress; i.e., there is no initial increase of stress with oxide
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`thicimess [Equation (2)] for thin oxides as there is for the
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`1-atm dry and wet cases. This is because the oxide
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`viscosity at 5 atm is already low enough that oxide flows
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`viscously during the entire time of oxidation. While Figure
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`2(b) implies a beneficial efiect of reducing viscosity, it
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`should be kept in mind that the large decreases in oxide
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`stress with high-pressure wet oxidation are at present only
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`predictions. Ho [15] has made this prediction by using
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`available data for the dependence of oxide viscosity on
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`hydroxyl concentration at one atmosphere, and
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`extrapolating to the case of higher atmospheres. However,
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`we have found that such predictions based on
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`extrapolation are very sensitive to data fitting of the data
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`obtained at one atmosphere. We mention in the following
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`discussion some preliminary data indicating that high-
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`pressure wet oxidation may not lead to significant
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`reductions in omde stress.
`We have also calculated the efiect of reducing the partial
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`pressure of oxidant. Consider the effect of reducing the
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`partial pressure of oxygen in a dry-oxidation process. The
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`viscosity of the oxide is unchanged, but the oxidation rate
`is reduced. Therefore, stress levels should be reduced. For
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`the case of dry oxidation at 0.1 arm, the curve for stress as
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`a function of oxide thickness practically coincides with the
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`Latin wet~oxidation curve in Figure 2(b); we have not
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`plotted the 0.1-atm case because of this close agreement.
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`The stress can be reduced simply by diluting oxygen with
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`an inert carrier gas. The penalty for this is that it increases
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`the time needed to grow the same amount of oxide, and
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`the extra time might be in excess of the allowable thermal
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`budget. Reducing the partial pressure of oxidant in a wet-
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`oxidation process leads to different results. If we d