Background Statement for SEMI Draft Document 6179

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Background Statement for SEMI Draft Document 6179

Reapproval of SEMI 3D1-0912, TERMINOLOGY FOR THROUGH SILICON VIA GEOMETRICAL METROLOGY

NOTICE: This Background Statement is not part of the balloted item. It is provided solely to assist the recipient in reaching an informed decision based on the rationale of the activity that preceded the creation of this ballot.

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Background

According to the SEMI StandardsProcedure Manual, a reapproval Letter Ballot should include the Purpose, Scope, Limitations, and Terminology sections, along with the full text of any paragraph in which editorial updates are being made.

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SEMI Draft Document 6179

Reapproval of SEMI 3D1-0912, TERMINOLOGY FOR THROUGH SILICON VIA GEOMETRICAL METROLOGY

This Standard was technically approved by the global 3DS-ICTechnical Committee. This edition was approved for publication by the global Audits and Reviews Subcommittee on August 30, 2012. Available at and in September 2012.

1 Purpose

1.1 Clear and commonly accepted definitions are needed for efficient communication and to prevent misunderstanding between buyers and vendors of metrology equipment and manufacturing services. The purpose of this Document is to provide a consistent terminology for the understanding and discussion of metrology issues important to through silicon vias (TSV).

2 Scope

2.1 The scope of this Document is limited to the definitions of terms used to specify measured quantities that characterize TSV. This Document does not describe measurement procedures, but rather attempts to define the measurands clearly, usefully, and unambiguously.

2.2 The focus of this Document is on geometry-related metrology and measurands important for definition and control of fabrication and inspection operations on structures that include openings for TSV.

2.3 This Document is intended for TSVs in silicon.

2.4 This Document does not attempt to discuss statistical considerations, which are covered in SEMIE89 and elsewhere.

2.5 This Document adapts structure and wording from other SEMI Documents, particularly SEMIP35.

NOTICE:SEMI Standards and Safety Guidelines do not purport to address all safety issues associated with their use. It is the responsibility of the users of the Documents to establish appropriate safety and health practices, and determine the applicability of regulatory or other limitations prior to use.

3 Referenced Standards and Documents

3.1 SEMI Standards and Safety Guidelines

SEMI E89 — Guide for Measurement System Analysis (MSA)

SEMI P35 — Terminology for Microlithography Metrology

3.2 Other Documents

SEMI International Standards: Compilation of Terms (Updated 0811)[1]

International Technology Roadmap for Semiconductors, 2009 Edition, Chapter on Interconnect

NBSIR 79-1758 — The Calibration of a Roundness Standard (

NOTICE: Unless otherwise indicated, all documents cited shall be the latest published versions.

4 TSV Metrology Issues

4.1 Geometrical Measurements — Any measurement of length or position contains an unknown error, the influence of which on subsequent application of the measurement data is best described by numerical measurement uncertainty. The problem of measuring geometric dimensions on TSV is complicated by the fact that the TSV is a 3-dimensional object with (usually) an irregular and complex shape. The metrology problem is further complicated because TSV are microscopic, and only magnified images can be measured.

4.2 In efficient practice, TSV opening geometry is specified using only a few selected parameters, because the additional data needed to fully describe the complex TSV shape can be considered immaterial to the subsequent applications of the measurement data, and are seldom available.

4.3 The first rule of metrology is to define exactly what is to be measured. The approach used here, in light of the previous paragraphs, is to define specific applications of widely used terms such as depth and diameter for TSV. These definitions will rely on standard geometrical constructions such as straight lines, circles, ellipses, etc. as needed for clarity. The differences between the ideal and the actual shape of a TSV opening may contribute to the overall measurement variability or uncertainty of some dimension of the TSV being characterized.

4.4 Challenges from the ITRS Roadmap — The International Technology Roadmap for Semiconductors, 2010 Update, anticipates minimum TSV diameters ranging from 8 to 0.8µm in global and intermediate interconnect levels in 3D-SICs from 2009 to 2015. Among the five most critical challenges beyond the 16nm node is a practical approach for 3D chip stacking; Table INTC5 states that engineering manufacturable solutions that meet cost targets for this technology is key. Addressing this challenge will require advances in metrology. This Document is part of the groundwork for that metrology.

5 Definitions

5.1 through silicon via opening (TSV) — a hole or recess in a silicon wafer that begins at the wafer top surface and may or may not: a) extend fully through the wafer from one surface to the other; b) contain a conductor; c) contain additional layers such as an insulator between the conductor and the silicon, a diffusion barrier on the silicon surface within the hole, or others.

5.1.1 Discussion — The International Technology Roadmap for Semiconductors defines a through-silicon-via as an electrical conductor. Here we are defining specifically terms for the metrology of the TSV opening, within which the complete electrical conducting TSV will be constructed.

5.2 wafer top surface—the wafer surface where the TSV opening is initially created and where the excavation of the TSV begins. This surface is defined to be horizontal; its perpendicular is defined to be vertical. Figure 1 shows a sketch of a TSV at the wafer top surface.

5.3 wafer bottom surface — the wafer surface opposite the wafer top surface.

5.4 area—geometrical area of the TSV opening in a particular horizontal plane. In Figure 1, the area of the TSV opening is the area covered by the light upward diagonal line pattern.

5.4.1 top area—area of the TSV opening at the top surface of the wafer.

Figure 1
TSV with Sidewall at Top Wafer Surface

5.5 depth — the distance, measured from the wafer top surface vertically toward the wafer bottom surface, between the top surface and the deepest point of the TSV opening.

5.5.1 intermediate depth—the distance measured from the wafer top surface to a selected level less than the full depth of the TSV opening. For example, the TSV opening area at an intermediate depth of 50% of the wafer thickness might be of interest.

5.5.2 full depth TSV—a TSV opening that extends from the top surface to the bottom surface of the wafer.

5.6 reported horizontal dimensions—the scheme and numerical values used to specify the breadth of a TSV. The breadth of a TSV may be specified in a variety of ways depending on circumstances such as the fabrication method and the measurement tools that are available. This Standard adopts the major and minor diameters, as defined below, as the reference geometrical construction for the horizontal dimensions of the TSV opening. Other possible examples of reference dimensions, not defined in this standard, include the diameter of a circle in a horizontal plane that can be inscribed in the TSV opening, the side of a square that can be inscribed in the TSV opening, etc. The geometrical terms and definitions herein are intended to be consistent, to the extent feasible, with their standard usage in the literature on geometry, in order to promote the use of existing mathematical methods and software and to minimize confusion.

5.6.1 center—the center of a TSV opening in any horizontal plane is the centroid (‘center of gravity’) of the area of the TSV.

5.6.2 chord—a straight line that extends from one point on the sidewall of the TSV opening in a specific horizontal plane to another such point in the same plane.

5.6.3 major diameter— the length of the longest chord of the TSV opening in any particular horizontal plane. There is no requirement that this diameter pass through any geometrical center or centroid of the TSV opening. There is no relationship implied between the length of the major diameter and the area of the TSV opening. Figure 2 indicates the major and minor diameters of a TSV opening. The unit for this dimension shall be the micrometer.

5.6.3.1 Discussion — This definition implies that all diameters of the TSV are observable within the field of view of the measurement instrument. If only a subset of the TSV diameters is within the field of view, for instance, by use of a vertical cross section, the reported diameter value will be affected by the geometrical relationship between the measurement plane and the TSV.

5.6.3.2 center point of the major diameter — the point on the major diameter equidistant from its ends.

5.6.4 minor diameter — the length of the shortest chord thatpasses through the center point of the major diameter.Figure 2 shows an example. Under this definition the minor diameter need not be at 90° to the major diameter, and no relationship is specified or assumed between the length of the minor diameter and the area of the TSV opening. The unit for this dimension shall be the micrometer.

Figure 2
TSV opening showing major and minor diameters as defined here. The minor diameter is the shortest diameter that passes through the center point of the major diameter.

5.6.5 Diameters of TSV openings may be of interest in planes at various intermediate depths along the TSV. Whenever a diameter is given, the plane in which it is measured should be specified. Some examples follow.

5.6.5.1 top major diameter—the major diameter of the TSV opening being characterized, measured at the top surface of the wafer.

5.6.5.2 bottom major diameter—the major diameter of the TSV opening being characterized, measured at the bottom surface of the wafer.

5.6.5.3 90% depth major diameter— the major diameter of the TSV opening being characterized, measured in the horizontal plane at 90% of the depth of the TSV.

5.6.5.4 top minor diameter— the minor diameter of the TSV opening being characterized, measured at the top surface of the wafer.

5.6.5.5 bottom minor diameter— the minor diameter of the TSV opening being characterized, measured at the bottom surface of the wafer.

5.6.5.6 90% depth minor diameter— the minor diameter of the TSV opening being characterized, measured in the horizontal plane at 90% of the depth of the TSV.

5.7 critical dimension (CD) —this Document defines all the CDs of the TSV opening based on the length of the minor diameter at various depths within the TSV opening.Whenever a CD is given, the plane in which it is measured shall be specified. Some examples follow.

5.7.1 top CD— CD (length of minor diameter) of the TSV opening at the top wafer surface.

5.7.2 CD at 90% depth — CD (length of minor diameter) at an intermediate depth of 90% of the depth of the TSV.

5.7.3 bottom CD— CD (length of minor diameter) of the TSV opening at the bottom wafersurface. Use of this term implies that the TSV opening is a full thickness TSV.

5.8 roundness—ratio of the length of the minor diameter to the length of the major diameter in the same plane. Because of the use of the term ‘aspect ratio’ in the International Technology Roadmap for Semiconductors (ITRS) in connection with the depth of the TSV opening, the use of the term ‘aspect ratio’ in connection with the in-plane dimensions of the TSV opening is deprecated. A definition of aspect ratio that is intended to be consistent with the ITRS usage is given below in 5.13.

NOTE 1:See the NIST document listed in §3.3, above, on calibration of a roundness standard, for an example of a treatment of roundness. This document treats roundness as “…radial deviations of the workpiece from the center of rotation…”, although it does not state a formal definition of roundness.

5.8.1 top roundness—roundness at the top surface of the wafer.

5.8.2 bottom roundness—roundness at the bottom surface of the wafer.

5.8.3 roundness at 90% depth—roundness at the an intermediate depth of 90% of the depth of the TSV.

5.9 sidewall surface texture—the local character of the surface formed by the sidewall of a TSV opening.

5.9.1 scallop— a periodic sidewall feature created by certain etch processes.

5.9.2 scallop size—the peak to valley distance perpendicular to the TSV sidewall at a particular scallop (see Figure3).

5.9.3 sidewall topography (ST)— the quantitative measure of the sidewall surface texture. Wikipedia indicates that topography is concerned with surface shape and features, local detail, and the three-dimensional quality of the surface. The term is used in that sense here.

5.9.3.1 For scallop-based etch processes, the sidewall topography is given by Equation 1:

(1)

5.9.3.2 For other etch processes, the sidewall surface topography is determined by use of idealized geometrical shapes that are drawn within and outside of the representation of the TSV sidewall. The ellipse is expected to be the most useful such shape for measurements in horizontal planes through the TSV. The circle is a special case of the ellipse. The rectangle may be chosen for TSV openings that conform better to this shape. The rectangle or the trapezoid may be the most useful such shape for measurements on vertical planes through the TSV. The sidewall topography is defined by Equation 2 for rectangles or trapezoids truncated at the top and bottom (Figure 3) and Equation 3 for circles or ellipses (Figure4), where ST is the sidewall topography, Aouter is the area of the circumscribed ellipse or rectangle, and Ainner is the area of the inner ellipse or rectangle.

(2)

(3)

5.9.3.2.1 Discussion — Figure 3 shows circumscribed and inscribed rectangles for a TSV opening as measured in a vertical plane through the TSV. Figure 4 shows circumscribed and inscribed ellipses for a TSV opening as measured in a horizontal plane through the TSV. There is no requirement for any particular geometrical relationship between the two constructed reference geometries. Their centers, orientations, and values of roundness may be the same or different.

Figure 3
TSV showing scallop size and inscribed and circumscribed rectangles in a vertical plane through a TSV. These are examples of geometrical constructions that may be used in evaluating sidewall topography.

Figure 4
TSV with inscribed and circumscribed ellipses, for use in evaluating
sidewall topography in a horizontal plane through the TSV.

5.10 pitch— distance between the centroid of a TSV and the centroid of an adjacent TSV.

5.10.1 pitch, average — average spacing between centroids of a specified group of TSV arranged along a line.

5.10.2 pitch, minimum—minimum spacing between centroids of any two neighboring TSV within a group of TSV arranged along a line.

5.10.3 tilt — angle in degrees between the top surface normal of the wafer and the line connecting the center of the TSV at the top wafer surface with the deepest point of the TSV.

5.10.3.1 Discussion — An angle of 0° indicates an absence of tilt.

5.11 sidewall angle—angle in degrees between a line through the edge of the TSV and the horizontal plane.

5.11.1 Discussion — Sidewall tilt is the same as sidewall angle. Considerations about the effect of the plane of the cross section on the measured result, as discussed in §5.6.3 (as revised) apply here as well, if a vertical cross section is used. An angle less than 90° indicates a TSV diameter that decreases with increasing depth, while an angle of more than 90° indicates a TSV with a diameter that increases with depth. The line should accurately track the direction of the top 90%, or more, of the sidewall of the TSV.