Controlling thickness and composition of gate stack layers in logic and memory devices is critical to ensure transistor performance meets requirements, especially at 10nm node due to the 3-d geometry of devices and tight process budget. It has become necessary to measure and control each layer in the gate stack before and after dielectric and metal gate deposition sequences. A typical gate stack can have 5-7 layers including the interfacial layer, high-k dielectric, metal gate stack, work function layers, and cap layers. Similarly, PMOS channel strain is controlled using a graded Si<sub>x</sub>Ge1-<sub>x</sub> stack grown epitaxially over fins in the source/drain regions. This graded stack can have 2-4 layers of different thicknesses and Ge concentrations. This paper discusses the benefit of using spectroscopic ellipsometry with multiple angles of incidence to accurately and precisely determine the thickness of individual layers in critical gate layer stacks at various process steps on planar and grating surfaces. We will also show the benefit of using an advanced laser-based ellipsometer, for ultra-precise measurement of the gate interfacial layer oxides.
We describe two types of active optical devices developed for
use as free-space optical interconnects FSOIs for chip-to-chip communications.
The design of both types of devices—membrane and freestanding
structures—includes both optical and mechanical components.
The optical component contains porous silicon PSi with customized
optical properties fabricated by electrochemical etching of silicon. The
mechanical part of the devices is composed of metal/nitride bimorph
thermal actuators. The membrane devices form concave mirrors when
actuated, and can be used to focus the incoming optical signals and
correct any optical misalignment within the input/output I/O fabric. The
freestanding devices have out-of-plane optical components, whose tilting
angle is controlled by the current applied to the actuator. These devices
can function as either reflectors or tunable optical filters. By incorporating
the developed PSi diffractive optical element DOE into the freestanding
structure, another type of freestanding device is realized for beamsplitting
applications. Details of the fabrication, testing, and integration of
these PSi-based devices are presented.
Porous silicon (PSi) is a promising material for the creation of optical components for chip-to-chip interconnects because
of its unique optical properties, flexible fabrication methods and integration with conventional CMOS material sets. In
this paper, we present a novel active optical filter made of PSi to select desired optical wavelengths. The tunable
membrane type optical filter is based on a Fabry-Perot interferometer employing two Bragg reflectors separated by an
adjustable air gap, which can be thermally controlled. The Bragg reflectors contain alternating layers of high and low
porosities. These layers were created by electrochemical etching of p+ type silicon wafers by varying the applied current
during etching process. Micro bimorph actuators are designed to control the movement of the top DBR mirror, which
changes the cavity thickness. By varying the applied current, the proposed filter can tune the transmitted wavelength of
the optical signal. Various geometrical shapes and sizes ranging from 100μm to 1mm of the active filtering region have
been realized for specific applications. The MOEMS technology-based device fabrication is fully compatible with the
existing IC mass fabrication processes, and can be integrated with a variety of active and passive optical components to
realize inter-chip or intra-chip communication at the system level at a relatively low cost.
Among the major challenges confronting the current initiatives to incorporate optical interconnect capabilities for
chip to chip I/O is to define, develop and implement the necessary components required for a complete pipeline
from source to receiver. For next generation integrated circuits, the need for multifunctionality and multidimensional
integration has resulted in new demands on interface technology to yield massively parallel data and
clock lines. At this point, such methods are primarily limited to static reflectors, filters and gratings for interface
and optical routing. One of the crucial elements is to develop a high performance and flexible optical network to
transform an incoming optical pulse train into a widely distributed set of optical signals whose direction, alignment
and power can be independently controlled. This coupling can be achieved using several methods including active
(primarily, MEMS-based) beam steering arrays. For chip to chip applications, the overwhelming majority of the
recent research and development effort has been focused on source and detector technologies, but less attention
has been devoted to flexible, reconfigurable beam steering modalities. A variety of approaches for such beam
steering and distribution of both timing and data lines has been examined. This paper will present an overview of
active, silicon components under development at the College of Nanoscale Science and Engineering for arraybased
I/O management with an emphasis on reconfigurable diffractive devices and adjustable, porous silicon-
based components which combine optical beam steering, filtering and focusing capabilities. Design details along
with initial performance data from prototype components will be presented.
Porous silicon (PSi) is an attractive material for fabrication of multilayer optical devices such as Bragg
reflectors, Fabry-Perot resonators and other novel (optical) components. Such devices are characterized by a
periodic modulation of the refractive indices in alternating layers and can be classified as 1D photonic crystals. 2D
photonic bandgap structures can be also obtained using a variation of applied potential on the back side of the
sample during electrochemical formation of the multilayers. This technique allows a fabrication of spatially
distributed filters on the millimeter size scale. In this paper, a new method is presented which uses a front side
protective mask for the creation of 2D photonic bandgap structures on the micron scale. The devices obtained by this
technique can be used for the creation of spatially distributed filters. The front side protective mask controls lateral
undercut in multiple ways depending on the mask material. By varying the design and material of the protective
mask, PSi interference filters with desired optical parameters across a field of view can be realized.
In this paper, a novel, simple method to produce 2D periodic multilayer structures is described. In
particular, the focus is on the changes in the photonic crystal cavities when various mask materials are used. In
addition, a new type of active optical components for a chip-to chip interconnection based on the combination of our
method and MEMS technology is presented.