Microstructure array and activation system therefor

An electronic transducer array and transfer device and method which provides for activation of selected transducers at selected times In one application, the device performs data transfer by a combination of suitably interconnected submillimeter transducers (4) capable of sensing and/or actuating microscopic data-storage cells, and electronic switching (402, 602, 702) to activate selected individual transducers. One embodiment of the invention provides for magnetic transducers for reading (304) and writing (302) on a magnetic medium (8). Another embodiment of the invention provides tunneling electron transducers (10) arranged in an array.

BACKGROUND OF THE INVENTION 
The present invention relates primarily to the field of semiconductor and 
microstructure devices and their fabrication, and more particularly to a 
method of and apparatus for data storage and data transfer that limits or 
eliminates the need for motion of macroscopic electromechanical elements 
in one particular embodiment. 
Much as the transistor replaced the vacuum tube, various other electronic 
functions are now being replaced by solid state devices. The attendant 
rate of miniaturization now allows one to make wires and other features 
that are as thin as 20 nm. Devices that have been produced range in size 
from micron to submicron. These microscopic sensors and actuators will be 
referred to hereinafter as microdevices. Important applications include 
microrobots for uses ranging from industrial production to microsurgery; 
optical devices for the generation, modulation and sensing of light; and 
complete microsystems for biological and chemical processing. 
The focus has been on physical, chemical, and biological microstructures 
for use in microsensors, transducers, tactile and vibrational sensing 
arrays, and membranes. The capability required for fabricating such small 
devices has been enabled by advances in lithography, molecular-beam 
epitaxy, and metal/organic vapor deposition. These materials-processing 
and microfabrication techniques have been combined successfully with the 
technology of semiconductor manufacturing to produce various microdevices 
capable of functioning as electromagnetic sensors and actuators. 
Microdevices are disclosed in, for example, the proceedings of the "IEEE 
Solid-State Sensor and Actuator Workshop," Hilton Head Island, S.C., Jun. 
6-9, 1988; "IEEE Workshop on Micro Electromechanical Systems," Salt Lake 
City, Utah, 20-22 February 1989; and "IEEE Workshop on Micro Electro 
Mechanical Systems," Napa Valley, Calif., 11-14 February 1990, all of 
which are incorporated herein by reference for all purposes. 
The steps required for the fabrication of microdevices integrated with 
microelectronics on a chip or as a hybrid are well known to those skilled 
in the art. For example, the technique is being used to construct the 
entire transducer for a scanning tunneling microscope on a silicon chip, 
as described in Kenny et al., "A Micromachined Silicon Electron Tunneling 
Sensor," IEEE (1990), previously incorporated herein by reference for all 
purposes. 
Electronic mass storage devices such as floppy disc drives, hard drives, 
and magnetic tape are well known. Three factors have provided the driving 
force in the quest for ever improved electronic mass storage: high 
information density, short access time, and long-term stability. The 
dominant technology for electronic-data mass storage over the past thirty 
years has been magnetic recording. The success of magnetic storage 
technology can be attributed at least in part to steady advances in 
providing the desired data capacity (for example, 10.sup.7 bits/cm.sup.2 
on commercially available disks, and one order of magnitude larger on a 
recent demonstration disk) at a competitive price, albeit at the loss of 
speed, and to its substantially unlimited number of erasure cycles. While 
meeting with substantial success, difficulty has been encountered in 
providing a technology that performs satisfactorily with respect to all 
three factors. A typical design of the storage hierarchy involves 
tradeoffs, as a result of which most systems include a combination of 
(expensive) semiconductor memories, to provide a better match to the 
processor speed, and (slower) magnetic storage, to provide larger 
capacities for long-term storage. 
The basic elements of a magnetic storage system comprise a magnetizable 
storage medium, a transducer that can write information to and/or read 
information from this medium, means for the medium and transducer to move 
with respect to each other, and suitable associated electronics. In a 
magnetic recording system, the transducer is called a head. The two most 
commonly used head technologies are based on inductive and flux-sensing 
methods and are described in, for example, Mee et al., Magnetic Recording 
Handbook, McGraw-Hill, 1990, which is incorporated herein by reference for 
all purposes. 
The inductive head in a magnetic record/erase system includes a coil of 
wire wound around a magnetic core, and it relies on Faraday's law of 
induction. In the read process, the relative head/medium motion causes the 
head to pick up the time rate of change of the medium magnetization in the 
transition region, which induces a current in the coil. In the write 
process, a current passing through the coil creates a magnetic field in 
the head which is used to impress magnetized regions onto the storage 
medium. Unlike the inductive read transducer, the flux-sensitive read 
transducers do not require any motion relative to the storage medium. 
Flux-sensitive transducers include those based on a change in resistance 
(magnetoresistive effect), change in electric field (Hall effect), and 
modulating the reluctance of a ring core (flux gate). Much of the 
remaining description of read transducers herein relies on 
magnetoresistive transducers; this is not intended to be limiting. The 
magnetoresistive head relies on the changes in resistance of a magnetic 
material that accompanies a change in magnetization. It depends on the 
magnetic flux itself rather than on the rate of change of flux, as is the 
case with inductive heads; its output therefore depends only on the 
instantaneous fields from the media and is independent of the relative 
head/medium velocity or the time rate of change of the magnetic flux. The 
sensing element is biased with a magnetic field to optimize the linearity 
of its output. Many biasing schemes have been utilized, the largest class 
of which provides the biasing field through an auxiliary microstructure in 
close proximity to the magnetoresistive element. Furthermore, it is a 
read-only device, so that it has to be combined with an inductive write 
head. Magnetoresistive tape heads are available commercially, disk heads, 
on a demonstration basis. 
Despite its dominant position, magnetic-storage technology suffers from 
several basic problems, arising mainly not from storage itself but rather 
from the present method of transfer of data between mass storage and the 
computer by means of moving heads. Disadvantages include: 
(1) The relatively slow speed of access. It takes on the order of tens of 
milliseconds to transfer a block from disk storage. 
(2) It is vulnerable to shock and vibration. 
(3) Materials problems arising from the relative head/medium motion. 
Materials choices generally represent a compromise between the desired 
electrical performance and tribological (wear and friction) constraints. 
(4) The practical limitation on the density of magnetic storage is 
currently set by the size of the read/write head; it is presently two 
orders of magnitude short of the theoretical limitation in many 
embodiments. In principle, each magnetic domain can encode one bit. In 
practice, locating or addressing individual domains presents problems that 
have proven insurmountable to date, because of the size of the head. 
Practical considerations normally dictate that in magnetic-recording 
systems based on prior-art head/disk technology each bit contains a large 
number of domains, which precludes reaching the theoretical storage 
capacity. 
(5) The magnetization in the direction normal to the surface of the medium 
falls off exponentially with distance. There is a corresponding loss of 
sensing signal with increased head/medium spacing. Designs thus represent 
a compromise between the close spacing essential for high-density storage 
and the need to maintain the stability required to avoid contacts. 
Computer applications require large amounts of data transfer between 
internal computer memory and an external storage device, such as a disk. 
There is generally a large disparity between the internal processing speed 
of the computer and speed of input/output (I/O). Typical computer 
instruction times range from the order of a microsecond down to tens of 
nanoseconds; a typical operation to transfer a sector of data is of the 
order of tens of milliseconds. Although comparison between the transfer of 
a sector and a single instruction is clearly not a direct measure of the 
relative times required for I/O and for processing, it is well known from 
practical experience that typical I/O times in data-transfer-intensive 
applications can be several orders of magnitude longer than typical CPU 
processing times. This large speed disparity between processing and I/O 
reflects the vast difference between the time constants that characterize 
these two functions. CPU processing time is dictated by transistor 
switching times, which are orders of magnitude shorter than the 
characteristic times of motion of the macroscopic electromechanical parts 
essential to prior-art head/disk mass storage systems. In such 
applications, I/O, not processing time, is the limiting factor in 
throughput. Overcoming this critical limitation in the overall speed of 
computing is one of the major problems in computer-system design. 
Magnetic disks store data in concentric circles called tracks. Digital data 
are stored serially around the track. Each track is divided into sectors. 
A sector is a group of contiguous bits, which are generally transferred 
between memory and disk in one I/O operation. The data are accessed by 
read/write heads mounted on the ends of access arms. In most disk units, 
the heads are positioned over a given sector by a combination of two 
mechanical motions: the disk rotates to provide angular position, and the 
access arm moves radially to provide radial position. The combination of 
disk rotation and access-arm motion allows the head to be positioned over 
any point (any sector of a disk and track) of a disk. 
Accordingly, the time required to move data between a disk and internal 
computer memory (access time) has three major components. These components 
represent three separate actions in the data storage and retrieval 
process. 
(1) Seek time (or access motion time) is the time required for the access 
arm to position its read/write head over the proper track. 
(2) Rotational delay (or latency) is the time it takes for the rotating 
disk to bring the desired sector under the access arm. 
(3) Data transfer time (or data movement time) is the time required to 
transfer data between the disk and main memory. 
The access time required to read (or write) on the disk is the sum of the 
three times: 
access time=seek time+rotational delay+data-transfer time 
The sum of the average seek time and rotational delay is referred to 
hereinafter as positioning time: 
positioning time=seek time+rotational delay 
As indicative of typical times, the HP 7935H has an average seek time of 24 
ms, rotational delay of 11.1 ms, and data-transfer time of 1.0 ms for a 
kilobyte sector; the corresponding times for the IBM 3380 are 16 ms, 8.3 
ms, and 0.33 ms, respectively. The average positioning time, which is seen 
to be 30-80 times longer than the actual data-transfer time from a sector, 
is clearly the limiting factor in access time. 
The technology of data storage and retrieval by prior-art magnetic-storage 
systems relies on the relative motion of the head and the storage medium. 
During transport of the recording medium past the head the relative motion 
of the two permits writing or reading; in general, this motion causes a 
transfer between a temporal signal in the read/write head and a recorded 
spatial pattern in the medium. 
The fundamental process, in which temporal input data are translated into a 
recorded spatial magnetization pattern in the medium during a write, 
involves several steps. 
(1) Information (audio, video, or data) to be recorded magnetically is 
encoded as a time-varying electrical signal. 
(2) The signal current with the encoded pattern is applied to the 
writing-head windings. 
(3) This current magnetizes the head. 
(4) The fringe magnetic field from the head creates, on the moving medium, 
a spatially varying pattern of magnetization that reproduces the pattern 
encoded in the electrical signal. 
The reading process uses either the same head or another head to reconvert 
the recorded magnetization pattern into a time-varying electrical signal 
that can be amplified to a useful level, for example, to feed data to a 
computer or to drive a loudspeaker or a receiver. 
The rotation of the disk past the head actually serves a dual role in these 
devices. 
(a) In the read process, the motion of the windings in the head through the 
magnetic-field lines from the storage medium generates current in these 
windings. This is the fundamental mechanism underlying the actual transfer 
of a given bit between the medium and an inductive head. This aspect of 
the motion does not come into play for a magnetoresistive head. 
(b) The relative motion of the head along the track accesses the data in 
the sector serially. In the recording process, this produces a 
magnetization pattern according to the input current applied to the head. 
If the input signal is at a frequency f and the medium is moving at a 
relative velocity v, a magnetization pattern (0s and 1s) will be recorded 
at a fundamental wavelength given by 
EQU .lambda.=v/f. (1) 
In general, all temporal signal variations are translated into spatial 
variations by the relation 
EQU x=vt, (2) 
where x denotes the pattern coordinate along the medium and t is the 
temporal coordinate of the input signal. This aspect of the head/medium 
motion is required for both inductive and magnetoresistive heads. 
In addition to their dependence on the speed v, the phenomena associated 
with the write process depend, among other parameters, on the head-medium 
separation d (otherwise known as the flying height). See, for example, 
"Special Section on Magnetic Information Storage Technology," Proceedings 
of the IEEE, November 1986, Vol. 74, which is incorporated herein by 
reference for all purposes. Apart from differences attendant to the head 
functioning as a sensor rather than as an actuator, the signal produced 
when the medium is read is a function of most of these same parameters, 
including v and d. 
Semiconductor random access memories (RAMs) are also well know to those of 
skill in the art. A RAM generally comprises a set of memory cells 
integrated on a chip with a number of peripheral circuits. RAMs are 
described in, for example, Porat et al., Introduction to Digital 
Techniques, John Wiley, 1979, which is incorporated herein by reference 
for all purposes. In general, RAM circuits perform several functions, 
including addressing (selection of specific locations for access), 
providing power, fanout (transmission of a signal to a multiplicity of 
directions), and conditioning required to generate a useable output 
signal. In RAM memories, the addressing scheme permits direct access to 
the desired cell, with access time independent of the cell location. 
Selected portions are then extracted for use. RAMs are generally fast 
enough to be compatible with a CPU, but they are generally too expensive 
to be used for mass storage. Further, RAMs are generally volatile in the 
sense that a source of power must be provided to refresh the memory 
periodically. They cannot, therefore, be used for long-term storage. One 
alternative to RAM includes ROM such as EPROMs (Electronically 
Programmable Read Only Memory). While such memories do not require a 
refresh cycle, they have the obvious disadvantage of being progammable 
only once. Other nonvolatile semiconductor memories that can be written 
repeatedly, such as EAROM (Electrically Alterable Read Only Memory) or 
EEROM (Electrically Erasable Read Only Memory), do not provide nearly the 
reliability of magnetic memories for long-term storage. 
The scanning tunneling microscope (STM) is an instrument used for measuring 
surface properties. It comprises a sharp needle, usually made of tungsten, 
that can probe the electronic structure of conducting surfaces by means of 
the tunneling effect. The probe is placed in close proximity to the 
surface and physically scanned over it. The instrument provides a tool for 
characterizing static surface properties of, for example, conductors. It 
is unique in providing surface information on an atomic scale and has 
opened opportunities in applications ranging from biological systems to 
telerobotics. Derivatives of the STM have extended the capabilities of 
tunneling sensors to the measurement of nonconducting as well as 
conducting surfaces. 
There are however whole classes of phenomena involving transient surface 
effects that are of wide interest but which take place on vastly shorter 
time scales than the time needed to scan a surface physically by an STM 
probe. Many of these effects bear directly on the development of new 
electronic materials, devices and circuits. There is thus a need to 
develop novel methods for the measurement of dynamic surface effects that 
derives from the confluence of several factors the importance of their 
application; the facts that surface properties are in general 
significantly different from and not nearly as well known as bulk 
properties; and the difficulty of measuring dynamic surface effects on an 
atomic scale by conventional methods. 
From the above it is seen that improved transducer arrays are desired. 
SUMMARY OF THE INVENTION 
An array of submillimeter transducers (sensors/ actuators) are formed with 
support electronics on a chip or as a hybrid, wherein the individual 
transducers act at specifically selected positions and times for reading 
or writing data on a storage medium. An addressing scheme provides access 
to any individual transducer directly, with access time substantially 
independent of the transducer's location in the array. The individual 
transducers on the array are activated at selected addresses and 
specifically related times and their actions are converted electronically 
into a function of the transducer system as a whole. The choices of the 
positions and times at which the transducers are activated are determined 
by the particular application of the transducer array. A further 
understanding of the nature of these choices may be had with reference to 
the following specific embodiments of the invention. 
According to one embodiment, the invention utilizes magnetic storage 
technology with the intrinsic speed of electronic switching that 
characterizes access to semiconductor memories. This is accomplished by 
replacing the macroscopic moving head that currently serves as the 
read/write transducer for magnetic storage by an array of interconnected 
submillimeter magnetic transducers on a chip. 
In one preferred embodiment, the invention performs data transfer by a 
combination of (i) suitably interconnected submillimeter transducers 
capable of sensing and/or actuating microscopic data-storage cells, and 
(ii) electronic switching to activate selected individual transducers. The 
spatial points at which the transducers are activated are the ones 
corresponding to the memory locations that are to be read or written. The 
times at which the selected transducers are activated are chosen to be 
substantially the same so that an entire block of data can be moved at a 
time. In a preferred embodiment, the electronically switched devices 
transfer data to and from the selected transducers at substantially the 
same time. According to one aspect of the invention, the transducers are 
interconnected in an array with electronics for coordinated operation of 
the array as a whole, with capabilities that are unavailable to the 
transducers acting alone, by activating selected individual transducers at 
specific times. 
According to another aspect of the invention, an array of 
scanning-tunneling-microscope probes is utilized in the measurement of 
dynamic surface effects by performing the surface scanning electronically 
rather than physically. The selection of spatial points at which the 
probes are activated is determined by the points over which the dynamic 
effect to be measured propagates. The times at which the selected probes 
are activated are chosen to coincide with the propagation of the effect 
over the selected spatial points 
A further understanding of the invention may be had with reference to the 
following description and drawings.