KR100605545B1 - Method for manufacturing a dense structure with high aspect ratio using a localized electrochemical deposition - Google Patents

Abstract

   CLAIMS 1. A method of manufacturing a high aspect ratio three dimensional microobject using localized electrochemical deposition, comprising: placing a conductive microelectrode near a surface of a conductive substrate to form a gap; Introducing an electrochemical medium such that the conductive microelectrode contacts the surface of the conductive substrate; Applying an electrical potential across the electrochemical medium to precipitate the three-dimensional object; Selecting an optimal distance between the conductive electrode and the three-dimensional object; And moving the conductive electrode while maintaining a constant distance between the fine electrode and the three-dimensional object calculated under the optimum process conditions. The optimum distance between the microelectrode and the three-dimensional object is confirmed using a microradio device using coherent X-rays, and the positioning of the conductive electrode is performed by a stepping motor.

LECD, electrode, conductive surface, coherent x-ray, microradioactive device

Description

Method for manufacturing a dense structure with high aspect ratio using a localized electrochemical deposition

1 is a schematic diagram of an apparatus for fabricating a high aspect ratio microstructure of the prior art.

2 is a schematic representation of an apparatus for producing high aspect ratio microstructures in accordance with the present invention.

3A and 3B are photographs showing LECD growth of high aspect ratio Cu microstructures monitored by real time microradiology using coherent X-rays. Here, FIG. 3B shows that when the microelectrode (tip) reaches about 35 μm from the tip of the growing microstructure, the growth rate rapidly increases as the shape of the growing tissue transitions from dense to porous. At this time, the applied voltage was V = 4.5V.

FIG. 3C is a photograph of FE-SEM analysis showing changes in tissue morphology for the same growth as in FIG. 3B.

Figure 4a is a graph showing the precipitation rate according to the distance L between the microelectrode and the microstructure at an applied voltage of 4.5 V according to the present invention. Here the arrow shows the threshold and the interpolation graph is a graph of precipitation rate versus 1 / L showing hyperbolic dependence.

Figure 4b is a graph of the current and precipitation rate in the electrolyte cell with time according to the present invention.

5 is a graph showing the change of the "critical" value of L according to the applied voltage V in the two solutions according to the present embodiment.

6A and 6B are photographs showing the transition from dense dense growth (left) to dense porous growth (right) at an applied voltage of 3.5 V in solution 1.

6C and 6D are photographs showing the transition from dense dense growth (left) to dense porous growth (right) at 10 V applied voltage in solution 1.

The “critical” distance here is 8 μm in the first case of FIGS. 6A and 6B and 80 μm in the second case of FIGS. 6C and D. It can be seen that the threshold distance is much larger in the first case than in the second case.

The present invention relates to a method for producing microstructures having high aspect ratios using localized electrochemical deposition.

Prior art

An alternative electrodeposition that does not require the use of a mask is local electrochemical precipitation (LECD).

Madden and Hunter used to deposit a variety of micrometer-sized nickel structures, using the tip of a very fine microelectrode (JD Madden and IW Hunter, J. Microelectrochem. Systems, 5, 24 (1996). )Reference).

Conventional tip-directed localized as described in AJBard, F.-RFFan, and MVMirkin, in Electroanalytical Chemistry, Vol. 18, AJ Bard, Editor, p.244, Marcel Dekker, Incorporated, New York (1994). In deposition, a solution between the ultrafine electrode tip and the substrate electrode submerged in an ionically conducting electrolyte when a bias voltage is applied between the ultramicro electrode (UME) tip and the substrate electrode is applied. Faraday current flows. If reducible metal ions are present in the electrolyte (e.g. Cu ²⁺ ions) and the substrate electrode potential is negative relative to the tip electrode, the flow of Faraday currents may be associated with the deposition of metal on the substrate. Causes oxidation at the tip. The magnitude of the Faraday current between the ultrafine electrode tip and the electrode is kept constant by feedback control means for monitoring the current between these electrodes and thus monitoring the mutual electrode spacing. By moving the tip electrode perpendicular to the substrate electrode, a metal structure of approximately tip diameter is produced. As another example, metal lines can be fabricated by scanning the tip over the surface of the substrate in a lateral direction.

EMEl-Giar, RASaid, GEBridges, and DJThomson also show in Local Electrochemical Deposition of Copper Microstructures, Journal of The Electrochemical of Society, 147 (2) 586-591 (2000), as shown in FIG. When a constant reference potential V0 (i.e., reference current I0) is set in the feedback circuit and precipitation starts and the Cu structure grows, the current is detected by the current monitor 10 so that the current It exceeds I0. (I.e., when the tip and the growing Cu structure are in contact), the feedback circuit is activated to move the tip of the microelectrode 20 upward and Cu growth continues until the growing Cu structure is shorted again. At this time, the microelectrode mounted on the micro position controller 30 is adjusted in three dimensions by using a fine stepping motor. Here 40 represents a vessel containing a solution containing the reactants.

Taken together, the conventional technique sets a reference current value between two electrodes and separates the distance between the microelectrode and the structure (allowing contact between the microelectrode and the structure) when more current flows between the electrodes. It can be seen that the technique for producing a microstructure using.

However, these conventional techniques allow for contact between the microelectrode and the structure.

The problem is that porous structures are made. If the structure has a lot of pores, there is a disadvantage that the resistance is increased electrically and the strength is structurally low.

       It is an object of the present invention to provide a method for rapidly producing fine metal structures having dense and high aspect ratios.

LECD is a new and exciting technology for the production of very high aspect ratio microstructures and nanostructures. The technique is based on electrochemical deposition:

In the device of the present invention, the microelectrode and the conductive substrate are placed in a container and a voltage is applied, and the microstructure is deposited while maintaining a constant distance between the microelectrode and the microstructure growing on the conductive substrate, thereby having a fine aspect ratio. Get the structure. The fabrication process is monitored in real time using a microradio device. Through such monitoring, since the distance between the microelectrode and the substrate having the growing microstructure is maintained at a constant distance using a stepping motor, it is possible to manufacture a dense microstructure having a high aspect ratio.

Method for producing a high aspect ratio dense microstructures according to the present invention using the same device is as follows.

1) Place the conductive microelectrode in close proximity to the conductive substrate submerged in the container. Microelectrodes are typically made of platinum or platinum or iridium. However, the microelectrode can be made of any conductive material. For example, it can be made of metal, carbon and conductive polymers.

2) Appropriate voltage is applied between the microelectrode and the conductive substrate.

3) Precipitation of micro and nano structures begins.

4) Select the optimal distance between the tip of the structure and the tip of the microelectrode while the micro and nano structure is growing. At this time, the measurement of this distance was carried out by a microradioactive apparatus having coherent X-rays. The configuration of the microradioactive apparatus having coherent X-rays is largely composed of an X-ray beam source, a sample stage, and an image detection apparatus. Microradiological devices with coherent X-rays are described in Y. Hwu, W.-L. Tsai, A. Groso, G. Margaritondo and JHJe, J. Phys. D35 , R105 (2002), which is described in detail and has recently made another breakthrough in understanding the dynamics of materials science processes.

In the present invention using the microradiology, the distance between the microelectrode and the deposited structure can be monitored in real time, and the structure of the deposited structure can be monitored in real time.

Using such a coherent X-ray microradiological apparatus, an optimal distance can be set.

Experiments were performed on a new 7B2 X-ray microscopy beamline from Pohang Light Source (PLS), South Korea. Microscopic features of the fabricated structures were measured by field emission scanning electron microscopy (FE-SEM).

5) The three-dimensional structure is manufactured by moving the microelectrode so that the optimal distance (micro, distance between the microelectrode from the tip of the nanostructure) adopted above is kept substantially constant during the growth. Using the system shown in FIG. 2, the process can be observed in real time using a microradiological instrument to observe that the distance between the microelectrode and the growing microstructure changes as the process progresses and the distance is measured. can do. If the distance is smaller than the threshold value, the stepping motor is used to move the position of the microelectrode to maintain the optimum critical distance (optimal distance).

This "bottom-up" approach offers significant advantages over standard "top-down" manufacturing techniques such as LIGA (Lithographie, Galvanoformung, Abforming). For example, LECD has the advantage of being inexpensive and having no minimum size limitation.

LECD can be applied to a variety of materials such as microns, sub-microns, nanoscale metals, metal alloys, conductive polymers and semiconductors. This simple approach can be an important manufacturing technology for microelectronics, telecommunications, microrobots, life sciences, microsensors, actuators and general small devices.

Until now, however, no one has observed the LECD process in real time with adequately high spatial resolution. Thus, the knowledge of the LECD process was limited: The inventors note that the distance between the electrode and the resulting structure, which is affected by the applied voltage and the concentration of metal ions, is important, and the distance between this microelectrode and the deposited structure, which has not been known so far, is important. Was found to affect the deposited structures. By keeping the distance between the microelectrode and the structure constant, it has been found that a three dimensional object with the dimension being micrometer can be obtained with a very low degree of porosity and a smooth and fine grain. .

Example

The present invention relates to new methods for producing metals, polymers and semiconductors in high aspect ratios and three dimensions and to new methods for depositing metals, semiconductors, ceramics and polymers. Deposition and etching are achieved by an electrochemical reaction localized on the conductor surface.

The method of the present invention selects a solution containing a suitable conductive substrate and reactant (s) which will precipitate the desired product upon electrochemical oxidation and reduction. Microelectrodes have characteristic features in micro or nano units that are small relative to the size of the substrate, and by arranging these features (microelectrodes) in close proximity to the substrate and applying an appropriate portal current, Precipitation on the substrate is localized. The microelectrodes are moved relative to the substrate in three dimensions to form the structures, thereby stacking material from the substrate. An exemplary apparatus for carrying out the process is shown schematically in FIG.

The conductive substrate 1 and the microelectrode 2 are immersed in the solution 4 placed in the container 3.

The conductive substrate 1 used a pure copper rod as a cathode.

The microelectrode 2 is typically made of platinum or platinum iridium. However, microelectrodes can potentially be made of any conductive material. For example, it can be made of metal, carbon and conductive polymers. However, it is not limited thereto. The microelectrode was prepared by sealing a platinum wire having a diameter of about 50 μm with a ceramic tube and an epoxy and then polishing it.

In this example, two types, CuSO 4 · 5H 2 O (250 g / L) (solution 1), CuSO 4 · 5H 2 O (180 g / L), and H 2 SO 4 (75 g / L) (solution 2) were used.

 The position of the microelectrode 2 is controlled by three stepping motors 5.

The position of the microelectrode 2 relative to the substrate 1 was adjusted by moving the microelectrode 2 in the X, Y, and Z directions using the motor 5. In an exemplary embodiment microstepping motors were used.

When the distance between the microelectrode 2 observed by the image acquisition apparatus 6 and the microstructures on the growing substrate reaches a threshold, the image acquisition apparatus 6 is a stepping motor controller 7. ) Outputs a motor driving signal, and the stepping motor controller 7 drives the stepping motor 5 to move the microelectrode 2. Here, the image collecting device is an X-ray microradiology apparatus.

The microelectrode is disposed in proximity to the conductive substrate in order to localize the precipitation. The distance between the microelectrode and the conductive substrate suitable for localizing the precipitation depends on the shape of the microelectrode, the solution and reactant (s) used, the voltage applied between the microelectrode and the substrate, and the material (to some extent) of the microelectrode and the substrate. Depends.

In the present embodiment, as shown in FIGS. 3A and 3B, the position of the microelectrode is fixed in the solution 1 so that the distance L between the microstructure and the microelectrode decreases as the microstructure grows. As the distance L became smaller, the precipitation rate increased and when the distance L reached about 35 μm, the deposition rate rapidly increased, and as shown in FIG. 3B, the grown structure became porous. The distance (interval) between the microstructure and the microelectrode at the time when the tissue transitions from dense to porous is defined as the critical distance.

As can be seen in Figures 6a, b and 6c, d, the critical distance means the distance at the time the tissue transitions from dense to porous. If the distance between the microelectrode and the structure is larger than the critical distance, the structure of the dense tissue can be obtained, but the deposition rate becomes slow. However, if the distance L is smaller than the critical distance, the speed becomes so fast that a porous structure is obtained. Therefore, considering the precipitation rate and the porosity of the tissue, a distance longer than the critical distance (for example, about 105% of the critical distance) is an optimal distance. The position of the initial microelectrode is set based on this optimum distance. The measurement of this critical distance was done by microradiological instruments with coherent X-rays. The critical distance could be measured by expanding and observing the deposition process in real time and simultaneously measuring the change in current.

 The constant maintenance of the optimum distance between the growing structure and the microelectrode during the fabrication process was also moved while zooming in real time using a microradiological apparatus with a coherent X-ray, an image collector.

When the microelectrode is arranged at an optimal initial position, a voltage is applied between the microelectrode and the conductive substrate to cause precipitation on the conductive substrate.

In the case of depositing copper on a copper plate using CuSO4 · 5H2O (250g / L) (solution 1), CuSO4 · 5H2O (180g / L), and H2SO4 (75g / L) (solution 2) using a platinum microelectrode For example, a voltage between about 4.0V and about 4.5V is suitable.

The experimental cell for the study of microradiology of LECD was fabricated by processing Teflon blocks and sealed with a Homton film (permeable to X-rays and stable to most chemical reactions) along the X-ray path. The spacing between the two cell windows through which the X-ray passes is optimized to about 5 mm to avoid unnecessary X-ray absorption by the electrolyte.

 Figure 4a shows the shape change of the structure according to the change in the deposition rate made by the distance between the microelectrode and the structure. In conventional electrochemical precipitation, the growth rate is mainly affected by electron transfer and mass transfer of metal ions from solution to the cathode, which is more affected by the latter. Precipitation depletes metal ions near the cathode, which must continue to be replenished by mass transfer in solution. There are two main mechanisms of this resupply mechanism: diffusion and migration. While diffusion prevails in conventional electroplating, movement must be considered in LECD because the distance between the microelectrode and the structure is small, resulting in very strong local electric fields.

The shift produces a current density proportional to the electric field E (= V / L, where V is the applied voltage). Without considering the edge effect, the moving current will be proportional to L ⁻¹ . For large values of L, this term can be ignored and diffusion prevails. As L decreases, the shift term increases approximately with hyperbolic law. This explains the data of FIG. 2A: The inset shows that the porous precipitate follows the hyperbolic law very well, except for very small L-values that make it difficult to measure the precipitation rate.

  4b shows that the time variation of the current and the precipitation rate are related to each other.

The "threshold" value of distance L corresponds approximately to the point overtaking diffusion as the predominant precipitation factor.

This analysis correctly predicts the effect of applied voltage V. If the spreading term is approximately constant and the moving term is approximately proportional to V / L, the "threshold" value of L defined above increases linearly with V. This is roughly consistent with the experimental voltage dependence of FIG. 5 for both solutions. 6a, b and 6c, d show that when the applied voltage V becomes larger, the transition of the tissue form, ie from the dense phase to the porous phase, changes at a much larger critical distance L.

This analysis accounts for the found nonlinear dependence of deposition rate and growth tissue morphology on the distance L between the microelectrode and the microstructure. This finding has generated significant repercussions in the practical use of LECD. Faster growth rates are certainly desirable. However, if the growth rate is too high due to the distance between the microscopic electrode and the structure, the product becomes porous. Therefore, the ideal method of LECD is to identify the optimal distance and keep it constant during growth.

Tests performed using a novel technique of real-time microradiology with coherent X-rays resulted in the growth of microelectrode (anode) and high aspect ratio in localized electrochemical deposition (LECD). It has been found experimentally clearly that there is a "threshold" value that determines the organizational state of the fabricated structure with respect to the distance between copper microstructures (cathodes). This threshold is separated into two different manufacturing zones: As the distance between the microelectrode and the microstructure becomes smaller, the deposition rate increases rapidly and the precipitates are porous rather than dense-important for the quality of the precipitates. Results in

The present invention aims to provide experimental evidence for the role of the distance between the microelectrode and the structure. The present invention aims to quantitatively explain the role of this distance with the interaction between metal-ion diffusion and transport in the precipitation process.

According to the present invention, a threshold exists in the distance between the microelectrode and the structure, which means the distance at which the porous structure starts to be produced. By moving the microelectrodes while keeping the distance above this threshold value, a three-dimensional structure having a dense structure can be produced.

By the technique of the present invention, copper lines can be grown between pads, for example in an integrated circuit package, and an antenna of about 2 cm can be grown on a millimeter wave transmission line cantilever.                     

The technology will also be promising in the microelectronics industry as a technology for interconnect repairs and altering interconnects on dense PC boards.

Claims (7)

As a method for producing high aspect ratio dense microstructures using local electrochemical precipitation,  (a) placing a tip-shaped microelectrode near the surface of the conductive substrate;  (b) introducing an electrochemical medium such that the microelectrode and the surface of the conductive substrate are in electrochemical contact;  (c) applying an electrical potential between the microelectrode and the conductive substrate across the electrochemical medium to precipitate microstructures;  (d) selecting an optimal distance between the microelectrode and the microstructure to allow the microstructure to grow densely; And  (e) moving the microelectrodes while maintaining the optimum distance such that the dense microstructures have a high aspect ratio. The method of claim 1, wherein the optimum distance between the microelectrode and the microstructure in step (d) is confirmed by using an image collecting device. The method of claim 2, wherein the image collecting device is a microradiographic device. 3. A method according to claim 1 or 2, wherein the positioning of the microelectrode is performed by a stepping motor. The method of claim 1, wherein the microstructures are at least partially selected from the group consisting of electroprecipitable metals, polymers, and semiconductors. 4. The method of claim 3, wherein the microradiologic device comprises an X-ray beam source, a sample stage, and an image detection device. The method of claim 1, wherein the optimal distance is a distance between the microstructure and the microelectrode at the time when the microstructure is transferred from the dense tissue to the porous tissue.

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