Electroless-plated deposit process for silicon based dielectric insulating material

Abstract

Process for thin-film electroless-plated deposition onto the substrate, which is either hydrophilic or hydrophobic silicon based dielectric material (e.g., silicon dioxide, silicate glass, or polysiloxanes), is a substitute for the conventional sensitization/activation route by pre-treating the surface of the dielectric substrate by plasma (H2/N2, N2, H2, or O2), immersing the substrate in a basic aqueous solution for removing hydrogen, immersing the substrate in a basic aqueous solution of metal ions for adsorbing the metal ions thereon, reducing the metal ions on the surface of the substrate, and immersing the substrate in an electroless-plating solution to deposit an electroless-plated metal film. The substrate formed of hydrophobic polysiloxanes which is properly pretreated by gaseous plasma can be immersed in a basic solution with strong oxidizing capability and/or performing a selective pattern by plasma treatment. Thus the metal film self-aligned patterns can be achieved electrochemically by this process.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to deposition processes of dielectric insulating material and more particularly to an improved electroless-plated deposition process for silicon based dielectric insulating material such as silicon dioxide (SiO₂) or polysiloxanes. The present invention provides new thin film deposition features b y a self-aligned, integrated plating technique based on plasma physics and colloidal-related chemistry to fabricate patterns of ultrathin (<20 nm) Co-based barriers and copper films in a selective manner on silicon based dielectric insulating material such as silicon dioxide (SiO₂) or polysiloxanes films by using the all-electrochemical integrated plating process, which consists of selective plasma treatment of the surface of substrate, solution immersion for improved wettability, and electroless plating.

2. Description of Related Art

A wide variety of silicon or carbon based dielectric insulating materials are employed as deposits on metal films in the microelectronics, optoelectronics, and semiconductor industry. For example, polyamide is widely employed as material in fabricating printed circuit boards (PCBs) in which through-holes of the PCB are formed by electroless-plated nickel (or copper) deposition. Also, silicon dioxide (SiO₂) is the major material of optical fibers of which surface has to be coated with a protection thin layer in order to prevent the possible attacks from water, contaminants, or the like through either physical vapor deposit (PVD) or chemical vapor deposit (CVD) process. It is known that silicon (Si) has replaced germanium (Ge) as the widely employed semiconductive material because silicon oxide (SiO₂) can be easily grown on the substrate by the thermal oxidation means. Undoped silicate glass (USG) is also a widely employed material as shallow trench isolation in the fabrication of integrated circuit (IC) devices such as metal oxide semiconductors (MOS). Phosphosilicate glass (PSG) and borophosphosilicate glass (BPSG) are also popular pre-metal dielectric materials for manufacturing MOS devices and the like. There are a number of dielectric materials having a low dielectric constant such as SiO₂ and polysiloxanes, which have been widely employed for the insulation of copper or aluminum alloy conductors in conjunction with the IC devices. A metal spacer as diffusion barrier must be deposited between a dielectric layer and a metal conductor for preventing possible interdiffusional behavior and chemical reaction at interfacial regions from occurring therebetween. The Co or Ni based films formed by electrochemical thin layer deposition techniques such as electroless-plating or electroplating have manifested a greater potential to be used as such metal spacers above-mentioned.

However, all existing dielectric insulating materials cannot catalyze the growth of electroless-plated metal film. Some techniques including an one-phase active palladium (Pd) seeding, an immersion plating, a grafting, and a radiation decomposition have been proposed to grow seeds (i.e., dies) as a substitute for the sensitization/activation process. Till now the sensitization/activation process is still the dominant art for growing dies in the IC fabrication. The sensitization/activation process involves the steps of acidifying (i.e., surface roughing) a dielectric substrate and growing seed in one-phase or two-phase treatment. For a two-phase treatment, the substrate is firstly exposed to SnCl₂/HCl acidic solution for sensitization in which Sn²⁺ ions to be adsorbed on the surface of substrate. Subsequently the substrate is exposed to PdCl₂/HCl acidic solution for activation in which Pd²⁺ ions can be reduced to be neutral by Sn²⁺ ions. For a one-phase treatment, the substrate is immersed in a mixed SnCl₂/PdCl₂ acidic solution to form Sn—Pd compound ions. Next procedure is followed by acceleration, of which purpose is to remove Sn²⁺ ions from the surfaces of dies. However, the conventional electroless-plating method is fairly time consuming. Another drawback thereof is that Pd²⁺ ions tend to form agglomerates at the size range of 100˜200 nm. As a result, it is impossible of growing an electroless-plated metal film having a thickness no more than 100 nm. Hence, a novel technique should be pursued to overcome the above drawbacks of the prior art.

SUMMARY OF THE INVENTION

The objective of the present invention is to provide a process for all-electrochemical integrated plating technique for the deposition of very thin metal film onto a substrate such as silicon based dielectric material being either hydrophilic (water adsorptive) or hydrophobic (water repellent). The all-electrochemical integrated plating process can make Co, Ni, Ag, Pd ions reduced into neutral metal ions which in turn can be served as ultrafine catalyst (2˜4 run) for growing electroless-plated Co, Ni, Cu or Fe film formed of an elemental metal, or a binary or tertiary alloy with the film thickness ranging from 20 nm to several thousand nanometers. The process technique of the present invention consists of the steps of (i) plasma treatment of the surface of the substrate by glow-discharged plasma of H₂/N₂, N₂, H₂, or O₂; (ii) immersing the substrate in a basic aqueous solution; (iii) removing hydrogen from the surface of the substrate; (iv) immersing the substrate in a basic aqueous solution having metal ions adsorbed onto the surface of the substrate; (v) reducing the adsorbed metal ions on the surface of the substrate; and (vi) immersing the substrate in an electroless-plated solution for depositing an electroless-plated metal film. The plasma source of H₂/N₂ can assure the environmental friendliness and better compatibility for the IC process, as opposed to the chlorofluorocarbon compounds (CFC) gases as dry etchant that pose a potential to public hazard. By utilizing the present invention, the expensive palladium chloride and tin chloride used in the prior sensitization/activation process are eliminated. Furthermore the capability for attaining a self-aligned, integrated plating method of the present invention to fabricate patterns of ultrathin (<20 nm) metal films in a selective manner on dielectric films using electroless plating far exceeds that of the conventional sensitization/activation processes.

In one aspect of the present invention, the silicon based dielectric material is selected from silicon dioxide, silicate glass, or polysiloxanes so as to form an active surface on the substrate and form a negatively charged surface on the substrate after performing the step (ii) for adsorbing the metal ions for electroless-plated catalysis.

In another aspect of the present invention, the silicon based dielectric material selected from either the hydrophilic silicon dioxide or the hydrophilic silicate glass is immersed in a basic aqueous solution for depositing the electroless-plated metal film irrespective of the plasma treatment.

In still another aspect of the present invention, the silicon based dielectric material selected from the hydrophobic polysiloxanes undergoes the step (i) for forming a hydrophilic surface, plasma is formed of one of ionized gas including N₂/H₂, N₂, H₂, and O₂, and the basic aqueous solution as a strong oxidizing agent for depositing the electroless-plated metal films.

In a further aspect of the present invention, the substrate formed of the hydrophobic polysiloxanes, which is selectively treated by glow-discharged plasma of H₂/N₂, N₂, H₂, or O₂ in the early processing procedure to remove hydrophobic radicals at the surface, is immersed in a basic aqueous solution of strong oxidizing agent for forming a negatively charged surface and converting it into surface silanol groups (Si—OH). The surface of the Si—OH ‘passivated’ dielectric film turns out to be hydrophilic and can be ion exchanged with divalent metal ions in the metal-containing solution. Consequently, the adsorbed metal ions can then be reduced to metal atoms by chemical reducing agents. Metal atoms dispersed on the surface act as catalytic sites for electroless deposition to form a self-aligned electroless-plated metal film pattern on the surface of the substrate.

The above and other objectives, features and advantages of the present invention will become apparent by the following detailed descriptions taken with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating a film deposition process according to the present invention, which consists of plasma treatment with colloid chemistry, following deprotonation of the plasma-treated (OH— terminated) films and adsorption/reduction of metal ions;

FIG. 2 is a side view showing the shape of water drop onto a hydrophobic dielectric substrate before and after plasma treatment, where the improved wettability and reduced contact angle are evident by plasma treatment;

FIG. 3 is a flowchart illustrating a self-aligned, selective electroless-plated metal deposition process according to the present invention, which consists of (1) the unselected area is covered by mask; (2) the unmasked area is treated by gaseous plasma (O₂ or H₂/N₂); (3) the selected area becomes hydrophilic after the mask removal; (4) all-electrochemical integrated process using a self-aligned seeding process that can be patterned selectively to induce the sequential growth of electroless barrier and metal films

FIG. 4 is a high-resolution X-ray near-edge absorption spectra obtained from as-deposited HOSP™ dielectric layers (a), sequentially treated by Ni²⁺ ion adsorption (b), adsorbed Ni²⁺ ion reduction (c), and electroless Co plating for 10 min (inset). The minor peak at ˜852 eV in (a) is due to X-ray guiding facility according to the present invention;

FIG. 5 depicts TEM bright-field micrographs and selected-area diffraction patterns (with major rings indexed) for (a) the initial dielectric material e.g. as-cured HOSP™ dielectric layers after sequential treatments by (b) Ni²⁺ ion adsorption/reduction as illustrated in FIG. 1 and (c) electroless Co plating for 2 minutes in diffraction pattern taken by TEM.

FIG. 6 depicts a SEM micrograph of a Sio₂ substrate through a process as illustrated in FIG. 1 by electroless-plated Co metal film for about 10 minutes;

FIG. 7 shows three-dimensional AFM micrographs for (a) the initial dielectric material e.g. as-cured HOSP™ dielectric substrates after a sequential treatment including (b) Ni²⁺ ion reducing and electroless Co plating for (c) 1 and (d) 2 min; and

FIG. 8 shows illustration of self-aligned MOS capacitive structure comprising silicon based dielectric layer, Co—W—P barrier, and Cu metal film on plasma-treated areas of HOSP™ dielectric substrates using the all-electrochemical integrated deposition process, which is illustrated in FIG. 3

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 in conjunction with FIG. 2, there is shown a thin film deposition process by electroless-plating in accordance with the present invention in which the principles of plasma treatment of the surface of the hydrophobic dielectric substrate and the all-electrochemical integrated deposition process are utilized. The process begins in step 1, where the plasma treatment on the surface of a dielectric insulating material will form dangling bonds and a chemically active surface. This step is particularly essential for hydrophobic dielectric substrates in a subsequent catalysis for metal ion (i.e., very small particle) adsorption. The plasma treatment retains a strong effect with respect to the hydrophobic radicals (e.g., S₁—CH₃, Si—H, etc.) of polysiloxanes dielectric polymer as shown in FIG. 2. In the left side of FIG. 2, a substrate, not processed by plasma, has a contact angle of about 96 degrees measured from the shape of water drop attached on the substrate. As a result, it is difficult of adsorbing very small metal particles having water component onto the surface of the hydrophobic dielectric substrate. Advantageously, as shown in the right side of FIG. 2, a fine water drop can be adsorbed onto the surface of the substrate in a contact angle of about 24 degrees measured with respect to drop shape after processing the substrate with plasma.

In step 2, select a basic aqueous solution having an appropriate pH value and immerse the silicon based dielectric material therein. As a result, a great number of OH radicals are adsorbed onto the active surface as shown in part (2a) of FIG. 1. Next, hydrogen removal (deprotonation) takes place to form a negatively charged surface of the dielectric substrate having Si—O— thereon, as shown in part (2b) of FIG. 1.

In step 3, immerse the dielectric substrate having a negatively charged surface in a basic aqueous solution having metal ions (e.g., Co or Ni) for adsorbing metal ions by Columbic forces and/or exchanging ions at the surface of the substrate. As a result, ultrafine nanoparticles of atomic scaled metal ions are formed on the surface of the substrate.

In step 4, expose the substrate to an annealing environment with reducing atmosphere or immerse the same in a reducing solution for the reduction of positively charged metal ions on the surface into neutral metal particles. This step would provide the catalytic effect for the subsequent electroless-plating process.

In step 5, immerse the dielectric substrate in an electroless-plated solution for depositing a thin film of an elemental metal (e.g., Co, Ni, Cu, etc.), a binary alloy (e.g., Co—P, Ni—P, etc.), or a tertiary alloy (e.g., Co—W—P, Ni—W—P, etc.).

Due to the drawbacks associated with growth of palladium catalyst seeds by current sensitization/activation process, the present invention using the all-electrochemical integrated plating technique excels in various aspects in terms of better control of very small film thickness, production cost, environmental friendliness and better compatibility adapted to the current IC process. FIG. 3 is a flowchart illustrating a self-aligned, selective electroless-plated metal deposition process according to the present invention. The process begins in step 1 where a hard mask or photoresist pattern is used to form a selective masking pattern on the hydrophobic substrate. In step 2, remove hydrophobic radicals from the surface thereof by performing a plasma treatment as illustrated in step 1 of FIG. 1. In step 3, remove the mask to leave a patterned hydrophilic area. In step 4, form the catalytic metal particles on the order of nanometer and develop the electroless-plated metal thin film by sequentially performing steps 2 to 5 of FIG. 1. Alternatively, perform step 4 prior to removing the mask in step 3, namely, steps 3 and 4 can be reversed.

Following is Table I to depict the differences between the present invention and the prior sensitization/activation process regarding seeding technique.

TABLE I
Comparisons between ultrafine catalytic seeding in the invention and
the sensitization/activation process
	Seeding Techniques
	Ultrafine	Sensitization/
	Catalytic	Activation
Description	Seeding	Seeding	Remarks
Plating solution	Basic	PdCl2/SnCl2
	metal ions
Process cost	Low	High	Pd metal is relatively
			expensive.
Solution stability	High	Low	Solution of
			sensitization/activation
			process should be
			constantly
			replenished.
Environmental	High	Low	HCl and HF solution
Friendliness			are highly corrosive.
Seed feature	2-5 nm	>100 nm	The particles formed
			by
			sensitization/activation
			process are highly
			agglomerated.
Self-aligned	Yes	No	Difference may result
Capability			in hydrophilic species
			and hydrophobic ones
Plating thickness	≧20 nm	≧100 nm

Following is Table II for illustrating process conditions of plasma treatment, electrochemical solution preparations, and their corresponding process steps.

TABLE II
Process conditions of plasma treatment, electrochemical solution
preparations, and their corresponding process steps.
Step	1	2	3
Description	Plasma treatment for the	In the basic	Ni(NO3)2/
	hydrophobic substrate to	aqueous solution	Co(NO3)2
	become hydrophilic
Process	Oxygen/nitrogen/	T = 25˜80° C.,	T = 25° C.,
Conditions	hydrogen	t = 10 min	t = 10 min
	plasma or combination
	thereof
Step	4	5	6
Description	Highly reducing	Preparation of	Cleaning/
	solution/hydrogen	electroless-plated	drying
	undergoing a	solution
	reduction reaction
	at high-temperature
Process	T = 80° C.,	Based on plating	Ultrasonic
Conditions	t = 60 min	operation	cleaning/
		conditions	drying
			by nitrogen
			spraying

Surface Treatment by Plasma

This is an essential process for hydrophobic dielectric material (e.g., polysiloxanes). Plasma is formed of ionized gases such as N₂/H₂, N₂, H₂, or O₂. Plasma formed of O₂ is advantageous for having the highest efficiency of removing hydrophobic radicals, however, it is disadvantageous for damaging the ring structure (e.g., ring Si—O) of polysiloxanes due to excessive input energy. In contrast, gaseous plasma N₂/H₂, N₂, or H₂ is advantageous for effectively removing hydrophobic radicals without damaging the ring structure of polysiloxanes. Furthermore, N₂/H₂ is preferred as compared with N₂ or H₂ plasma gaseous species.

Basic Aqueous Solution Preparation and Metal Ion Adsorption

Selecting a basic aqueous solution with desired components, types, concentration, and temperature would rely on the surface properties of substrate in order to perform a effective surface activation and thus the proficient adsorption of metal ions on the surface thereof.

Metal Ion Reduction

In addition to the prior high-temperature hydrogen reduction, a chemical solution reduction is embodied in the present invention. SiO₂ and polysiloxanes as representative silicon based dielectric materials will be described below. Catalytic metal ion adsorption, reduction, and electroless-plated film deposit are described in cooperation with soft X-ray synchrotron radiation absorption spectroscopy, transmission electron microscopy (TEM), atomic force microscopy (AFM), and scanning electron microscopy (SEM). Finally, an exemplary example is described for the illustration of a self-aligned patterned process.

Referring to FIG. 4, synchronous radiation peaks L2 of 845 eV and L3 of 875 eV in an X-ray absorption spectrum has verified that after undergoing steps in FIG. 1, SiO₂ or polysiloxanes dielectric substrate has adsorbed metal ions (e.g., Ni) thereon and has grown electroless-plated Co metal film. Curve (a) represents an initial substrate in which a substantially flat peak is shown and the peak of 852 eV is generated b y an analytic instrument, namely, the X-ray guiding facility. Signal of Ni²⁺ ion is shown after performing Ni²⁺ ion adsorption. After performing the metal ion reduction, the peak will change into peaks of neutral Ni metal, as indicated in curves (b) and (c) of FIG. 4. This means that Ni²⁺ ions have been reduced into neutral Ni ions which in turn can be served as catalyst for growing electroless-plated Co, Ni, or Cu metallic film formed of an elemental metal or a binary or tertiary alloy. The inset graph in FIG. 4 represents a curve to show the existence of electroless-plated Co film.

Referring to FIG. 5, the speckle feature of the TEM dark-field micrograph in part (a) of FIG. 5, along with the diffuse electron diffraction pattern (inset), indicates that the unprocessed dielectric films are amorphous. After the adsorbing/reducing treatment, bright-field images (see part (b) of FIG. 5), together with dark-field images and selected-area diffraction patterns, clearly reveal that a high density of ultrafine (2-4 nm) Ni crystalline precipitates are embedded in the dielectric film. Judging from the indexed diffraction pattern inset in part (b) of FIG. 5, these nickel precipitates have a face-centered cubic (fcc) structure and a lattice constant of 3.71 Å, ˜5% larger than that given by the X-ray diffraction source data (3.52 Å). They can provide the platform for the formation of electroless Co-based grains, ultimately coalescing to a very thin (20 nm) continuous barrier of grain size approximately 20 nm, as show in part (c) of FIG. 5.

Referring to FIG. 6, it depicts a SEM micrograph of the cross-section of a SiO₂ substrate being processed by electroless-plated Co deposit for about 10 minutes in which the thickness of electroless-plated Co film is approximately 100 nm.

Referring to FIG. 7, it shows a series of three-dimensional AFM images for the samples giving TEM micrographs in Referring to FIG. 5. The part (a) of FIG. 7 reveals that the dielectric substrate not being processed is featureless and has a mean roughness (R_a) of only 0.65 nm, concurring with the TEM image depicted in part (a) of FIG. 5. The surface morphology and R_a resemble those of part (a) of FIG. 7 for samples after the sequential treatments of the N₂/H₂ mixed plasma (R_a=0.62 nm) and immersion of a basic aqueous solution (R_a=0.57 nm). Subsequently after the Ni²⁺ adsorbing (R_a=0.97 nm) and reducing steps, part (b) of FIG. 7 indicates that ultrafine catalytic sites are heavily incorporated on the surface of the dielectric substrate, rendering a sharp rise in R_a (2.4 nm). The ultrafine (2˜4 nm), densely distributed Ni catalytic particles allow 70% coverage of the Co-based grains after plating for only 1 min and result in a significant increase in R_a to 8.1 nm (see part (c) of FIG. 7). Moreover, the thickness for just forming a continuous barrier film (R_a being substantially reduced to 3.69 nm) is only ˜20 nm (see part (d) of FIG. 7), in accordance with the TEM result (see part (c) of FIG. 5). Generally, the seed particles, produced using the sensitization/activation, displacement, or other related processes, tend to form aggregates exceeding 20 nm. Thus, these methods are generally incapable of initiating the plating of electroless barriers of extremely fine thickness (e.g., <20 nm). Given the results of the present invention, clearly, it is possible to fabricate electroless-plated metal film of thickness only 20 m or even less using this all-electrochemical integrated plating technique by properly optimizing the surface activating procedure.

There is no need of further plasma treatment for the surface of hydrophilic SiO₂ dielectric material. Simply it is immersed in a known basic aqueous solution for forming a negatively charged surface of the dielectric material having Si—O— thereon (see FIG. 1). However, hydrophobic polysiloxanes substrate has to be processed by plasma treatment to form a hydrophilic surface. Also, the hydrophobic polysiloxanes substrate has to be immersed in a basic aqueous solution having strong oxidizing agent in order to form an OH-terminated surface. Next, a deprotonation process is performed to form a negatively charged surface having Si—O⁻ thereon. After using a suitable strong oxidizing agent, and performing a selective treatment on the surface by plasma or immersing the hydrophobic polysiloxanes substrate in an oxidized basic aqueous solution, the steps shown in FIG. 3 can be utilized to form a self-aligned electroless-plated film formed of Co or Cu. The image in FIG. 8 illustrates a self-aligned MOS capacitive structure comprising silicon based dielectric layer, Co—W—P barrier, and Cu metal film on plasma-treated areas of HOSP™ dielectric substrates using the all-electrochemical integrated deposition process, which is presented in FIG. 3.

While the present invention herein disclosed has been described by means of specific embodiments, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope and spirit of the present invention set forth in the claims.

Claims

1. A process for ultrathin electroless-plated metal film on a dielectric substrate formed of silicon based dielectric material being either hydrophilic or hydrophobic, comprising the steps of:

(i) treating the surface of the substrate by plasma;

(ii) immersing the substrate in a basic aqueous solution;

(iii) removing hydrogen from the surface of the substrate;

(iv) immersing the substrate in a basic aqueous solution having metal ions for adsorbing the metal ions onto the surface of the substrate;

(v) reducing the metal ions on the surface of the substrate; and

(vi) immersing the substrate in an electroless-plated solution for depositing an electroless-plated metal film.

2. The process of claim 1, wherein the silicon based dielectric material is selected from silicon dioxide or silicate glass or polysiloxanes so as to form an active surface on the substrate and form a negatively charged surface on the substrate after performing the step (ii) for adsorbing the metal ions as catalytic seeding for electroless-plating deposition.

3. The process of claim 2, wherein the silicon based dielectric material selected from either the hydrophilic silicon dioxide or the hydrophilic silicate glass is immersed in a basic aqueous solution for depositing the electroless-plated metal film irrespective of the plasma treatment.

4. The process of claim 2, wherein the silicon based dielectric material selected from the hydrophobic polysiloxanes undergoes the step (i) to form a hydrophilic surface, plasma is formed of one of ionized gas including N2/H2, N2, H2, and O2, and the basic aqueous solution is a strong oxidizing agent for depositing the electroless-plated metal film.

5. The process of claim 4, wherein the substrate formed of the hydrophobic polysiloxanes is immersed in a basic aqueous solution with strong oxidizing capability for developing a negatively charged surface and performing a selective treatment on the surface of the substrate by plasma to form a self-aligned electroless-plated metal film pattern with very small (<20 nm) thickness.