US 4423087 A
A thin film capacitor having a dual bottom electrode is provided. The bottom electrode comprises a first layer of metal and a second layer of platinum, said metal of the first layer being of the nature of forming a stable intermetallic phase with the platinum during heat treatment. The metal of the first layer is typically selected from the group consisting of Hf, Zr, and Ta. The thin film capacitor is suitable for the decoupling capacitor of VLSI.
1. A method for forming a thin film capacitor comprising the steps of:
depositing on a substrate a first layer of metal selected from the group consisting of hafnium, zirconium, and tantalum,
depositing a second layer of platinum on said first layer,
depositing a dielectric layer on said second layer, and
forming a counter electrode.
2. The method of claim 1, wherein said dielectric layer being a dual layer of Si3 N4 and Ta2 O5.
3. The method of claim 1, wherein said dielectric layer is deposited at a temperature where intermetallic phase is formed at the interface of said first and second layers.
This invention relates to thin film capacitors and particularly to decoupling capacitors for each integrated circuit chip to remove undesirable noise. More particularly, this invention relates to the bottom electrode structure for thin film capacitors.
The prior art, U.S. Pat. No. 4,002,545 entitled "Method of Forming A Thin Film Capacitor" granted Jan. 11, 1977 to F. P. Fehiner et al., proposed a thin film capacitor comprising a dual bottom electrode, a tantalum oxide film formed on the bottom electrode, and a counter electrode. The duplex bottom electrode consists of a first layer of any electrically conductive material which is compatible with tantalum and tantalum oxide, such as aluminum, nichrome, chromium, and gold, and a second layer of tantalum.
U.S. Pat. No. 3,969,197 entitled "Method For Fabricating A Thin Film Capacitor" granted July 13, 1976 to N. J. Tolar et al., discloses a thin film capacitor having a dual bottom electrode which consists of a first layer of molybdenum or tungsten and a second layer of chromium, on which is deposited a dielectric film of tantalum oxide. The materials used are selected so that they all have substantially the same coefficient of thermal expansion. These dual bottom electrodes of the prior art are not satisfactory for use in the VLSI devices, particularly when they are used for decoupling capacitors since they are subject to corrosion or oxidation during high temperature deposition of CVD Si3 N4 or SiO2.
U.S. patent application Ser. No. 182,740 entitled "Capacitor structures With Dual Dielectrics" by J. K. Howard, filed Aug. 29, 1980 and assigned to the assignee of this invention relates to a thin film capacitor wherein the dielectric film is comprised of a first dielectric layer of silicon nitride of aluminum oxide and a second dielectric layer selected from the group consisting of Ta2 O5, HfO2, TiO2, PbTiO3, BaTiO3, CaTiO3, and SrTiO3. The bottom electrode was polycrystalline silicon which could easily withstand the 800° C. process for CVD Si3 N4. However, if this capacitor structure is used for a thin film decoupling capacitor, polysilicon is not acceptable since the sheet resistance of phosphorus doped polysilicon is approximately 20-40 ohms/square, whereas the sheet resistance requirement for the bottom electrode in the decoupling capacitor is 10-30×10-2 ohms per square.
U.S. Pat. No. 3,723,838 entitled "Nitrogen-Doped Beta Tantalum Capacitor" granted Mar. 27, 1973 to H. Y. Kumagai and U.S. Pat. No. 3,621,347 entitled "Semi-conductor Device Comprising A Field Effect Transistor Having An Insulated Gate Electrode and Circuit Arrangement Comprising Such a Semiconductor Device" granted Nov. 16, 1971 to J. A. Van Nielen generally relate to thin film capacitors.
Since large scale integrated circuit devices operate on a very high frequency such as 1 GHZ, the capacitor properties must be stable. The dissipation factor, tan S, is frequency dependent:
tan S=tan S'+WCR
tan S'=intrinsic loss due to dielectric
R=series resistance of electrodes+leads
Thus, to achieve a stable and low dissipation factor at high frequency, it is important that the resistivity of the electrodes be low to minimize series resistance.
It is also important that the bottom electrode structure is thermally stable so that it can be used as the substrate for high temperature deposition of dielectrics. The bottom electrode is required to have the properties of providing a smooth metallic surface to reduce pin hole type defects, providing an oxidation resistant surface at high temperature, and providing good adhesion to the underlaying substrate.
Therefore, it is the object of this invention to provide a thin film capacitor which is stable at high frequencies.
It is another object of the invention to provide a thin film capacitor having a low dissipation factor.
It is a further object of the invention to provide a thin film capacitor having a bottom electrode which has low resistivity and is stable to high temperature treatment.
It is a still further object of the invention to provide a process for forming a thin film capacitor having a bottom electrode which has low resistivity and is stable to high temperature treatment.
According to the invention, the thin film capacitor includes a bottom electrode of dual structure which comprises a first layer of metal which, when subjected to a high temperature anneal, interdiffuses into platinum layer to form a very stable intermetallic phase having lower resistivity than platinum, and a second layer of platinum. Because of the very stable intermetallic phase, the growth of such phase is self-limiting and the first layer metal does not reach the surface of the platinum layer during heat treatment. The metal for the first layer is typically selected from the group consisting of zirconium, hafnium and tantalum. When the dual bottom electrode is subjected to the high temperature (500°-1000° C.) anneal, Pt and Zr, Hf or Ta interdiffuse to form a very stable intermetallic phase ZrPt3, HfPt3, or TaPt3. The resistivity of these phases is approximately 8μ ohm cm, which is less than that of Pt, 10.6μ ohm cm. Some properties of these phases are included in a publication by L. M. Percora and P. J. Ficalora (Journal of Electronic Material 6, 531 1977). When the capacitor of this invention is formed on a silicon dioxide layer on a semiconductor body, the first metal layer reduces the thickness of the SiO2 at the interface to form an adhesive bond to the substrate during heat treatment. This is because the metal of the first layer forms more stable oxides than SiO2 and oxygen is displaced from SiO2 to form a metal oxide of the first layer such as ZrOx, TaOx and HfOx.
The dielectric layer for the capacitor of this invention can be any conventional material like Ta2 O5. However, for high density capacitance applications, the dual dielectric layer as proposed in U.S. patent application Ser. No. 182,740 is appropriate. The dual dielectric layer comprises a first dielectric layer of silicon nitride or aluminum oxide, or silicon dioxide formed on the bottom electrode, and a second dielectric layer selected from the group consisting of Ta2 O5, HfO2, TiO2, PbTiO3, BaTiO3, CaTiO3 and SrTiO3. Typically, the dual dielectric layer is composed of a thin CVD Si3 N4 layer deposited at 800° C. onto the bottom electrode material and reactively sputtered Ta2 O5 deposited onto the Si3 N4. The dielectric constant of Ta2 O5 is high (ε˜22-25) and provides high capacitance and the Si3 N4 layer or SiO2 layer exhibits a high breakdown field (˜10 MV/cm), thus providing low leakage (˜10-12 A at 2 V) and high breakdown voltage. A particular capacitor requirement can be achieved by varying the oxide/nitride thickness ratio using the equation for capacitors in series.
The counter electrode for the capacitor of the invention can be any conductive material such as Al, Cu, Al-Cu, etc.
The details of this invention will be described in connection with the accompanying drawings, in which:
FIG. 1 is a cross-section of the bottom electrode structure;
FIG. 2 is a cross-section of the bottom electrode showing interfaces where reactions occur;
FIG. 3 shows a cross-section of a capacitor device array;
FIG. 4 shows an Auger depth-composition profile for Pt-Hf deposited on oxidized silicon;
FIG. 5 shows the Auger depth-composition profile after anneal at 800° C. in vacuum;
FIG. 6 is a cross-section of a capacitor device array including solder pads for flip-chip bonding.
A cross-section of the bottom electrode structure is shown in FIG. 1. The silicon wafer 1 having silicon dioxide layer 2 formed thereon is cleaned with 10:1 buffered HF for 10 seconds, N2 blow dried and loaded in a deposition system. In one embodiment, layer 3 of Zr, Hf, or Ta is electron beam evaporated to a thickness of 500-700 Å at 3 Å/sec at a pressure under 2×10-6 Torr. After layer 3 is deposited, the substrate is transferred to a metal sputtering system where Pt is sputter deposited to form Pt layer 4 of a thickness of 10,000-20,000 Å at 2-5 Å/sec under approximately 10-7 Torr base pressure with 1 KW 10 m Torr Argon. Upon heat treatment such as for dielectric deposition and/or post deposition anneal, reactions occur at the interface A between SiO2 and layer 3, and at the interface B between layer 3 and layer 4 (FIG. 2).
At interface A, the metal of layer 3, i.e., Zr, Hf or Ta reduces SiO2 to form an adhesive bond to the substrate. During the high temperature anneal, Pt of the second layer and Zr, Hf, or Ta of the first layer interdiffuse at interface B to form a very stable intermetallic phase ZrPt3, HfPt3, or TaPt3. The resistivity of these phases is approximately 8μ ohm cm, which is less than 10.6μ ohm cm of Pt. FIG. 4 shows an Auger depth-composition profile for the bottom electrode structure comprising 500 Å Hf layer deposited on SiO2 and 3000 Å Pt deposited on the Hf layer. To simulate CVD Si3 N4 deposition, the bottom electrode structure was annealed at 800° C. FIG. 5 shows the auger depth-composition profile after anneal. Pt-Hf reaction and reduction of SiO2 (dip in oxygen profile) are shown.
The important feature of this profile is that Hf is retained at the interface after anneal and does not reach the Pt surface. It is postulated that HfPt3 forms and limits further diffusion of Hf and/or Hf is stabilized by reduction of SiO2. Surface sensitive Auger profiles were obtained on 10,000 Å Pt on Zr films after 800° C. anneals and Zr was not detected.
The series resistance was measured for a sample having a bottom electrode comprising 3000 Å Pt and underlaying 500 Å Hf deposited on 5000 Å SiO2 formed on N+ silicon substrate. The sheet resistivity Rs=ρ/t measured before anneal was 0.706 ohms/square and after anneal at 800° C. to simulate Si3 N4 it was reduced to 0.592 ohms/square.
The Pt-Hf-SiO2 reaction was evaluated further by Auger electron spectroscopy. A composition-depth profile shown in FIGS. 4 and 5 reveal Pt-Hf-SiO2 system before anneal and after 800° C. anneal. The dip in the oxygen peak represents reduction of SiO2 by Hf to provide excellent adhesion. The Pt peak is altered in Hf and represents PtHfx compound phase. The phase growth is self-limiting, as it grows thicker it limits further penetration of Hf, thus little Hf penetrates the phase even at 800° C. The resistivity of HfPt3 is known to be very low (8μ ohm cm) and probably accounts for the drop in the sheet resistivity after heat treatment. The self-limiting nature of the Hf-Pt phase is expected since these phases are some of the most stable structures.
Three kinds of samples were prepared for comparison. The first sample is composed of a dual bottom electrode consisting of 7000 Å Pt layer and underlaying 500 Å Hf layer which is deposited on 5000 Å SiO2 layer formed on a silicon substrate. The second sample includes a dual bottom electrode consisting of 7000 Å Pt layer and underlaying 500 Å Zr layer which is deposited on 5000 Å SiO2 layer on a silicon substrate. The dual (i.e. composite) bottom electrode of the third sample comprises 7000 Å Pt layer and underlaying 500 Å Cr layer which is deposited on 5000 Å SiO2 layer on a silicon substrate. Table I shows the results of sheet resistivity measurements made before and after 800° C. anneal to simulate CVD Si3 N4 deposition.
TABLE I______________________________________Rs (As-Deposited) Rs (Anneal) Δ(%)______________________________________Pt/Hf 275 mΩ/square 317 mΩ/square +15%Pt/Zr 237 mΩ/square 260 mΩ/square +9.7%Pt/Cr 260 mΩ/square 519 mΩ/square +99.6%______________________________________
The data indicates that the sheet resistivities of Pt/Hf and Pt/Zr electrode structures are much lower than that of Pt/Cr structure after heat treatment. The sheet resistivity increased by 15% and 9.7% with heat treatment as to Pt/Hf and Pt/Zr structures respectively while it increased 99.6% with Pt/Cr structure.
An improved sheet resistivity is obtained by interrupting the deposition process after deposition of Zr, Hf, or Ta, opening the system to air before deposition of Pt. It is postulated that interdiffusion between Pt and underlaying metal is reduced by this interruption. However, the effect of breaking vacuum appears to be slight.
Table II shows the results of sheet resistivity measurements before and after heat treatment.
TABLE II______________________________________ Rs (As-Deposited) Rs (Anneal)* Δ(%)______________________________________Pt/Hf (SPD) 167 mΩ/square 213 mΩ/square 27.5%Pt/Hf (BV) 172 218 26.7%Pt/Zr (SPD) 167 205 22.7%Pt/Zr (BV) 166 199 19.8%Pt/Ti (SPD) 219 337 53.8%______________________________________ *150Å Si3 N4 deposited at 800° C. and stripped prior to Rs measurement.
For the samples of Table II, 150 Å Si3 N4 layer is deposited on the dual bottom electrode structure at 800° C. and stripped prior to sheet resistivity measurement. In the Table, SPD indicates that Pt and Hf or Zr were deposited in a single pump down. BV indicates that vacuum was broken and the system was exposed to air prior to Pt deposition.
Thin film capacitor samples having the dual bottom electrode structure which comprises 7500 Å Pt layer and underlaying 600 Å Zr layer were annealed at 1025° C. in Argon. These samples show two times increase after processing at a temperature higher than 1000° C. The sheet resistivity being 0.24 ohm/square before anneal and 0.43 ohm/square after anneal. The Pt/Zr or Pt/Hf electrode structures are stable at deposition temperature higher than about 1000° C. This property could be useful in forming metal gates for FET applications.
FIG. 3 shows the cross-section of a capacitor device array which has a Pt/Zr bottom electrode. The capacitor area was approximately 4.13×10-2 cm2. The capacitor comprises of the dual bottom electrode of Zr layer 3 of 500 Å and Pt layer 4 of 7500 Å, Si3 N4 layer 5 of 173 Å deposited on Pt layer 4. Ta2 O5 layer 6 of 225 Å deposited on layer 5, and a 7000 Å counter electrode 7 of aluminum containing 4% of copper. Approximately one hundred devices were tested prior to SiO2 overlay and mostly yielded the 10 nanofarad capacitance with leakage less than 10-10 -10-11 amp. at 3 V.
FIG. 6 shows a cross-section of a semiconductor device which was processed to include solder pads 50 and 60 for flip-chip bonding. A dual bottom electrode 10 is formed on a semiconductor silicon substrate 1 having SiO2 layer 2 formed thereon, followed by deposition of a dielectric layer 20, preferably comprising of a dual layer of Si3 N4 and Ta2 O5. A counter electrode 7 of any conventional conductive material such as Cu, Al, Al-Cu, etc., is deposited and formed into the desired electrode pattern with conventional method. After SiO2 layer 30 is deposited, contact holes are opened through SiO2 layer 30 and dielectric layer 20 with conventional photolithographic and etching technique to expose portions of the bottom electrode 10 and the counter electrode 7. Solder pads 50 and 60 are formed to contact electrode 7 and 10 respectively for flip-chip bonding. The solder pads include Cr-Cu-Au layer 40 at the interface with the underlaying electrodes.
While the invention has been particularly shown and described with reference to the embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and detail may be made therein without departing from the spirit and scope of the invention.