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Some bipolar memory cells require a low-leakage high-capacitance densi capacitor. Described here is a capacitor structure to meet increased requirements of such cells.
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Capacitor Structure for Bipolar Memory Device
Some bipolar memory cells require a low-leakage high-capacitance densi
capacitor. Described here is a capacitor structure to meet increased
requirements of such cells.
The device requirements include N/+/ poly Si contact to the N+,emitter, as
shown in Fig. 1. The N/+/ poly thickness is approximately 1500 Angstrom. The
capacitor must be designed around this bottom poly Si electrode.
Next, hafnium (Hf) metal is evaporated or sputtered to coat the N+ poly, prior
to reactive sputtering or RF sputtering of HfO(2) (hafnium dioxide), as shown in
Fig. 2. The Hf layer is approximately 1000 Angstroms, and the HfO(2) is
approximately 300-600 A thick. The top electrode Al or Au is deposited over the
HfO(2) to complete the capacitor. The films are usually annealed in O(2) prior to
Al (top electrode evaporation).
Experiments have shown that if the Hf film is not included in Fig. 2, oxygen
diffuses through HfO and oxidizes the Npoly. This decreases the capacitance
because of the low dielectric constant of SiO(2). The Hf layer also planarizes the
Npoly surface which reduces the number of breakdown sites. A reactively
sputtered HfO(2) film (540 Angstroms) over 1000 A of Hf yields an average
capacitance of 640 picofarads, and leakage of less than 10/-12/ amperes up to 5
volts. Typical cell requirements would be 700 picofarads and 10/-9/ amperes up
to 3-4 volts (aluminum electrodes). A 300 Angstroms HfO(2) film yields 904