Systems and methods to maintain optimum stoichiometry for reactively sputtered films
The present invention relates to systems and methods for preparing reactively sputtered films. The films are generally thin transition metal oxide (TMO) films having an optimum stoichiometry for any useful device (e.g., a sub-stoichiometric thin film for a memristor device). Described herein are systems, methods, and calibrations processes that employ rapid control of partial pressures to obtain the desired film.
1. A method for calibrating a plasma vapor deposition (PVD) tool for deposition of a transition metal oxide (TMO) film having a specified sub-stoichiometric composition, comprising:
(a) stepping a rate of flow of reactive oxygen into a deposition chamber of the PVD tool through a range of flow-rate values,
(b) obtaining a feedback reference measurement from a gauge at each of a plurality of the stepped flow-rate values, wherein the gauge provides feedback reference measurements indicative of a reactive oxygen partial pressure within the deposition chamber, and wherein the feedback reference measurements can be compared to a target value for feedback control of the TMO film;
(c) identifying a transition region over which the feedback reference measurements exhibit a plateau as the reactive oxygen flow rate is varied, said plateau having a beginning at a first endpoint value of the reactive oxygen flow rate and an end at a second endpoint value of the reactive oxygen flow rate;
(d) designating a first setpoint equal to the feedback reference measurement from the gauge at the first endpoint value of the reactive oxygen flow rate, and
(e) designating a second setpoint equal to the feedback reference measurement from the gauge at the second endpoint value of the reactive oxygen flow rate;
(f) accessing a predetermined mapping between accessible compositions of the TMO film and their corresponding positions on the range from the first setpoint to the second setpoint, the predetermined mapping being independent of any particular feedback reference measurements and independent of any particular reactive oxygen partial pressures;
(g) from the mapping, selecting a position on the range from the first setpoint to the second setpoint that corresponds to the specified sub-stoichiometric composition;
(h) from the designated first and second endpoints, predicting a feedback reference measurement value that corresponds to the selected position on the range from the first setpoint to the second setpoint; and
(i) designating the predicted feedback reference measurement value as the target value for feedback control of the TMO film.
2. The method of claim 1 , further comprising, before depositing the TMO film, measuring film stoichiometry as a function of a first and of a last feedback reference measurement.
3. The method of claim 1 , wherein each of the feedback reference measurements comprises an O 2 optical emission measurement.
4. The method of claim 1 , wherein the obtaining a feedback reference measurement from the gauge at each of a plurality of the stepped flow-rate value comprises obtaining the feedback reference measurements from a gauge that is configured to respond within less than 250 milliseconds to changes in oxygen partial pressure in the deposition chamber.