Semiconductor device manufacturing: process – Making passive device – Resistor
Reexamination Certificate
2000-08-04
2002-11-05
Smith, Matthew (Department: 2825)
Semiconductor device manufacturing: process
Making passive device
Resistor
C438S382000, C438S385000, C257S359000, C257S379000, C257S516000
Reexamination Certificate
active
06475873
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to semiconductor processing, and in particular, to a method of forming a thin film resistor disposed on an underlying dielectric layer having a well-controlled thickness useful for reliable laser trimming.
BACKGROUND OF THE INVENTION
Thin film resistors are employed in many integrated circuits. Thin film resistors are used in integrated circuit to implement the desired functionality of the circuit, including biasing of active devices, serving as voltage dividers, assisting in impedance matching, etc. They are typically formed by deposition of a resistive material on a dielectric layer, and subsequently patterned to a desired size and shape. Deposition of the resistive material can be performed by any deposition means, such as by sputtering. Often, a thin film resistor is subjected to a heat treatment process (i.e. annealing) to improve its stability and to bring the resistance to a desired value.
Generally, obtaining a precise resistance value for a thin film resistance is not a straightforward process. First, the resistive material, size and shape of a thin film resistor are typically selected so that the overall resistance is slightly below its final value. Then, the thin film resistor undergoes an annealing process to raise the resistance up to its final value. However, the desired resistance value for the thin film resistor may not be achieved using this process. As a result, laser trimming is performed on the thin film resistor while measuring the resistance, or another circuit parameter associated with the resistance, to precisely set the thin film resistor at the desired value. In order to precisely laser trim a thin film resistor, a high degree of thickness control is typically necessary for the dielectric layer to which the thin film resistor is disposed on.
FIG. 1
illustrates a portion of an exemplary integrated circuit
100
having a thin film resistor formed in a conventional manner. The integrated circuit
100
consists of a silicon substrate
102
, a local oxidation of silicon (LOCOS) layer
104
, a glass layer
106
such as borophosphosilicate (BSPG) or phosphosilicate glass (PSG), a thin film resistor
108
which can be formed of chromium-silicon (CrSi), tantalum nitride (TaN), and other suitable materials, a dielectric overcoat
110
, metal contacts
112
for the thin film resistor
108
, and a metal contact
116
for an active device (not shown). Generally, the process of forming the integrated circuit
100
includes the step of forming the LOCOS layer
106
and active device, depositing the glass layer
106
on the LOCOS layer
104
, depositing and patterning the resistive material (e.g. CrSi) to form the thin film resistor
108
, depositing the dielectric overcoat
110
, and forming the contacts
112
for the thin film resistor
108
and the contact
116
for the active device.
There are several undesirable characteristics of the conventional integrated circuit
100
. First, the thin film resistor
108
is disposed over two dielectric layers, i.e. the LOCOS layer
104
and the glass layer
106
. As it was previously stated, to achieve precise laser trimming, a well-controlled dielectric thickness is required under the thin film resistor
108
. In the conventional integrated circuit
100
, however, there are two dielectric layers. It is relatively difficult to control the thickness of two dielectric layers since two variables are involved. Second, another undesirable characteristic of the conventional integrated circuit
100
is that contacts
112
for the thin film resistor
108
are made directly to the resistor
108
. Since the thickness of the thin film resistor
108
can be on the order of 50 to 100 Angstroms, the etching of the dielectric overcoat
110
to form the contacts
112
often results in the etching away of the thin film resistor
108
underlying the contacts
112
.
Thus, there is a need for a method of forming a thin film resistor that has improved laser trimming capability, and whose formation process is less prone to defects resulting from the formation of the contacts for the thin film resistor. Such improved method, among other aspects of the invention, is provided herein in accordance with the invention.
SUMMARY OF THE INVENTION
A new and improved method of forming a thin film resistor is provided herein that overcomes many of the drawbacks of prior art methods. More specifically, the new method of forming a thin film resistor provides for a well-controlled dielectric thickness under the thin film resistor which is useful for laser trimming purpose. The preferred thickness of the dielectric layer is approximately an integer of a quarter wavelength of the optical energy used to laser trim the resistor. The new method also provides contacts to the thin film resistor that do not directly contact the thin film resistor so as to prevent any adverse process effects to the thin film resistor.
More specifically, the method of forming a thin film resistor comprises the steps of forming a pair of spaced-apart polysilicon islands over a semiconductor substrate, forming a dielectric layer over and between the polysilicon islands, forming contact holes through the dielectric layer to expose respective first regions of the polysilicon islands, forming a layer of thin film resistive material that extends between respective first regions of the polysilicon islands, forming another dielectric layer over the polysilicon islands and over the thin film resistive material layer, and forming metal contacts through the second dielectric layer in a manner that they make contact to respective second regions of the polysilicon islands, wherein the first and second regions of the polysilicon islands are different.
Alternatively, the new method may further include forming a silicide layer between the polysilicon islands and the thin film resistor. This silicide layer can be formed by depositing a refractory metal and heat treating the refractory metal so that it reacts with a portion of the polysilicon to form silicide. The refractory metal can include cobalt, titanium, vanadium, nickel, etc. Also, as previously stated, the dielectric layer between the thin film resistor and the semiconductor substrate has a thickness of approximately an integer of a quarter wavelength of an optical energy used to laser trim the thin film resistor. This thickness optimizes the laser trimming of the thin film resistor. The dielectric layer can comprise an oxide or other suitable dielectric material. Further, the thin film resistive material can include chromium-silicon (CrSi), tantalum nitride (TaN), or other suitable thin film resistive material.
In addition to the formation of the thin film resistor, other integrated circuit structures and/or devices can be formed in combination with the thin film resistor. For instance, a shallow trench isolation can be formed within the semiconductor substrate. Additionally, an active device can be formed at least in part within the semiconductor substrate, isolated by the shallow trench. The active device can comprise a metal oxide field effect transistor (MOSFET), a complimentary MOSFET (CMOS), bipolar type transistors, diodes, and other semiconductor devices. Also, a Schottky diode can be formed using the same or different silicide layer used to line the polysilicon islands where the thin film resistor makes electrical contact. The silicide layer makes contact with the semiconductor substrate to form the Schottky diode.
Other aspects and features of the invention will be apparent in view of the following detailed description of the invention including the accompanying drawings.
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patent: 5414404 (19
Ellul Joseph P.
Kalnitsky Alexander
Scheer Robert F.
Blakely , Sokoloff, Taylor & Zafman LLP
Malsawma Lex H.
Maxim Integrated Products Inc.
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