- BACKGROUND OF THE INVENTION
This invention relates generally to semiconductor processing methods of forming resistors and diodes from semiconductive material, and to static random access memory (SRAM) cells incorporating resistors, and to other integrated circuitry incorporating resistors and diodes.
One of the common elements required in electrical circuit devices is the pull-up or pull-down resistor from an active device to one of the power supply buses, typically referred to as Vcc. The pull-up is simple if used to construct a circuit using discrete components in that all that is required is selecting a resistor of the desired resistance and tolerance, connecting it between an active device such as an open collector transistor and Vcc, and the transistor's output would be pulled up to Vcc once the transistor is forward biased. With the advent of integrated circuitry, however, fabricating a resistance onto a wafer substrate, such as silicon or gallium arsenide, takes special consideration, particularly when resistivity and tolerances play an important part in circuit operation.
For example, as SRAMs have evolved from the 4Kb memory arrays to more densely packed array sizes, tolerances of pull-up resistances had to be tightly controlled. In order to minimize standby current, many fabrication processes adopted use an active device as the pull-up. In CMOS fabrication, it is common to see a PMOS transistor acting as the current path between a memory cell access transistor and the power supply bus. In this manner, the PMOS transistor can be gated “on” only when the desired line is to be pulled up to Vcc and turned “off” otherwise. This in essence eliminates leakage current and minimizes standby current for the SRAM device as a whole.
The main drawback to using an active device for a pull-up device is the amount of space required to fabricate the device. Now that the SRAM generation has grown to the 1Mb array size, die space is a critical factor to consider. Technology has basically pushed all types of integrated circuits to be more densely packed, and pull-ups are a common element in many circuit designs.
BRIEF DESCRIPTION OF THE DRAWINGS
Although the invention primarily arose out of concerns associated with resistor fabrication in SRAM circuitry, the artisan will appreciate applicability of the inventive technology elsewhere, with the invention only being limited by the accompanying claims appropriately interpreted in accordance with the doctrine of equivalents.
Preferred embodiments of the invention are described below with reference to the following accompanying drawings.
FIG. 1 is a diagrammatic cross-sectional view of a semiconductor wafer fragment at one processing step in accordance with the invention.
FIG. 2 is a view of the FIG. 1 wafer fragment at a processing step subsequent to that shown by FIG. 1.
FIG. 3 is a view of the FIG. 1 wafer fragment at a processing step subsequent to that shown by FIG. 2.
FIG. 4 is a view of the FIG. 1 wafer fragment at a processing step subsequent to that shown by FIG. 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 5 is a schematic representation of SRAM circuitry in accordance with an aspect of the invention.
This disclosure of the invention is submitted in furtherance of the constitutional purposes of the U.S. Patent Laws “to promote the progress of science and useful arts” (Article 1, Section 8).
The invention includes several aspects of forming resistor and diode constructions. The invention also includes several aspects of SRAM and other integrated circuitry incorporating diodes and resistors produced according to the inventive methods, and produced according to other methods.
A semiconductor processing method of forming a resistor construction from semiconductive material is first described with reference to FIG. 1. Such illustrates a bulk semiconductor substrate 12, such as monocrystalline silicon, having a conductive diffusion region 14 formed therein. Region 14 constitutes a node to which electrical connection to a resistor is to be made. An electrically insulative layer 16, typically borophosphosilicate glass (BPSG), is provided outwardly relative to node 14. A first opening 18, preferable in the configuration of a substantially vertical passageway, is provided therethrough over node 14. First opening 18 has an opening width “A”. Node 14 can be provided before provision layer 16 and opening 18, or after provision of layer 16 and opening 18.
Referring to FIG. 2, a first layer 20 of semiconductive material is chemical vapor deposited over electrically insulative layer 16 and within first opening 18 over node 14 to a thickness which is less than one-half first opening width “A” to less than completely fill first opening 18 with semiconductive material. Such thereby defines a remaining opening 22. An example semiconductive material of first layer 20 is silicon, such as amorphously deposited silicon or polycrystalline silicon. During or after deposition, first layer 20 can be provided with a first conductivity enhancing dopant impurity of a first “p” or “n” type. Layer 20 can also remain undoped at this point in the process. Accordingly, semiconductive material layer 20 has a first conductivity enhancing dopant concentration falling within a range of from 0 to some first value, with the first value typically being less than 1×1018 ions/cm3. Greater values, such as 1×1020 ions/cm3, would effectively inherently make layer 20 undesirably sufficiently electrically conductive to provide negligible resistance effect.
Referring to FIG. 3, a second layer 24 of semiconductive material is provided by chemical vapor deposition, or other technique, to within remaining opening 22 and inside of first layer 20 to completely fill remaining opening 22 with semiconductive material. Thus, opening 18 is likewise completely filled with semiconductive material. Second layer 24 is provided in one example with a second conductivity dopant concentration which is greater than the first concentration. Thus, a conductivity enhancing impurity concentration gradient is provided within opening 18 which varies from a low concentration at an inner elevation 26 within opening 18 to a higher concentration at an outer elevation 28 within opening 18.
The conductivity enhancing dopants provided in first and second layers 20 and 24 can comprise the same or different conductivity “n” or “p” types. Further even if of the same type, the dopants might be different, for example one being arsenic and one being phosphorus which are both “n” type. When of the same type, the semiconductive material effectively filling opening 18 defines a substantially vertically elongated resistor within opening/passageway 18, which is an ohmic electrical contact with node 14. Where the conductivity enhancing dopants provided in first and second layers 20 and 24, respectively, are of different type, the result will be formation of a substantially vertically elongated diode provided within passageway 18 and in ohmic electrical contact with node 14. The diode will effectively comprise two regions 20 and 24 of semiconductive material which in combination completely fill passageway 18.
In such instance, it may be desirable to provide each of regions/layers 20 and 24 with about the same concentration level of dopant impurity, and at greater than 1×1020 ions/cm3, to form a highly conductive diode. Alternately, it might be desirable to provide the two regions/layers with different type and different concentration level dopant impurities to effectively define a leaking or leaky diode, which then effectively functions as a resistor. Accordingly, in one aspect of the invention, a leaky diode construction also constitutes a vertically elongated resistor within opening 18.
Ultimately, an outer layer of electrically conductive material is provided outwardly of insulating layer 16 and patterned into a conductive line, with the elongated resistor or diode extending between node 14 and the formed conductive line. Such might be accomplished by a number of methods. For example, the construction of FIG. 3 could be etched back by chemical mechanical polishing, or other means, back to the upper surface of insulating layer 16. Subsequently, a metal or other highly conductive material can be deposited and patterned to form a line. More preferably, second layer 24 of semiconductive material is provided to have a dopant concentration which is at least 1×1020 ions/cm3 and is deposited to a sufficient thickness to enable it to be patterned into a conductive line 30 (FIG. 4). Accordingly in such instance, conductive line 30 comprises both first and second layers 20 and 24, with outer layer 24 forming a highly conductive part thereof. A silicide layer, such as WSix (not shown), might also be provided outwardly of layer 24. In either event, the formed diode or resistor extends between node 14 and patterned conductive line 30. Processing in accordance with the above described preferred method provides the advantage of provision of a line and diode or resistor without added masking steps for the resistor or diode.
Alternate techniques are contemplated for provision of substantially elongated vertically oriented resistors or diodes which fill opening/passageway 18. For example in provision of a resistor, opening/passageway 18 might be filled in a substantially continuous chemical vapor deposition step. During such deposition, the conductivity enhancing impurity would be provided to the reactor at a rate varying from, for example, a first lower rate to a second higher rate such that the outermost portion of the deposited layer has the desired high conductivity attributes, whereas lower regions have the desired resistive attributes. The final resultant rate could be provided to produce a dopant concentration at the outer regions of the deposited layer which is at least 1×1020 ions/cm3 to facilitate production of a desired highly conductive line outwardly of insulating layer 16.
A similar process could be utilized for formation of a diode. For example, a substantially continuous chemical vapor depositing step could be utilized to fill passageway/opening 18 and provide a layer thickness outwardly of insulating layer 16 sufficient for formation of a conductive line, and define an elongated diode within the passageway. For example, the chemical vapor depositing step could include first feeding a conductivity enhancing impurity of a first type into the reactor during deposition to provide semiconductive material of the first conductivity type at an inner elevation within the opening. During deposition, the dopant feed to the reactor would be changed from the first feeding to a second feeding of a conductivity enhancing impurity of a second type to provide semiconductive material of the second conductivity type at an outer elevation within the opening. Subsequent provision of a conductive metal line by mere patterning, or by provision of other conductive layers and patterning, could be provided.
Alternately in formation of a resistor, semiconductive material might be deposited atop insulating layer 16 and within opening 18 to be inherently undoped or very lightly doped as deposited. Subsequently, a conductivity enhancing dopant impurity might be driven into the semiconductive material layer at least outwardly of the electrically insulative layer to a peak concentration of greater than or equal to 1×1020 ions/cm3. Subsequently, this semiconductive material layer would be exposed to annealing conditions effective to diffuse dopant impurity within the semiconductive material from outwardly of the electrically insulative layer into the semiconductive material within the opening to effectively form an elongated resistor in ohmic contact with node 14. The semiconductive material layer outwardly of the electrically insulative layer would thereafter be patterned into a conductive line, with the elongated resistor extending between node 14 and the conductive line. Example annealing conditions include 950° C. for 20 seconds in an N2 atmosphere. Alternately, the wafer might be exposed to sufficient thermal conditions throughout processing to inherently provide such desired dopant driving to deep within passageway 18.
For diode formation, an alternate process is also contemplated. Specifically, a single semiconductive material layer can be chemically deposited to within opening 18 and over layer 16 to completely fill such opening. As deposited, the semiconductive material layer would be provided with conductivity enhancing dopant impurity of a first type having an average concentration of about 1×1018 ions/cm3. After the chemical vapor depositing step, a conductivity enhancing dopant impurity of a second type can be provided into the outermost portions of the deposited layer by ion implantation to a peak and overwhelming concentration of at least 1×1020 ions/cm3. The substrate is then exposed to annealing conditions effective to diffuse second type dopant impurity within the semiconductive material from outwardly of the electrically insulative layer into the first type semiconductive material within opening 18 to effectively form an elongated diode within the opening.
Integrated circuitry incorporating the above constructions whereby a substantially vertically elongated resistor or diode extends between a node and an outer conductive line is also contemplated.
FIG. 5 schematically illustrates one example of integrated circuitry of an SRAM cell in accordance with the invention utilizing at least one of the subject resistors. Such comprises a pair of first and second pull-down transistors 50 and 52, respectively. These include respective drains 53, 54; respective sources 55, 56; and respective gates 57, 58. Gate 57 of first pull-down transistor 50 is electrically coupled to drain 54 of second pull-down transistor 52. Likewise, gate 58 of second pull-down transistor 52 is electrically coupled to drain 53 of first pull-down transistor 50. A ground node 58 and a Vcc node 60 are provided. A first resistor 62 and a second resistor 64 electrically are coupled with Ad Vcc node 60 via a patterned line. Drain 53 of first pull-down transistor 50 electrically couples with Vcc node 60 through first resistor 62. Drain 54 of second pull-down transistor 52 electrically couples through second resistor 64 to Vcc node 60. A pair of cell access transistors 66 and 68 are also provided.
In the context of the previously described construction, the described and illustrated resistor/leaking diode would constitute one or both of resistors 62 and 64. Node 14 would constitute one of drains 53 or 54. Node 14 could alternately be the outer surface of gate 57 or gate 58. Patterned line 30 would be configured to extend to the suitable power Vcc node.
In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.