Method for improving uniformity in deposited low k dielectric material

Abstract

A method for forming a low k dielectric material block is provided. In one example, the method includes depositing a low k dielectric layer over a semiconductor substrate and curing the deposited low k dielectric layer. The curing may be performed using a remote plasma process in which an excitation gas is excited in a selected region remote from the deposited low k dieletric layer to carry radiation energy and transfer to the low k dielectric layer when the excitation gas contacts the low k dielectric layer.

Description

BACKGROUND

[0002] The present disclosure relates generally to the fabrication of semiconductor devices, and more particularly, to a method for improving uniformity in deposited materials.

[0003] Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. Since then, integrated circuits have generally followed the two year/half-size rule (often called Moore's Law), which means that the number of devices on a chip doubles every two years. Today's fabrication plants are routinely producing devices having 0.35 μm and even 90 nm feature sizes.

[0004] In the process of reducing the size of devices on integrated circuits, it has become necessary to use conductive materials having low resistivity and insulators having low dielectric constants (k≦4.0) to reduce the capacitive coupling between adjacent metal lines. A conductive material of interest is copper which can be deposited in submicron features by electrochemical deposition. Dielectric materials of interest are silicon oxides that contain carbon. Combination of silicon oxide materials and copper has led to new deposition methods for preparing vertical and horizontal interconnects since copper is not easily etched to form metal lines. Such methods include damascene methods depositing vertical and horizontal interconnects wherein one or more dielectric materials are deposited and etched to form the vertical and horizontal interconnects that are filled with the conductive material.

[0005] Dielectric layers can be deposited, etched and filled with metal in multiple steps. Exemplary methods for depositing dielectric layers include damascene methods where lines/trenches are filled concurrently with vias/contacts. In a “counter-bore” scheme, a series of dielectric layers are deposited on a substrate, then vertical interconnects such as vias/contacts are etched through all of the layers and horizontal interconnects such as lines/trenches are etched through the top layers. A conductive material is then deposited in both the vertical and horizontal interconnects.

[0006] The deposition of low k material may use several conventional methods, and may be subsequently cured through a radio frequency (RF) plasma curing process. The RF plasma curing process not only causes chemical reactions on the deposited material, but also bombards the material with ions. One problem that has emerged is that the bombardment may cause degradation in the uniformity of the deposited material.

[0007] Accordingly, what is needed is a process to form a low k material that has better uniformity in the deposition process.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIG. 1 illustrates a sectional view of a via/interconnect structure.

[0009] FIGS. 2-5 illustrate sample processing steps to deposit low k dielectric material without using an etch stop layer according to one example of the present disclosure.

[0010] FIG. 6 illustrates a sectional view of a gradient low k dielectric material undergoing an RF plasma curing process.

[0011] FIG. 7 illustrates a sectional view of a relatively homogenous low k dielectric material undergoing a remote plasma curing process.

DETAILED DESCRIPTION

[0012] The present disclosure relates generally to the fabrication of semiconductor devices, and more particularly, to a method for improving uniformity in deposited materials.

[0013] It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the disclosure. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

[0014] Embodiments of the present disclosure provide a method for depositing and etching of low k dielectric layers (i.e., k less than or equal to about 4, often less than about 3). The methods may be suited for selective etch processes such as damascene schemes that deposit conductive materials, such as copper, within interconnects formed in the low k dielectric layers.

[0015] The term “low dielectric constant” or “low k”, with respect to the low dielectric constant (k) dielectric material being treated by the process as disclosed herein, means a dielectric constant (k value) that is less than the dielectric constant of a conventional silicon oxide dielectric material. For example, one large group of the low k materials is the carbon-containing low k material, which is a silicon oxide-containing material having one or more carbon-containing groups with one carbon atom in each carbon-containing group bonded to a silicon atom.

[0016] Whereas silicon oxide has a dielectric constant of approximately 4.0, many low-k dielectrics have dielectric constants less than 3.5. Examples of low-k dielectric materials include organic or polymeric materials. Another example is porous, low density materials in which a significant fraction of the bulk volume contains air, which has a dielectric constant of approximately 1. The properties of these porous materials are proportional to their porosity. For example, at a porosity of about 80%, the dielectric constant of a porous silica film, i.e. porous SiO2, is approximately 1.5. Still another example of a low-k dielectric material is carbon doped silicon oxide wherein at least a portion of the oxygen atoms bonded to the silicon atoms are replaced by one or more organic groups such as, for example, an alkyl group such as a methyl (CH3) group.

[0017] FIG. 1 illustrates a sectional view of a via/interconnect structure 100 manufactured by conventional processing methods. In this structure, a via 102 is embedded or surrounded by a low k dielectric layer 104. The low k dielectric layer 104 may contain silicon, oxygen, carbon, and hydrogen elements, and have a low dielectric constant (k less than or equal to about 4.0). The low k dielectric material may be spin on low k dielectrics (doped) or a CVD layer deposited by oxidation of an organosilicon compound containing C—H bonds and C—Si bonds. The via 102 is separated from the low k dielectric layer 104 by a barrier layer 106 such as silicon nitride or silicon carbide, that protects the dielectric layers from diffusion of a conductive material such as copper filling in the via 102. Underneath the low k dielectric layer 104, there is a etch stop layer 108, which helps to control the thickness of the low k dielectric layer while the entire structure is going through some etching process. There may be a second low k dielectric layer 110, such as spin on low k dielectrics (doped or undoped) or a CVD layer deposited by oxidation of an organosilicon compound. The second low k dielectric layer may also sit on a prior dielectric substrate 112 wherein the via 102 makes contact with a feature such as a metal line 114.

[0018] The first dielectric layer 104 is preferably deposited to a thickness of about 1,000 to about 5,000 Å. The second dielectric layer 110 is then deposited to a thickness of about 500 to about 4,000 Å. Deposition of low k dielectric layers can be performed using conventional processes for depositing silicon oxides, such as by oxidation of tetraethylorthosilicate (TEOS), also known as tetraethoxysilane. Alternatively, the low k dielectric layers can be produced by oxidation of an organosilicon compound containing both C—H bonds and C—Si bonds, such as methylsilane, dimethylsilane, trimethylsilane, trimethylsiloxane, or any other similar materials. The dielectric layer is cured at low pressure and high temperature to stabilize properties. Carbon or hydrogen which remains in the dielectric layer contributes to low dielectric constants, good barrier properties, and reduced etch rates. The carbon and hydrogen contents of the deposited dielectric layers is controlled by varying process conditions.

[0019] FIGS. 2-5 illustrate sample processing steps to deposit the low k dielectric material without using an etch stop layer according to one example of the present disclosure. FIG. 2 illustrates that the metal contact 114 is formed on an existing substrate 112, and a layer of low k dielectric material 200 of a predetermined thickness is deposited on top of the substrate 112 and metal 114. The initial thickness of the low k dielectric material 200 can be approximately 6500 Å. The general process for depositing the low k dielectric material can be the similar to the conventional methods. However, it would be preferred that a low temperature condition is used. In one example, the deposition is performed at the temperature below 300° C., preferably below 50° C. such as around 35° C. At this relatively cold temperature range, the deposited layer is partially polymerized during deposition and polymerization is completed during subsequent curing of the layer.

[0020] The subsequent curing process is done by a process referred to as a low power curing process. In addition to setting the energy level, the power of the curing process is actively controlled by varying the curing time,. For example, a plasma curing is performed for about 3 to 10 minutes at a temperature of about 400° C. In some embodiments of the present disclosure, the plasma energy level is set around or below 2000W and the curing duration is about 6 minutes. The plasma curing can be a remote plasma curing as well as a radio frequency plasma curing.

[0021] FIG. 3 illustrates the result of the plasma curing in which the one layer of low k dielectric material is now controlled and separated into two layers 300 and 302. The top layer 300 has its physical characteristics that is different from the second layer 302 due to the fact that these two layers have different compositions caused by the low temperature deposition and controlled low power curing. One feature for these two layers is that the top layer or the trench layer 300 has an etch rate much lower than the lower layer 302 due to its higher density than the lower one 302. The thickness of the trench layer 300 is about 2000 to 4000 Å.

[0022] It is thus noted that the above described process does not need an etch stop layer to be separately formed. The difference in the etch rate between the top layer and the lower layer of the low k dielectric material satisfy the needs to control the etch process, thus eliminating the need of the using an etch stop layer between the two low k dielectric layers and simplifying the entire low k material deposition process both in terms of processing time and cost. It thus provides a larger etch window about trench depth and profile control. And since the formation of both layers all will happen in one reaction chamber, it reduces the both the process time and costs. In addition, the increased density of the top layer low k material also enhances the material mechanical property.

[0023] In another embodiment, a dielectric layer may be cured to provide uniformity throughout the dielectric layer. A dielectric layer may be damaged when certain photo resist areas are removed. If not treated, this damage may cause the dielectric layer to become an absorption site that absorbs volatile material, such as moisture, which may increase the dielectric constant of the dielectric layer. In order to reduce the damage and aid in the conversion of the low k material to an amorphous silicon oxide, the deposited low k material may go through a bake process and then a plasma curing process. The bake process may use a temperature range from 150° C. to 250° C. for a time interval sufficient to liberate substantially all of any solvent or dispersant from the deposited low k material. By curing the low k dielectric layer, damage to the dielectric layer may be reduced and the conversion of the low k dielectric is completed.

[0024] In one example, the dielectric layer is cured at a temperature between approximately 250° C. and 450° C. for a period of time to initiate polymerization or cross-linking sufficient to convert the deposited low k material to a low dielectric constant layer. In some examples, the cure temperature approaches the decomposition temperature of the selected low k material. In order to speed up the curing process, the conventional methods may utilize RF plasma to bombard the low k material in order to transfer radiation energy thereto. However, the ion bombardment may damage the low k material by causing degradation in the uniformity of the deposited material, which may impact the quality of the low k dielectric layer.

[0025] FIG. 6 illustrates one such low k dielectric layer wherein poor uniformity has formed a deposited dielectric layer 400 with a gradient composition. In FIG. 6, the RF plasma 402 is from relatively high power energy sources (not shown) and affects at least a portion of the dielectric surface 404. Another portion 406 may be unaffected. The bombardments cause a gradient low k material composite in the deposited dielectric material 400, which destroys the uniformity of the material and may result in the dielectric material having a higher k value. For example, the upper portion 404 may be more dense than the lower portion 406. Although the conventional RF plasma curing may vary in its plasma power level, temperature, and pressure of the reaction chamber, cycle time, and frequencies to improve the result, the fact that the particles from the plasma source are bombarded onto the deposited low k dielectric material cannot be changed.

[0026] Referring to FIG. 7, in order to get a homogeneous composite of the low k dielectric layer, a remote plasma curing process 408 may be used instead of the direct RF plasma curing process. Plasmas, which are excited by action of electrical energies of different frequencies (e.g., from null (direct voltages) to microwave frequencies), may be used to treat substrate surfaces in semiconductor manufacturing. In general, the plasma may be produced in an excitation gas (e.g., H2) remote from the low k material and the excitation gas excites reactant low K materials. The excitation gas may comprise a non-coating forming gas or a mixture of several non-coating forming gases passing through a discharge zone, in which excited and atomic species are formed. The term “non-coating-forming gas” may be used to refer to such gases as noble gases, CH4, O2, H2 and N2O. This excitation gas is mixed with a coating-forming gas (e.g., H2), which may comprise a single coating-forming gas or several gases, in a discharge-free region remote from the excitation source.

[0027] The deposition process may use a plasma-CVD. More specifically, in a selected region, the excitation gas and the coating-forming gas may be mixed to cause an interaction between the excitation gas and the coating-forming gas, which causes a transfer of excitation energy of molecules or atoms and causes a homogeneous pre-reaction. The pre-reacted components then react heterogeneously with the substrate and form the low k dielectric layer deposited thereon.

[0028] A remote plasma curing process may be used that does not directly bombard the deposited material and does not prevent or negatively affect the desired chemical reactions from happening. The curing may be performed in Rapid Thermal Processing (RTP) equipment with a radiation source. In the present example, the curing process may last between approximately one to ten minutes and may occur at a temperature of approximately 250° C. to 450° C.

[0029] During the curing process, the substrate (e.g., the wafer) may be maintained in an environment in which only a very low oxygen level is permitted to avoid undesired oxidation. A nitrogen or other inert gas atmosphere may be used. In addition, the curing time may vary depending on the low k material selected for the deposition, as well as other considerations such as cure temperature, process gas ambient atmosphere, time-temperature process conditions and heating condition.

[0030] It is understood that several modifications, changes and substitutions are intended in the foregoing disclosure and in some instances some features of the disclosure will be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the disclosure.

Claims

1. A method for forming a low k dielectric material block comprising: depositing a low k dielectric layer over a semiconductor substrate; and curing the deposited low k dielectric layer using a remote plasma process in which an excitation gas is excited in a selected region remote from the deposited low k dielectric layer to carry radiation energy and transfer the energy to the low k dielectric layer when the excitation gas contacts the low k dielectric layer.

2. The method of claim 1 wherein the curing lasts from approximately thirty seconds to ten minutes.

3. The method of claim 1 wherein the curing is completed in Rapid Thermal Processing (RTP) equipment with a radiation source.

4. The method of claim 1 wherein the curing occurs at a temperature of approximately 250° C. to 450° C.

5. The method of claim 1 wherein the depositing is accomplished using a plasma chemical vapor deposition (CVD) process.

6. The method of claim 1 wherein the excitation gas is H2.

7. The method of claim 1 wherein the excitation gas is selected from a group consisting of noble gases, CH4, O2, H2, N2O, or a combination thereof.

8. A method for forming a substantially homogenous layer of dielectric material, the method comprising: depositing a low k dielectric layer over a semiconductor substrate; exciting a gas in a location remote from the deposited low k dielectric layer; and directing the excited gas towards the deposited low k dielectric layer, wherein energy contained by the gas operates to cure the low k dielectric layer when the gas contacts the low k dielectric layer.

9. The method of claim 8 wherein the curing lasts from approximately thirty seconds to ten minutes.

10. The method of claim 8 wherein the curing occurs at a temperature of approximately 250° C. to 450° C.

11. The method of claim 8 wherein the depositing is accomplished using a plasma chemical vapor deposition (CVD) process.

12. The method of claim 8 wherein the gas is H2.

13. The method of claim 8 wherein the gas is selected from a group consisting of noble gases, CH4, O2, H2, N2O, or a combination thereof.

14. The method of claim 8 further comprising: creating a trench; creating a via; and depositing a conductive metal into the trench and via.

15. The method of claim 14 further comprising performing a chemical mechanical planarization process after depositing the conductive metal.

16. A semiconductor device comprising: a low k dielectric material block having a first low k dielectric layer with a higher density on top of and contiguous to a second low k dielectric layer; a trench formed in the first low k dielectric layer; a via formed in the second low k dielectric layer; and a conductive metal deposited into the trench and the via.

17. The semiconductor device of claim 16 wherein at least one of the first and second layers is substantially homogenous.