Patent Description:
Some devices require different implant depths of an implant species in defined implant areas of the device. This typically involves multistep lithography, i.e. performing a first implant area lithography using a first temporary resist and a first implant energy, removing the first temporary resist, and then performing a second implant area lithography using a second resist and a second implant energy. Multistep lithography is expensive and inevitably results in indirect alignment of the implant areas due to the multistep lithography process involved. Hence, a more tolerant circuit design is needed which leads to a loss of device area and to a limitation of device performance.

Grey-tone lithography is an alternative approach to enable selective reduction of implant depth. Grey-tone lithography involves patterning the resist in the vertical dimension, i.e. modifying the resist thickness to obtain different implant depths. This approach avoids indirect alignment but suffers from a number of other problems, among them the lack of accuracy in vertical resist patterning.

<CIT> discloses a method of implanting an implant species into a substrate. A mask having a block array and an opening is used for tilted implantation. After implantation, a temperature driven implant species diffusion process is used to locate the implant species in the substrate at two different depth. <CIT> describes implanting an implant species in a first implant zone by vertical implantation and in a second implant zone by tilted implantation. <CIT> discloses a method of implanting an implant species into a substrate. A mask having a block array and an opening is used for tilted implantation.

According to an aspect of the disclosure, a method of implanting an implant species into a substrate at different depths includes forming an implant mask over the substrate. The implant mask includes a first implant zone designed as an opening having a first lateral dimension. The implant mask further includes a second implant zone designed as a block array. Further, the method includes implanting the implant species through the implant mask under an implant angle which is tilted against a block plane which is perpendicular to the first lateral dimension. Thereby, the implant species is located by the implant process at a first implant area which is formed by the implant species at a first depth in the substrate beneath the first implant zone and the implant species is located by the implant process at a second implant area which is formed by the implant species at a second depth in the substrate beneath the second implant zone. The first depth is greater than the second depth and the implant angle is chosen such that the implant species has in the second implant zone to pass through less implant mask material than in non-structured regions of the implant mask, wherein a bulk thickness of the block array and a thickness of the implant mask in non-structured regions is the same.

It is to be understood that the features of the various exemplary embodiments and examples described herein may be combined with each other, unless specifically noted otherwise.

A conventional multistep lithography process to obtain different implant depths is illustrated in <FIG>. Referring to <FIG>, a first implant mask <NUM> disposed over a substrate <NUM> is patterned by first lithography using a first reticle (not shown) to define a first opening 110_1 in the first implant mask <NUM>. A high energy first implant indicated by arrows is done to locate an implant species <NUM> at a first implant depth D1 in the substrate <NUM>. The substrate <NUM> may, e.g., comprise or be made of a semiconductor material layer 120_1 and a hard passivation layer 120_2.

The first implant mask <NUM> (e.g. a photoresist) is then removed and a second implant mask <NUM> (e.g. a photoresist) is disposed over the substrate <NUM>. The second implant mask <NUM> is patterned by second lithography using a second reticle (not shown) to define a second opening 150_1. A lower energy second implant indicated by arrows in <FIG> (which are shorter than the arrows in <FIG> to indicate the lower implant energy) is done to locate the implant species <NUM> at a second implant depth D2 in the substrate <NUM>, with D2 < D1. Subsequently, the second implant mask <NUM> is removed (not shown in <FIG>).

A high number of process steps is needed for obtaining the different implant depths D1, D2, making the overall process costly. Costs for reticles and metrology inline controls are double. Further, the two (or more) lithography steps can only get aligned indirectly, which results in the need of trading off substrate area and device performance.

<FIG> illustrates an alternative approach to end up at two different implant depths D1 and D2 of an implant species <NUM> in a substrate <NUM>. The process illustrated in <FIG> uses grey-tone lithography. Grey-tone lithography relies on varying the thickness of the implant mask (e.g. the resist thickness) while keeping implant energy constant. More specifically, grey-tone zones on the reticle are used to create an area 210_2 of reduced thickness of an implant mask <NUM> (which otherwise corresponds to implant mask <NUM> in <FIG>). A single high energy implant process indicated by arrows in <FIG> is then performed to locate the implant species <NUM> at the first implant depth D1 through an implant mask opening 210_1 (which corresponds to the first opening 110_1 in <FIG>) and at the second implant depth D2 through the area 210_2 of reduced implant mask thickness.

Referring to <FIG> an exemplary approach to end up at different implantation depths D1, D2 involves that an implant mask <NUM> is formed over the substrate <NUM>. The implant mask <NUM> includes a first implant zone 310_1 designed as an opening. The first implant zone 310_1, which corresponds to implant mask opening 210_1 in <FIG>, has a first lateral dimension L1.

The implant mask <NUM> further includes a second implant zone 310_2 designed as a block array. The block array comprises a number of blocks 310_2a, 310_2b, 310_2c, 310_2d. The blocks 310_2a, 310_2b, 310_2c, 310_2d may all have an equal lateral dimension (block width). The blocks 310_2a, 310_2b, 310_2c, 310_2d may be arranged relative to each other under a block pitch of a second lateral dimension L2. The first lateral dimension L1 may, e.g., be greater than the second lateral dimension L2, i.e. L1 > L2.

The first lateral dimension L1 may, e.g., be a minimum lateral dimension of the opening of the first implant mask zone 310_1 in a direction perpendicular to a block plane which is perpendicular to the first lateral dimension L1. That is, the opening may, e.g., be equal to or wider than L1 along other cross sections perpendicular to the block plane.

The blocks 310_2a, 310_2b, 310_2c, 310_2d may have a height equal to the thickness of the implant mask <NUM>. More specifically, the implant mask <NUM> may have a constant thickness, or, if e.g. substrate topology is covered by the implant mask <NUM>, an upper surface of the implant mask <NUM> adjacent the first implant zone 310_1 may level with an upper surface of the blocks 310_2a, 310_2b, 310_2c, 310_2d of the block array.

The block array may include a number of M blocks 310_2a, 310_2b, 310_2c, 310_2d,. , with M equal to or greater than, e.g., <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>.

Accordingly, the block array may have a lateral dimension in a direction perpendicular to the block plane (i.e. parallel to L1 and L2) which may be (much) larger than L2, e.g. equal to or larger than M times L2 (L2 is the block pitch). For example, the lateral dimension of the block array may be of the same order than L1.

In general, the blocks could have an equal length (in a dimension along the block plane) or could have different lengths. The length of a block could be equal to or greater than, e.g., <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM> times the width of the block. That is, the blocks may either be designed as lamellas, wherein a block may be termed a lamella if the length of the block is, e.g., equal to or greater than <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM> times the width of the block. Many of the examples disclosed further below exemplify block arrays as lamella arrays. However, the blocks may also be designed as blocks with similar or equal dimensions in length and width, e.g. may even be shaped as squares, see the examples shown in <FIG>. Further, multiple block arrays could be combined (e.g. grouped or interlaced with one another) to form a (composite) block array. Such (composite) block array could be designed as a segmented block array, a serrated or staggered block array or may form any other pattern of composed or superimposed block arrays (<FIG> illustrate a few possible examples of such "irregular" types of block arrays).

The implant species <NUM> is implanted through the implant mask <NUM> into the substrate <NUM> under an implant angle α tilted against the block plane, see <FIG>. The implant angle α is different from <NUM>°. The implant angle α is chosen such that the implant species <NUM> has to pass through less implant mask material in the second implant zone 310_2 than in non-structured regions of the implant mask <NUM>, even though the (bulk) thickness of the block-structured second implant zone 310_2 and the thickness of the implant mask <NUM> (in non-structured regions) is the same.

Differently put, the combination of a tilted implant with areas of a segmented implant mask structure (as, e.g., represented by the second implant zone 310_2 of the implant mask <NUM>) causes the implant species <NUM> (e.g. a dopant such as boron, indium, etc. for p-type doping and/or phosphorus, arsenic, antimony, etc. for n-type doping) to be closer to the surface of the substrate <NUM> than under large implant mask openings such as, e.g., represented by the first implant zone 310_1. Stated differently, the patterning of the second implant zone 310_2 in the implant mask <NUM> acts like a "subresolution pattern" for a tilted implant or a series of tilted implants.

As a consequence, the implant species <NUM> implanted through the first implant zone 310_1 of the implant mask <NUM> is located at a first depth D1 which is greater than the second depth D2 at which the implant species <NUM> implanted through the second implant zone 310_2 is located. Non-structured regions of the implant mask <NUM> may completely shield the substrate <NUM> from the implant species <NUM>, i.e. may act as blocking zones of the implant mask <NUM>.

More specifically, when implanting the implant species <NUM> through the first and second implant zones 310_1, 310_2, the implant species <NUM> is located in the substrate <NUM> with a specific distribution in depth. A first implant area 340_1 associated with the first implant zone 310_1 and a second implant area 340_2 associated with the second implant zone 310_2 may both be defined as areas in the substrate <NUM> having an implant species concentration above a certain (absolute or relative) threshold. The first and second implant depths D1, D2 may then, e.g., be defined geometrically based on the shape of the respective implant areas 340_1, 340_2 (e.g. corresponding to the centerlines thereof) or based on the implant species <NUM> concentrations (e.g. D1 and D2 may correspond to the depths of maximum implant species concentration in the first and second implant areas 340_1, 340_2, respectively).

Comparing <FIG> and <FIG>, the block array approach combined with tilted implant (<FIG>) may have the same effect as using an area of reduced implant mask thickness 210_2 in the grey-tone lithography approach of <FIG>. Similar to the grey-tone lithography approach, the multiple depths D1, D2 may be obtained by using a single implant mask <NUM>.

The first implant area 340_1 in the substrate <NUM> defined by the implant species <NUM> embedded in the substrate <NUM> through the first implant zone 310_1 of the implant mask <NUM> may have bent-up edges. Similarly, the second implant area 340_2 in the substrate <NUM> defined by the implant species <NUM> embedded in the substrate <NUM> through the second implant zone 310_2 of the implant mask <NUM> may have bent-up edges. The bent-up edges are caused by the tilted implantation which results in that implant species <NUM> which hit the implant mask <NUM> in the vicinity of the edges of the first or second implant zones 310_1, 310_2 pass through different lengths of implant mask material (e.g. photoresist material) depending on the implant angle and the distance from the edge of the respective first or second implant zone 310_1, 310_2.

While in <FIG> the tilted implantation is inclined against the vertical direction under the implant angle α, a further implant process may be performed through the implant mask <NUM> under the negative implant angle, i.e. under the implant angle -α (not shown, the substrate <NUM> may be rotated by <NUM>° to set the implant angle -α). Alternatively or in addition, further implant process(es) may be performed through the implant mask <NUM> under implant angle(s) different from α and/or -α.

The semiconductor material layer 120_1 of the substrate <NUM> may comprise or be of a bulk semiconductor material, e.g. Si, SiC, SiGe, GaAs, GaN, AlGaN, InGaAs, InAlAs, etc. The (optional) hard passivation layer 120_2 of the substrate <NUM> may comprise or be of an electrically insulating dielectric material, e.g. silicon oxide, silicon nitride, etc..

<FIG> illustrates a perspective view of an exemplary second implant zone 310_2 of an implant mask <NUM> disposed over the substrate <NUM>. As shown in <FIG>, the block array may be formed by a number of linear, parallel blocks 310_2a, 310_2b, 310_2c, 310_2d,. , which are, e.g., lamella-shaped in <FIG> illustrates another perspective view of the exemplary second implant zone 310_2 of the implant mask <NUM>.

In general, the blocks 310_2a, 310_2b, 310_2c, 310_2d,. need not to be linear (or straight). It is also possible that the blocks (e.g. lamellas) 310_2a, 310_2b, 310_2c, 310_2d,. have a bent or curved shape (but may still have the other features described above such as, e.g., a constant pitch p, a constant height, a constant width w etc.).

In general, however, the pitch p of the blocks 310_2a, 310_2b, 310_2c, 310_2d,. and/or the width w of the blocks 310_2a, 310_2b, 310_2c, 310_2d,. need not to be the same. Rather, it is possible that the pitch p and/or the width w of the blocks 310_2a, 310_2b, 310_2c, 310_2d,. may have multiple certain values and/or may gradually change. This, e.g., would allow to create a second implant area 340_2 where the implant species <NUM> (e.g. dopant) is located at multiple certain depths and/or is located at a gradually changing depth.

The implant process can be tailored in multiple ways, e.g. by varying the implant mask thickness t, the pitch p (also referred to as L1 in <FIG>) of the blocks 310_2a, 310_2b, 310_2c, 310_2d, the implant angle α and the block width w. As illustrated in <FIG>, the implant angle α may be chosen to match the pitch p and the aspect ratio t/(p-w) of the blocks 310_2a, 310_2b, 310_2c, 310_2d. More specifically, a virtual line VL parallel to the implant direction may "touch" e.g. the upper right corner of a block (here: block 310_2c) and then pass through the neighboring block (here: block 310_2d) and leave this block at the lower right foot edge.

This relationship of dimensioning the block array and setting the implant angle results in that each implant species <NUM> always passes through the same amount of implant mask (e.g. resist) material irrespective of where it hits the block array within the second implant zone 310_2 of the implant mask <NUM>. Thus, the second implant area 340_2 formed in the substrate <NUM> by the embedded implant species <NUM> should ideally have a straight or linear shape (except of the bent-up edges). However, as described further below, the second implant area 340_2 will have a slightly wavy profile due to unavoidable "non-ideality", e.g. tolerances (variations in width w and/or height t and/or irregularities in pitch p) in the geometry of the block array and/or deviations from the desired implant angle and/or aberrations from implant parallelism, etc..

As depicted in <FIG>, an optimum implant angle may be written as α = arctan(p/t). More generally, the implant angle α may, e.g., be within the ranges of arctan(p/t)±<NUM>°, arctan(p/t)±<NUM>°, arctan(p/t)±<NUM>°, or arctan(p/t)±<NUM>°.

Further, while in the example shown in <FIG> the virtual line VL runs only through one block (here: block 310_2d), it is also possible that the virtual line VL intersects with multiple blocks 310_2a, 310_2b, 310_2c, 310_2d. If N blocks are intersected, the optimum implant angle may be written as α = arctan(Np/t). The implant angle α may then be within the ranges of arctan(Np/t)±<NUM>°, arctan(Np/t)±<NUM>°, arctan(Np/t)±<NUM>°, or arctan (Np/t)±<NUM>°, with N being an integer equal to or greater than <NUM>.

Generally, the implant angle α may, e.g., be between <NUM>° and <NUM>°, or <NUM>° and <NUM>°, or <NUM>° and <NUM>°, or <NUM>° and <NUM>°. The block pitch p may, e.g., be equal to or less than <NUM> pm, <NUM> pm, <NUM> pm, <NUM> pm, <NUM> pm, <NUM> pm, <NUM> pm, <NUM> pm, <NUM>, <NUM> pm, <NUM> pm, <NUM> or <NUM>. The thickness t of the implant mask <NUM> may, e.g., be equal to or greater than <NUM> pm, <NUM> pm, <NUM> pm, <NUM> pm, <NUM> pm, <NUM> pm, <NUM> pm, <NUM> pm, <NUM> pm, <NUM> pm, <NUM> pm, or <NUM>. All these quantities may be combined. The combinations may, e.g., be in accordance with the relationship between implant angle and implant mask geometry as described above.

Referring to <FIG>, a method of implanting an implant species into a substrate at different depths may comprise, at S1, forming an implant mask over the substrate, wherein the implant mask includes a first implant zone designed as an opening having a first lateral dimension, and a second implant zone designed as a block array. For instance, the block array may have a block pitch of a second lateral dimension, wherein the first lateral dimension is greater than the second lateral dimension. The formation of the implant mask at S1 can be done by standard resist lateral patterning tools, materials and processes which have been developed and optimized through decades of technological progress in the field of lithography.

At S2, the implant species is implanted through the implant mask under an implant angle tilted against a block plane. Thereby a first implant area is formed by the implant species at a first depth in the substrate beneath the first implant zone and a second implant area is formed by the implant species at a second depth in the substrate beneath the second implant zone, wherein the first depth is greater than the second depth. As many of the existing implant tools already allow for a tilted implant, S2 will typically not incur any additional cost to existing wafer processing methods or tools.

Generally, the process described herein may be modified in a variety of ways. For instance, the same implant mask may be used for a second (or further) implant having different implant energy and/or different implant angles and/or different implant species <NUM> (dopant). For instance, two or more tilted implants from the left and/or right side may be performed. Further, the whole process S1, S2 could be done more than once. For instance, after S1 and S2 the implant mask may be removed, another implant mask including another block array could be applied and the process of S1, S2 may be repeated with the other implant mask using different implant energy and/or different implant angles and/or different implant species <NUM> (dopant).

If compared to grey-tone lithography, a reticle to print small blocks is less expensive than a reticle with subresolution grey-tone patterns for vertical resist patterning. While for grey-tone lithography only a limited subset of photoresist materials is suited, the approach described herein may use a wide range of mask materials (i.e. a wider range of photoresists or other materials configured for shieling the substrate from the implant species <NUM>). The critical dimensions (CD) of the blocks in the photoresists are much easier and much more accurate to control than grey-tone modulated resist thickness. Additionally, a run-to-run control of the critical dimensions (e.g. pitch, aspect ratio, width, height) of blocks can easily be done, while a run-to-run control on grey-tone resist thickness is difficult to implement. In short, the "tilted implant - block array" approach described herein provides for many advantages if compared to conventional grey-tone lithography technology.

<FIG> illustrate the equivalence of existing grey-tone lithography (combined with vertical implant) and the block array approach combined with tilted implant relative to the vertical direction. The implant mask <NUM> (e.g. structured resist) of <FIG> includes an area of reduced implant mask thickness 610_2 which is laterally bounded by two implant mask openings 610_1. The area of reduced implant mask thickness 610_2 may be bar-shaped. This kind of grey-tone pattern translates into the implant mask <NUM> shown in <FIG>. The implant mask <NUM> (e.g. structured resist) includes a second implant zone 710_2 designed as an block array which is laterally bounded by two first implant zones 710_1 designed as longitudinal openings in the implant mask <NUM>. The footprint of the block array of implant mask <NUM> may coincide with the footprint of the area of reduced implant mask thickness 610_2. Further, the implant mask openings 710_1 may correspond in shape with the implant mask openings 610_1. It is to be noted that the thickness of the second implant zone 610_2 in <FIG> translates in a specific relationship between the implant angle α, the width w and the pitch p of the block array for a given thickness t (i.e. block height) of the implant mask <NUM>.

<FIG> illustrate top views on various "irregular" implant masks including block arrays of different patterns, i.e. on implant masks which are not formed by a regular lamella array. Instead, <FIG> illustrates a block array formed by lamella segments which form a staggered arrangement, i.e. the lamella segments are aligned offset to one another relative to the lateral dimension. <FIG> illustrates a block array composed of patterns of different block shapes, e.g. squares and rectangles (lamellas). Again, the blocks may be aligned in a staggered arrangement. <FIG> illustrates a block array also composed of patterns of different block shapes, wherein the blocks are (at least partly) separated from each other. <FIG> illustrates a block array in checkerboard pattern. Further, many other examples of "irregular" block arrays are feasible. As apparent from <FIG>, these examples may still use a constant pitch and/or a constant width. All these examples provide for similar shallow implant profiles as "regular" block array or lamella array, except that the implant profile could be slightly affected due to known lithographic line end shortening effects, i.e. the resist image of the end of small blocks or lamellas can get slightly rounded, which as a consequence will locally have some impact on the implant profile. It is to be noted that any of the above implant mask pattern features could be combined selectively or in aggregation with any of the features disclosed elsewhere in this application.

<FIG> illustrates implant profiles (in terms of areas of different implant species concentrations) obtained by computation which simulates the implanting of the implant species <NUM> through a block array in a direction parallel to the block plane (left portion from the dash-dotted line in <FIG>) and under an angle tilted against the block plane (right portion from the dash-dotted line in <FIG>). The simulation was performed for the example of p = <NUM> pm, w = <NUM> (i.e. the spacing between the blocks is <NUM>), a photoresist with t = <NUM> and a tilted boron implant. An implant angle α = <NUM>° was used, wherein the implant angle α was oriented perpendicular to the block plane in the right side portion of <FIG> and was oriented along the block plane in the left side portion of <FIG>. The tilted implant was done twice, i.e. a first tilted implant at implant angle α was performed, the substrate <NUM> was rotated by <NUM>° and then the tilted implant was performed again (now under an implant angle -α due to the rotation of the substrate <NUM>).

Exemplary implant profiles are indicated in <FIG> by exemplary implant areas of different concentrations of the implant species. Ranges of different concentrations are illustrated by letters a, b, c, d, e, with.

As apparent from <FIG>, the implant profile in the right side portion of <FIG> (where the blocks are hit under α = <NUM>°) is less deep than the implant profile in the left side portion of <FIG> (where only the top portions of the blocks but not the sidewalls of the blocks are hit by the implant species). Further, while the left side portion of <FIG> illustrates that the high concentration implant profile at a, b is shaped as a series of disconnected implant areas located between the blocks, the high concentration implant profile at a, b in the right side portion of <FIG> is a continuous, slightly wavy area located at a smaller distance under the surface of the substrate <NUM>. The waviness of the profile is caused by the block structure and therefore provides clear evidence of the way the reduced implant depth in the second implant area 340_2 (which is defined by the implant species <NUM> embedded in the substrate <NUM> through the second implant zone 310_2, 710_2 of the implant mask <NUM>, <NUM>) has been created. Though the waviness could be avoided in theory, it will show up in all practical embodiments either due to geometrical tolerances and/or irregularities or other causes or even as a desired implant feature.

<FIG> illustrates an implant profile (again in terms of areas of different implant species concentrations) obtained the same way as in the right portion of <FIG>, i.e. by implanting the implant species <NUM> through a block array under an angle tilted against the block plane. The wavy profile of the shallow implant at around depth D2 (e.g. along the highest concentration area a) is shown by phantom line PL1 for ease of representation.

The periodicity of the wavy profile (which may correspond to the block pitch) may be smaller than the first lateral dimension.

<FIG> illustrates by way of example that the second implant area 340_2 is shaped with bent-up edges, see for example the bent-up ends of the (high concentration) implant areas b.

The second implant area 340_2 has, e.g., a laterally expanding foothills zone of low implant concentration in downward direction, see phantom lines PL2. The laterally expanding foothills zone is caused by the tilted implant and therefore provides evidence of the way the reduced implant depth in the second implant area 340_2 (which has a significantly higher implant species concentration than the foothills zone) has been created.

<FIG> schematically illustrates a device <NUM> including a substrate <NUM> having at least a first implant area 340_1 and a second implant area 340_2 which are at different depths D1, D2, respectively. The first implant area 340_1 is formed by an implant species <NUM> at the first depth D1 in the substrate <NUM>, the first implant area having a first lateral dimension L1' (which may be similar to L1 or a little be greater than L1 due to the tilted implant). The second implant area 340_2 is formed by the implant species <NUM> at a second depth D2 in the substrate <NUM>. The first depth D1 is greater than the second depth D2. As already illustrated e.g. in <FIG> and <FIG> for the high concentration implant species profiles a, b, the second implant area 340_2 has a wavy profile with a periodicity L2 smaller than the first lateral dimension.

Further, the second implant area 340_2 may be shaped with bent-up edges as, e.g., illustrated in <FIG>, <FIG>, <FIG>.

The second implant area 340_2 may have a laterally expanding foothills zone of declining implant species concentration in downward direction as illustrated in <FIG>, see phantom lines PL2.

Claim 1:
A method of implanting an implant species into a substrate (<NUM>) at different depths, the method comprising:
forming an implant mask (<NUM>) over the substrate, the implant mask including
a first implant zone (310_1) designed as an opening having a first lateral dimension, and
a second implant zone (310_2) designed as a block array;
and
implanting the implant species through the implant mask under an implant angle tilted against a block plane which is perpendicular to the first lateral dimension, whereby
the implant species is located by the implant process at a first implant area formed by the implant species at a first depth (D1) in the substrate beneath the first implant zone,
the implant species is located by the implant process at a second implant area formed by the implant species at a second depth (D2) in the substrate beneath the second implant zone,
wherein the first depth is greater than the second depth and the implant angle is chosen such that the implant species has in the second implant zone to pass through less implant mask material than in non-structured regions of the implant mask, wherein a bulk thickness of the block array and a thickness of the implant mask in non-structured regions is the same.