Find control inside control

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C#

ASP.NET

hi i need help

i need to get a control from with using method FindControl(id);
but in my control is in HTML and it have 2-ID because he is custom controll

this is how i create the control

protected override void Render(HtmlTextWriter writer)
      {
   for(int i = 0 ; i < 3; i++)
{
                writer.AddAttribute(HtmlTextWriterAttribute.Type, "checkbox");
                writer.AddAttribute(HtmlTextWriterAttribute.Id , "ch"+i.ToString());
                this.AddAttributesToRender(writer);
                writer.RenderBeginTag(HtmlTextWriterTag.Input);
                writer.RenderEndTag();
}}

and this is how i call the control in my asp.net page

<CC:ServerControl1 ID="ServerControl"  runat="server" />

this is how it looks in pagesoure

<input type="checkbox" id="ch0" id="ServerControl" />
<input type="checkbox" id="ch1" id="ServerControl" />
<input type="checkbox" id="ch2" id="ServerControl" />

this is how i use the Method FineControl(id)

for (int i = 0; i < 3; i++)
        {

            CheckBox cb = this.FindControl("ch"+i.ToString()) as CheckBox;
            if (cb != null)
            {
                if (cb.Checked == true)
                {
                    ss[pom] = i;
                    pom++;
                }
            }
}

and all time the finecontrol give me null never dont give me that what i need?
can some one give me any sugestion how to do it???
thnx for help and sorry for my bad english!